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7

HYDROCARBONTRANSPORT

AND GAS STORAGE

7.1.1 General observations on the hydrocarbontransport

IntroductionStarting from the second half of the twentieth

century, the level of development in society hasbeen conditioned by and in correlation with theavailability of energy, which calls for a distributionnetwork particularly in those areas of the Westernworld where population density is high. In thiscontext, liquid hydrocarbons and natural gas play aprominent role, and their transport from the areas ofproduction to the market is a factor of strategicrelevance, and is sometimes considered to be thecause of political instability in the regions traversed(Rifkin, 2002).

The need to transport hydrocarbons over longdistances is usually resolved by means of twosolutions, i.e. transport by tanks, overland (on roadsor railways) and by sea (in tankers); and transportthrough pipelines across the country, in accordancewith the market principles of supply and demand andsometimes across the sea. The choice between the twotransport technologies depends on the investment andoperating costs and on the certainty of the continuityof the energy supply, both in technical terms (with particular emphasis on the reliability of theinfrastructure) and in strategic terms, in relation to thepolitical crises that arose at the beginning of the thirdmillennium in those parts of the earth richest inhydrocarbons (the Middle East, Russia and other ex-Soviet countries). The tank solution is flexible,with moderate investment costs due, largely, to theinfrastructure available, such as sufficiently suitableroads and railways, which are offset, however, by veryhigh operating costs (Kennedy, 1984). Flexibility, inparticular the freedom of the choice of market from

which to obtain supplies of hydrocarbons and, to acertain extent, the freedom of where to locatereception and storage terminals for the product, iscertainly a relevant factor in the choice, but can beoutweighed by problems of security andenvironmental impact that are sometimes highlycritical issues. The tank solution is often adopted fortransport of liquid hydrocarbons over medium/shortoverland distances and, increasingly in recent years,over long overseas distances for natural gas liquefiedby means of thermo-physical processes. The facilitiesdedicated to transport by road or rail already being inplace, the tank solution is inevitably adopted foroverland transport of liquid products over longdistances, between remote continental regions andcoastal consumers. The pipeline solution is certainlyan inflexible option in comparison with the foregoingand calls for very high initial investment, againstwhich, however, operating costs are not particularlyhigh. Due to the permanent nature of a pipeline’sinfrastructure, the political stability of the area to betraversed is a decisive factor in the choice of thisoption. In fact, even if buried or under sea (to alesser extent), a pipeline is always vulnerable, beingfixed, recognizable and of great length, makingactive and effective protection across hostileterritory unfeasible.

Currently, technology is focussed primarily onlong-distance transport of hydrocarbons, in view ofthe fact that new resources are nearly always foundfar from the markets, for example, in arctic and sub-arctic zones or in the innermost areas of thecontinents. Long-distance transport becomes acrucial factor in the exploration strategy of theinternational oil companies, in terms of its technicalfeasibility and the economic competitiveness of theavailable transport solution. Special mention must bemade of the exploitation of natural gas, a topic much

7.1

Transport by pipeline

771VOLUME I / EXPLORATION, PRODUCTION AND TRANSPORT

discussed in the various international forums onenergy at the start of the twenty-first century, as thereis a consensus that the energy policies is movingaway from the use of fuel oil towards an increasinguse of natural gas, both to meet the growing demandfor energy in the face of the simultaneous reductionin liquid stocks, and also because the environmentalimpact resulting from the combustion of natural gasis held to be tenable.

The hydrocarbons exploration and productionindustry has always considered long-distance transportof natural gas uncompetitive because of the hightransport costs per unit of mass (and therefore ofenergy) compared with liquid hydrocarbons, which forthe same volume have a significantly higher heatcapacity. This has led to a limiting of the explorationin search of reservoirs of natural gas itself, as a resultof which, the gas availability figures currentlypresented by the oil industry, by default, areapproximate. The energy crisis and the internationaltensions in the traditional markets at the end of thetwentieth century have, however, led to a review of theposition by oil companies. Gas transport, particularlyin large quantities (for example 10-20 billion cubicmetres per year) over long distances, through pipelineswhose costs are optimised and which offer long-termreliability, is becoming a central argument in the plansfor development of gas transport from remote regions,such as the arctic regions and the innermost regions ofthe Euro-Asiatic continent. Fig. 1 shows the bestoptions for bringing natural gas to the market, inrelation to distance and volume. It is a schedule usedby many oil companies, based on in-depth studiescarried out in various contexts (energy, finance,politics) to select the most appropriate technologies for the exploitation of gas fields locatedin remote regions.

The flexibility of the system of transport by tanker,based on onshore liquefaction and regasificationplants, has to be evaluated against the economy oflong-distance natural gas transport by pipeline.Transmission by pipeline uses no more than 10% ofthe energy contained in the gas transported, comparedwith the 30% used in the liquefied gas option.Moreover, a comparative analysis of the costshighlights that for long-distance pipelines the actualcost of transport in the year 2000 varied betweenUS$1.40 and US$1.80 per unit of energy transported(1 million BTU), while the transport cost of liquefiednatural gas was in the order of US$2.50-2.70 for thesame amount of energy transported.

Brief historical outlineThe general public is not particularly aware of the

transport of hydrocarbons by pipeline because thepipelines are generally buried or under sea and do notinterfere, or at least should not interfere, with humanactivities. It comes to light only when an accidentoccurs which causes damage to property or publichealth. For those working in the hydrocarbonsindustry, pipelines are an investment to be verycarefully managed, as they are support elements of aState’s energy policies, both when they are a principlemeans of importation from foreign markets, and whenthey are part of a distribution network for a finishedproduct in the country.

The use of pipelines for the transport of fluids hasa long history. In Mesopotamia and in Egypt, 5,000years before Christ, clay pipes were used for irrigationand drainage purposes. In China, in the Fifth centuryBC, bamboo pipes wrapped in cloth impregnated with wax were used to transport natural gas toBeijing, the capital of the empire, for the purpose ofillumination. The Romans, during the golden age ofthe empire, when creating large infrastructurecomponents such as the aqueducts, used lead pipes inthe most important branches of the network. Up to theEighteenth century, particularly in the architectureand urban development of the Renaissance era, manytechnically interesting examples of the use of pipesfor hydraulic connections can be found, both for thetransporting of water and for the transporting ofhydrocarbons used in public lighting, but usingtechnologies that were not different from those usedby the Romans. A significant advance was made, inthe Eighteenth century, with the introduction of pipesin melted iron for aqueducts and sewers, andsometimes for gas transport for lighting. In 1879,following the discovery of an oil field inPennsylvania, a first pipeline was laid, with adiameter of 15 cm, for the oil transport across thestate over a distance of about 180 km. In the same

772 ENCYCLOPAEDIA OF HYDROCARBONS

HYDROCARBON TRANSPORT AND GAS STORAGE

natural gaspipeline

0

25

20

15

gas

volu

me

(109

Sm

3 /yea

r)

10

5

01,000 2,000 3,000

distance (km)4,000 5,000 6,000

gas to liquidssyndiesel, dimethyl ether, methanol

AC/DC current

LNG

Fig. 1. The industry’s options for transportgas to market, based on distance and volume.

area, nine years later, another line of 20 cm diameterand about 120 km in length was laid for natural gastransport from Pennsylvania to the state of New York.It was the start of an industry that, in the years at theend of the nineteenth and the beginning of thetwentieth centuries, adopted highly innovativesolutions principally in the United States, inVenezuela and in the area to the west of the CaspianSea. In those years, the heavy engineering industrystarted to produce high resistance steel pipes, that atthe time incorporated screw joints which werecomplex and not very efficient for pipelines set up forlong distance transport of hydrocarbons. Theintroduction of submerged arc welding, around theyear 1920, substantially changed the scenario andfrom then began the production of large diameterpipelines, which we can call modern.

The majority of the pipelines in operation todaywere produced after the Second World War, often inresponse to a country’s very particular needs. This iswhat happened, for example, in the United Stateswhere, during the Second World War, between 1942and 1943, the Big Inch and the Little Big Inch wereconstructed for the transport of fuel oil from Texas tothe ports on the North-East coast, so as to avoid thedanger of attacks on tankers by German submarines.The Arab oil embargo of 1974, following the MiddleEast crisis provoked by the Yom Kippur War,accelerated the construction of the pipeline fortransport of fuel oil from the rich reservoirs ofPrudhoe Bay, in the part of Northern Alaska beside theBeaufort Sea, to the terminal of Valdez, in the extremesouth of Alaska on the Pacific Ocean, to which tankerscould have access even in winter (Williams, 1999). Atpresent, given the political instability of the MiddleEast, the oil companies are moving towards theexploitation of offshore gas fields in the Canadianarctic with transmission to the United States bypipelines that traverse areas which are difficult andparticularly sensitive from an environmental point ofview. (Cope, 2004).

While, in the first instance, we see only theacceleration of the development of an already-plannedfacility, the second and third instances are projectswhich present a challenge for the technology of theday. In the case of the so-called Trans-Alaska Pipeline,technology and engineering were heavily involved, forthe first time, in the construction of a pipeline in orderto overcome the difficulties presented by the project,such as permafrost, mountainous and volcanic areas,high seismic risk, a very vulnerable environment, etc.The result has certainly been satisfying (30 years ofoperation confirm that), beyond justifying the highestcosts ever incurred in the completion of such astructure. It is commonly acknowledged that this

project initiated the challenge of laying sub seapipelines over deep seabeds, as shown by thetechnological initiatives undertaken in the Seventies inthis sector of the oil industry (resulting in thecompletion of three pipelines in the Mediterraneanthat reached the depth of about 600 m, at that timeconsidered almost abyss-like for sub sea pipelines).

The development of the network of pipelines inEurope (including Russia) has been much moregradual than in the United States, and not necessarilylinked to important political events. A particular caseworthy of mention is Italy, which in the Seventiesbecame the first country in the Western world to movetowards an energy policy based on methane gas, withthe construction of the first gas transport lines acrossthe deep seabeds of the Mediterranean (the SicilianChannel and the Straits of Messina).

When speaking of sub sea pipelines, we are usuallyreferring to:• Lines dedicated to the transport of the product

(generally multiphase or oil), from a platform to amarine terminal for treatment and subsequentexportation, or from a platform to an onshoreterminal (oil and gas, sometimes multiphase). Thelengths are less than 100 km, for diameters rangingfrom 12'' to 18'' (ca. 30.5-45.7 cm).

• Lines crossing the great marine basins, such as theMediterranean and the North Sea, from one shoreto another (for example, between North Africa andSicily, between Norway and the rest of northernEurope); these are long lines for gas transport, overdistances between 100 and 1,000 km, and of largediameter, between 20'' and 44'' (ca. 50.8-112 cm).There are also subsea pipelines linking the

different areas into which a very large field issubdivided for extraction; they transport oil and/or gas(sometimes a multiphase product), are of smalldiameter, between 4'' (ca. 10.2 cm) and 16'' (ca. 40.6cm), and are less than 10 km long.

The oldest sub sea pipelines were for dischargesinto the sea, the first of which were set up in thenineteenth century. The first usages in thehydrocarbons industry were short lines for loading andunloading, that were built onshore and then towed intoposition and lowered onto the seabed with the help ofbarges and using not particularly sophisticatedequipment. This is a technology still in use for suchpurposes, obviously enhanced by more sophisticatedand powerful equipment. The industry for theexploitation of offshore oil and gas fields is, however,relatively recent. The first sub sea pipelines wereinstalled in the immediate post-war period, in the Gulfof Maracaibo (Venezuela) and in the Caspian Sea.They were short, of small diameter and laid inrelatively shallow waters. The need for hydrocarbon


TRANSPORT BY PIPELINE

transport over longer distances and deeper waterscame immediately afterwards. On this topic, what wasdone during the Second World War in connection withthe landing of Anglo-American troops in Normandy, isrelevant. The military asked a British oil company, thatwould later become British Petroleum, to install apipeline between Great Britain and France across theEnglish Channel. During the study for the project,code-named PLUTO (Pipe Line Under The Ocean),two types of pipeline were considered: one similar to asub sea was a lead pipe, reinforced on the outer side bylayers of steel wire covered with a resinous matrix; theother consisting of normal steel pipe with butt weldsand without any anticorrosion protection (Searle,1995). The choice fell on the former and the tests werecarried out rapidly in just a few weeks. The reinforcedlead pipe was constructed onshore and coiled ontofloating spools which were towed by tugboats andfrom which it was uncoiled and laid onto the seabed.The pipeline connected England’s Isle of Wight to theCotentin peninsular in France. The whole layingoperation took just 10 hours (an excellent performanceeven by current standards), and even today there is talkabout the effectiveness of the PLUTO solutioncompared with transport through tankers, in the portsbrought back into operation immediately after thelandings, by the military engineers of the Allies.

Today, when talking about transport ofhydrocarbons in sub sea pipelines in deep waters overlong distances, it is usually with reference to gas,which is transmitted at high pressure, always above10 MPa, so as to guarantee high flow rates, usingdiameters which are not particularly big andtherefore not difficult to install with the equipmentavailable (Bruschi, 2002). Even if suitable treated toavoid problems of corrosion, the transport of oil overlong distances sub sea is limited by pumping issues,which require the use of intermediate pumpingstations, and therefore costs that make such transportuncompetitive compared with traditional transport bytankers. The problem of thrust becomes almostinsurmountable when the seabed to be crossed isparticularly irregular, with repeated steep slopes(typical examples being the profiles of the pipelinesthat cross the Mediterranean); for this reason the onlyexamples of sub sea oil pipelines of any significantlength are found in the North Sea, on routes thatfeature a particularly even ascent between the 70 m of the Ekofisk production basin in theNorwegian sector of the North Sea, and the terminalin Great Britain.

Types and classifications of pipelinesA pipeline is a system that calls for a variety of

components such as valves, junction elements,

pumps (liquids) and compressors (gas), flow metres,inspection devices, transducers, cathodic protection,control systems, etc. Fig. 2 shows an outline of apipeline transport system with the variouscomponents highlighted. Figs. 3 and 4 show the flowchart of the decision-making process that leads tothe construction of a pipeline for transport of theproduct from a field in production: the former showsthe development of the various engineering phasesthat lead to the construction of an exportationpipeline; the latter shows the interaction between thevarious disciplines that participate in thedevelopment of an exportation pipeline project.

The exportation pipelines that start at a pumpingstation can be classified on the basis of:• Environment to be crossed, for example, overland

pipelines or sub sea pipelines, with sub-categoriesthat further qualify the principal characteristicssuch as the elevation profile, the nature of theterrain, etc.

• Product transported, whether liquid, gas ormultiphase, with sub-categories that qualify thefluidodynamics of the transport such as pressure,temperature, velocity, etc.

• The materials with which they will be constructed,the typical carbon-manganese steel or special



wellsgathering and injectionflow lines

plant piping exportation(transportation)

pipelines

users users

users users

terminal

compression stations

oil and gastreatment

distribution

distribution

Fig. 2. Schematic of the elements of a transportsystem, showing the various components.

corrosion resistant alloys, and the weldings withwhich they will be assembled.

• The technologies used in the construction, inparticular those relating to the laying operation andthe work needed for burial, both for overland andfor sub sea pipelines.The factors that differentiate a pipeline in

comparison with other transport systems are:• Its operation, in relation to the transport cost of

the units of energy and of the transport capacityacross environments that may be particularlyhostile.

• The environment which does not have any impactduring operation, once the operations relating tothe construction phase of a pipeline are completed(and which call for particular care, especially in themost vulnerable environments).

• Safety, highlighted by satisfactory breakdown ratesboth as far as transport of liquid products isconcerned and that of gas; the transmission of gasat high pressure can, however, present problems in



execution phasedefinition phase

basic design

risk analysis

development plans

development ofcontractual strategy

bidding process

pre-qualifications

bid contentsand structure

bid evaluation

technical strategy

commercial strategy

timing strategy

interface strategy

conceptual design

project budget

project timing

project specifications

data for basic design

detail engineering

material supply

construction

installation

tests

Fig. 3. The phases of the development of a pipeline project.

route safety

hydraulics

environment

mechanics

materials

projectsolution

managementsolution

corrosionand protection

availability

crossings terminalsand stations tests instrumental

control

Fig. 4. The disciplines involved in the creation of an oil and gas pipeline.

densely populated areas, and can only be overcomeby the use of high quality materials, carefulconstruction planning in terms of timing andtaking respect for the area, and by carefulmanagement of the transport to guarantee thesafety of the environment, of property and ofhealth.The engineering involved in the construction of

pipelines relates to:• Optimization of the pumping system, taking into

account the elevation profile of the route, thediameters that can be installed, the distance to becovered, the flow rate to be guaranteed.

• Analysis of the environment, as far as thehydrology, morphology and geoseismology isconcerned, the ground crossed and its stability, andthe environmental conditions close to thefoundation.

• A preliminary sizing and choice of materials, inrelation to the product to be transported and themechanical performances required for theoperation and the environment.

• Planning of the installation, taking into accountboth the ability to follow a planned trench and theintegrity of the pipeline during the laydownoperations, particularly for sub sea pipelines onwhich the installation process foresee high stressesin terms of external pressure; planning must takeinto account uneven seabeds where the pipeline hasto be laid, thus avoiding having it suspended forlong stretches.

• The planning of its operation which, on the basis ofthe foreseeable pressure and temperature profilesof the fluid transported, establishes the pipeline’scapacity to support the operating loads and theadditional environmental loads.

• Design of the control system for the transportprocess and the functional planning of theprocedures and of the equipment for inspection andthe ordinary and extraordinary maintenance of thepipeline.The level of the planning is always correlated to

the level of knowledge of the environmentalconditions and is dependent on the vulnerability of the environment to be traversed, on the peculiaritiesof the optimal route, the difficulties of the operating conditions, and the strategic importance.

Safety, consequences of fluid losses and riskThe safety of the hydrocarbons transport in

pipelines is measured on the basis of the probability ofthe occurrence of damage that can cause leakage ofthe contents, by partial cracks that appear on the walls(immediately noticeable during construction or duringthe hydraulic test, during operation following normal

instrumental measurements for checking the flow inthe pumping station and at the reception terminal, atthe conclusion of a routine inspection or anextraordinary inspection carried out because ofexceptional environmental events or of humanactivity), or even by large scale evidence of breakageand/or bursting.

The consequences of the loss of a product haveto do with the environment; those connected withthe loss of liquid products are particularly serious(pollution of the landscape and water table, impacton flora and fauna, disruption of human activitieslocally as a result of the passage of the pipeline, orglobally, in connection with product availability,injury or death of pipeline employees or of thirdparties involved accidentally, etc.). In the case ofgas losses, the danger of explosion and firesignificantly increases the potential effect on nearbyhuman activities, and hence the consequences to thehealth of employees and of whoever works in thevicinity (incidents of gas leaks with explosionand/or fire have occurred in the United States andin Europe).

Pipeline transport risk is defined as the product ofthe probability of leakage or bursting and themagnitude of the consequences of the same:• In the case of transmission of gas, the magnitude is

measured from the volume of gas that could bereleased into the atmosphere; for this reason shut-off valves are placed at regular intervals along thepipeline depending on the environmental andsocial characteristics of the areas traversed.

• In the case of transport of liquid, the magnitude ofthe consequences is tied to the volume released ina unit of time, to the atmospheric and hydraulicconditions of the locality where the leakage takesplace (such as the distance from the coast for subsea pipelines), to the time required and thetechnology available for stopping the leak and forenvironmental recovery, again in relation to thenatural and social characteristics of the areatraversed.Also to be taken into consideration, are the

consequences that an incident can have for thereputation of the company responsible for thetransport, which can translate into a financialimpact equal to the cost of the incident itself andthat can become a primary element in the riskanalysis carried out by the pipeline operator. In the1990s, the quantitative risk analyses (defined, asalready stated, as the product of the probability ofthe occurrence of an incident and the magnitude ofthe consequences resulting from it) became anecessary step in pipeline planning involvingmultidisciplinary expertise, such as:



• Expertise in environmental fluidodynamics, in thechemistry and thermodynamics of reactions.

• Expertise in product diffusion physics for the studyof consequences, in the mechanics of the processesof interference and damage, in geotechnics and thephysics/chemistry of the environment wheredamage occurs, in analysis of structural integrity,in metallurgy and in susceptibility to theenvironment of the materials, in active and passiveprotection etc., for the study of safety and hence ofstructural integrity.The safety of hydrocarbon transport by pipeline

falls within the competence of the planners, and isevaluated in two ways:• Analytically, when the quantitative safety

requirements to be followed in a project areidentified (in the functional tests, expressed withspecific equations, the minimum requirements arecorrelated, through a series of safety coefficients,to the probability of the occurrence of amalfunction that would lead to measurable damageand/or breakage with associated leakage of theproduct) in relation to the type of product beingtransported, to the anticipated fabrication andconstruction technologies, to the operatingconditions and monitoring of the relevantparameters, and to the environmental and socialconditions of the region to be traversed.

• Consumptively, when the results of incidents thathave occurred during construction and operationover the years in similar pipelines, and in thevarious technological, operative and environmentalcontexts, are collected and processed, by type andfrequency. With reference to the analytical method, a design

based on standards that explicitly pursue quantitativesafety objectives, through a series of safety factorsbased on the probability of not exceeding theoperating conditions, guarantees a nominal safety tothe project. Sometimes the request for specific checksnot covered by the standard, or the necessity to checkthe safety level of a pipeline in operation, calls for acomplete probability analysis, where:• The relevant operating conditions are identified,

and hence the functional relationships between theproject parameters, making it possible to interpretthe performances by means of models.

• For each of the functional relationships identified,the uncertainties that influence the relevantparameters in describing the transition from anoperational condition to a non-operational conditionare analysed, and the statistical distributions andtheir relevant parameters are defined on the basis ofinformation such as experimental comparisons,qualification tests and design.

• The measurements of uncertainty are used both tocalculate the probability of exceeding the limit ofan operating condition, and to calibrate safetyfactors to apply to the individual projectparameters, so that the probabilities of exceedingthe operating condition limit become less than acertain target value.The above matters were introduced in the 1990s, in

particular for sub sea pipelines, and have becomecurrent in the development of frontier projects plannedin the first decade of the third millennium (the projectsin Sakhalin Island and in North America).

As regards checking the performance of pipelinesover time, based on the analyses of the data ofincidents that occurred during some 30 years ofoperation of modern pipelines (1970-2000),government and private bodies have worked for yearsto gather such data and to rationalize them for criticalanalysis. In Europe (Bruschi, 2002) and in the UnitedStates, the data collected provides a complete pictureof the adequacy of the technology employed in theindustry in the sector under consideration, consistentwith what is considered acceptable in the industry. Ingeneral, the unit of measurement for performance isexpressed as the number of incidents per year perkilometre of pipeline. For example, in the database ofEGIG (European Gas pipeline Incident data Group;Bruschi, 2002) there are about 1,060 incidentsdocumented for a total exposure of 2.4 million km peryear. In the database of CONCAWE (CONservationof Clean Air and Water in Europe) a total of 394incidents are documented, for a total exposure of700,000 km per year. The DOT/OPS/RSPA(Department of Transport, Office of Pipeline Safety,Research and Special Program Administration)database of the United States covers incidents onpipelines for transport of oil and gas, both underseaand overland, including pumping and measurementstations. For overland pipelines, external interferencefrom human activities (excavations for work on civilutilities, ploughing and water wells in agriculturalactivity) is, by far, the most frequent cause ofincidents, especially in the case of small diameterpipelines. The covering is a critical factor, togetherwith the thickness of the pipeline and its location,rural, suburban or urban. Other relevant causes arecorrosion, mechanical causes (principally during theconstruction phase), operational errors and events ofnature.

It is difficult to make comparative analysesbetween the various databases, both because often thedefinition of an incident is not consistent and thedata on the incidents is not available at the same levelof detail, and because of the diversity of eachpipeline in terms of technology, project criteria,



equipment for construction and management, andoperating and environmental conditions. A weightedaverage of the transport performances, extracted fromthe statistics of the various databases, gives thefollowing indications in terms of incidents per yearper km of pipeline during the period 1970-2000: inthe case of gas pipelines, the rate of breakdowns hasgone down from 0.8-1.5 to 0.15-0.21 per 1,000 km ofpipeline; in the case of oil pipelines, the rate ofbreakdowns has gone down from 1.2-1.8 to 0.3-0.6per 1,000 km of pipeline. These values, whencompared with those derived from the chemical andenergy industries, prove very satisfactory and bearwitness to the role played over the years by thetechnological innovation of the petroleum productstransport industry.

For sub sea pipelines, the database set up by theBritish government body HSE (Health, Safety andEnvironment) relating to sub sea pipelines in the NorthSea is very detailed, especially regarding incidentsoccurring from the 1990s onward. A total of 542incidents are documented: 396 relate to pipelines inoperation (of which 209 were in rigid steel pipelinesand the others in flexible pipelines, constructed inplastic with metal reinforced casing and break pointssuch as valves and flanges); 65 of these incidents ledto a leakage of the product. As in the case of overlandpipelines, a large number of the incidents was causedby corrosion (�40%) and external interference(�39%), while only 6% resulted from environmentalcauses, notwithstanding the severe climatic conditionsin the North Sea. These statistics refer to an exposureof about 100,000-1,000,000 km per year and, althoughthey are much more detailed than the correspondingones for overland pipelines, they are neverthelessinadequate for defining an objective breakdown ratefor general application to risk analyses, because theyare limited by:• Lack of consistency of the sample in terms of

technology, materials and construction equipment,of project criteria, of operating conditions andenvironmental conditions (shallow waters, deepwaters, etc.).

• Definition of breakage (leakage or interruption oftransmission) and/or availability of transportfollowing the incident.However, a very strong signal can be picked up

from these data, noticeable also in those relating tooverland pipelines, regarding the role of the thicknessof the pipe in the pipeline’s ability to withstand themost frequent incidents: large diameter, high pressureand extra thick pipes, provide greater safety, but anincrease in thickness, usually adopted nearby safetyzones or where activities by employees are carried out,does not, in itself, guarantee safety. This means that

along a great part of the route, an increased thicknessof steel avoids loss of product caused by incidents, butis not sufficient to avoid incidents where high levels ofhuman activity are lacking in adequate procedures.

7.1.2 Routes across the environments

Route selection for overland pipelines

The execution of a pipeline for the transport ofhydrocarbons is influenced by the characteristics ofthe terrain which it has to traverse and by the need tominimize the impact on the environment, especiallyduring construction. The environmental considerationsconcerning the area in question relate tohydrogeomorphological issues, vegetation and thelandscape, but also include statutory restrictions inforce in the region. The choice of the best route mustreconcile the technical-economic requirements withthe need to protect the places through which it passes,within the limits of the variations of the parameterswhich determine its technical-economic feasibility.

The process of optimization is carried out througha series of phases involving proposals, checks andverifications (Mohitpour et al., 2000), which includethe preliminary selection of the shortest route based onthe study of maps and aerial photographs, the criticalevaluation and fine-tuning of the proposed path, thedefining of the most promising route, the visual andinstrumental inspection of the same, and theengineering analysis and the legal processes necessaryto obtain the various permits. A critical aspect of theselection process concerns the constraints in force inthe area which rule out certain stretches of the routeand impose a less than optimal solution from aneconomic and technical point of view, while stillfalling within the limits of acceptability. Gathering ofavailable maps of the area under examination, possiblyat a scale of 1:50,000, as well as aerial photographs isvery important. The latest technology has madeextremely sophisticated satellite imagery availablewith varying degrees of detail, which make the map-based engineering phase much more effective than waspreviously possible. In addition to geographical maps,thematic maps are also available (Champlin, 1973)which make it possible to define the overall nature ofthe area being traversed from a variety of points ofview, highlighting the existence of limiting factors.The preliminary selection generally leads to tracing ofvarious possible routes, which avoid the critical areasand take full advantage of the routes which featureconditions favourable to the realization and operationof the pipeline.



The elements which influence the choice aremanifold and varied in nature, of both legally-restrictive and technological character. In the case ofrivers, torrents, lakes and marshes for example, it isimperative to avoid construction across areas whichcould be affected by erosion or in areas in whichnatural evolution could, over time, impact on thepipeline. From the morphological and physiographicalpoint of view, routes across steep slopes are to beavoided as is terrain which might be eroded or is tootight for normal digging operations for the trench inwhich the pipeline is to be laid. The area’s seismicityand the presence of any faults (Champlin, 1973) can belimiting factors. As far as environmental considerationsare concerned, areas of faunal reproduction and thehabitats of protected species are to be avoided, as areareas and sites of historical and archaeological interestand of outstanding natural beauty. Critical factorsaffecting the choice of a pipeline’s route are not onlythe crossing of roads, railways or pipelines, and ofareas which are densely populated or subject torestrictions (national parks, nature reserves, importantwoodlands and areas of replanting), but also the ease oftemporary and permanent access for construction andoperation (Passey and Wooley, 1980). Each preliminaryroute is analysed in detail through an iterativeengineering process of successive checks andmodifications, based on assessment of the variousaspects, both restrictive and favourable, until a newrevised and corrected version is obtained.

The engineering activity is followed by a visualinspection of the site, and the critical areas arespecified by appropriate surveys which include adetailed topographical survey, analysis of slopes anddrainage channels, identification of any geotechnicalimplications concerning stability of the ground,analysis of the surface conditions such as themorphology, the type of terrain, the location of rock,

the presence of water courses and the vegetation. Inrural areas any agricultural zones are identified,specifying the type of agriculture. Moreover,identification is made of any areas which are sensitivefrom an environmental point of view or of particularhistorical and archaeological interest, the presence ofstructures, buildings or services, the availability ofexisting corridors and the possibility of access bywork-site vehicles, also taking account of seasonally-related construction problems. Based on this, all thealignments of the course and the safe distances fromroads and services are established. Today, the visualsurvey and the preparation for the final design phaseare supported by the use of a Geographic InformationSystem (GIS), which makes it possible to identify aposition and analyse objects and events present on theEarth’s surface (Fig. 5). This technology bringstogether the capability of recording and storing datawith the ability to process them through statisticalanalysis, and makes it possible to report the results inthe form of thematic maps or tables which can be



Fig. 5. Example of a digital image of the territoryfrom a satellite map (courtesy of R. Bruschi).

Fig. 6. Pipeline building on a slope: the axis of the pipeline is parallel to the steepest incline on the slope (courtesy of R. Bruschi).

overlaid onto maps. Once the critical areas of a sitehave been established, the next step is the detailedengineering analysis such as the geotechnical analysisof river crossings, the crossing of slopes andevaluation of the environmental impact (Fig. 6). In theevent of having to cross areas of hydrogeologicalinstability, slopes which show signs of activeinstability can be identified by the observation ofsome superficial evidence during the inspection phase:any potential problems of interaction with the pipelinemust be resolved in detail in order to make provisionfor the necessary preventive measures. In a situationwhere there are signs of recent instability, the nature ofthe activity must be analysed and understood. In theevent of deep-seated movements, it often becomesnecessary to seek an alternative route, as thestabilisation work required could prove to be too costlyor unfeasible. Occasionally, instead, where themovement is not too deep, the stabilisation work mayturn out to be relatively simple and it may be possibleto remedy the situation in order to make it conform tothe requirements of the project.

Instances of river crossings have a significanteffect on both the costs and the overall length of thepipeline, since it is essential to find the stretch wherethe riverbed is the most suitable. The presence ofparticularly tight rock carries high shaping costs, whilesandy beds can involve a large amount of excavation.It is preferable to carry out the crossing at right anglesto minimize the length, and to avoid having an inclineat the point of landing. Erosion of the banks can leadto damage to the pipeline and to its being exposed tothe effects of the external environment. Stretches witha high-flowing current are to be avoided because theymake construction difficult, whereas it is preferable tocross at a stretch where the river is straight as thisreduces the probability of erosion of the banks.

The various detailed analyses make it possible toadopt the best approach through a thorough evaluationof the costs and benefits. The costs of any stabilizationwork, of excavation and controlling of drainage (bothsurface and underground) must be compared with thecost of selecting an alternative route which crosses astable area or which does not require such remedialwork. The construction work provides for the creationof a strip of land which makes it easy to carry out thework well and facilitates access by service andemergency vehicles. Morphologically irregular areasand those where there are very steep slopes, call forthe carrying-out of excavation of the incline andtherefore, during the phase when the route is beingdefined, effort should be made to minimize theseoperations restricting as much as possible the extent ofthe modifications made to the original profiles.Preparation of the technical drawings needed for

construction call for definition of the profile of theland, specifying the dimensions and the length of thetrench needed, and the existence of roads, railways andplaces where services cross. This operation is carriedout through the sub-division of the course into evensegments and the indication of all the unusualsituations, providing all the elevations and linearmeasurements and angles needed to define completely,and in detail, the geometry of the pipeline’s axis. Adetailed profile of each crossing point and of everyunusual situation which calls for a separate study atthe design stage must be produced. The engineeringsurvey can be carried out at the same time as the legalsearches and has the objective to produce thesupporting documentation for the possible purchasingactivities or for obtaining the concessions necessaryfor use of the land for the construction of the pipelineand its future operation.

Geo-risks for onshore pipelines

A pipeline for the transport of hydrocarbons is astructure with a linear extension which coversdistances in the order of several hundred kilometres.Its construction involves the traversing of entireregions with environmental and territorial conditionswhich differ totally from zone to zone and which mustbe addressed within the scope of the project. Inparticular, as it deals with a structure in direct contactwith the ground, the geomorphological, geotechnical,hydraulic and seismic aspects are of fundamentalimportance and have a major influence on the project.

Landslides and hydrogeological instabilitiesA landslide is a downward movement of a mass of

earth on an slope and subject to the force of gravity.The movement is caused by a variation of anycondition which is capable of disturbing the temporarybalance of the system, such as a variation in the levelof the water table, the presence of materials which losetheir own properties of resistance in the presence ofwater, the structure of the material, the topography,seismic activity, etc. (Abramson et al., 1996).

During the phase in which the route is beingselected, it is of fundamental importance to identify allthe unstable or potentially unstable slopes in order toavoid crossing them. The type and distribution ofnatural landslides are very varied and are dependenton the morphology of the terrain and its localcharacteristics, in addition to the sub-soil hydraulicconditions. Even if an aerial photograph might be ofvalue in identifying the areas at risk of landslides,many instabilities are too small or, in any case, toodifficult to identify using this technique. A visualinspection is always necessary and has the objective of



identifying all the signs typically associated withactive movement of the ground or with landslideswhich occurred in the past and which could recur withthe installation of a pipeline (Abramson et al., 1996).Typical signs of hydrogeological instability are thepresence of steep escarpments, crevices and fissuresupstream of a slope, swellings and accumulations atthe foot of a slope; the presence of bent trees; damageor changes to the alignment of structures (roads,telegraph or electrical poles, pipelines). These andother signs may also contribute to the identification ofthe boundaries of the area subject to landslide.

The most dangerous type of landslide for apipeline project is when it is deep, in which the failureplane is situated a long way below the trench bottom.This situation should be avoided, even though,technically, it is possible to design and build a pipelinewhich crosses a potentially unstable area withoutcausing new movements. In this case, a stability studyof the incline in question is fundamentally importantand presupposes a meticulous analysis of the area inmorphological, geological and geotechnical, physicaland hydraulic terms, with the objective ofunderstanding the forces which control the movement.Once the forces involved and the processes which actto improve or disturb the existing conditions ofequilibrium have been identified, it is possible to beginthe design of the pipeline and of any work needed tostabilize the slope. Identifying the existence ofpotential hydrogeological instabilities also has adetermining influence during the operation stage ofthe pipeline, affecting the management andmaintenance activities which often have to besupported by an appropriate and careful monitoringprogramme which controls the geotechnical andphysical conditions that underlie the forces ofinstability (Fig. 7). In particular, an inclinometer forthe direct measurement of the shifting of the terrainand a piezometer for measuring the variations of theunderground water table are used, in conjunction witha pluviometer for measuring the amount of rainfall inthe area under examination. The study of a landslideoften involves the use of numerical modelling which,through a series of simulations based on differentvalues of the model’s basic parameters, contributes tofinding the most probable values of the parameters

involved and to the understanding of thephenomenology which characterizes the movement ofthe ground. Numerical modelling is also findingsignificant use in the pipeline’s operating phase:combined with the monitoring activity, it cancontribute to the prevention of damage resulting fromcatastrophic events, enabling creation of a forecastanalysis of the pipeline’s possible structural responseto the induced stresses.

There is an international nomenclature whichclassifies the landslides on the basis of type in a mannerwhich is consistent throughout the world (Abramson etal., 1996). As far as the behaviour of a landslide isconcerned, the principal factors which govern theequilibrium of the system are: a) the destabilizingforces which cause the movement, e.g. the weight of thematerial; b) the forces due to the movement of water inthe pores within a slope; c) the dip of the failure plane;d) the resistance of the terrain along the failure plane;e) the reduction of resistance along the failure plane dueto interstitial pressure. The first three factors contributeto destabilization, while the final two (resistance forces)tend to maintain the equilibrium. As far as thedestabilizing forces are concerned, the weight of thematerial involved is generally known or can, in anycase, be calculated with sufficient accuracy. The forcesproduced by the flow of underground water or causedby an earthquake are more difficult to quantify. A gooddatabase of previous seismic events, if available, makesit possible to formulate a forecast of expected eventsand to estimate the level of risk associated with them.The most important variable in defining thedestabilising forces is the dip of the potential slidingsurface. The greater the dip, the greater the probabilitythat a landslide might occur. The resistance forces arelinked to the shear resistance of the material in thefracture zone and, in particular, to the effective angle of friction of the terrain along the sliding surface. The material’s shear resistance is reduced by the effectof the interstitial water pressure present on the slipsurface, since it causes a reduction in the effectivetensional state of the ground and therefore of themechanical resistance capacity (principle of effectivetensions; Terzaghi, 1925).

The causes of geological instabilities are thereforevaried and diverse. Natural events can act in such away as to increase the destabilizing forces (e.g.accumulated deposits, seismic forces) or of reducingthe forces of resistance (for example, erosion at thefoot of a slope, increase of interstitial pressure, etc.).In particular, instances of heavy rainfall can cause amassive infiltration of water into the sub-soil,increasing the interstitial pressure in the ground andreducing its resistance characteristics, create seriouserosion as a result of the run-off of surface water and



Fig. 7. Interaction betweenhydrogeologicalinstability and a pipelinecrossing a slope.

the increase of the flow-rate of the water courses, andconstitute a potential overload, saturating the grounditself. The activities connected with the construction ofthe pipeline can also disturb the balance of a slope.Opening a site road for the normal constructionactivities often involves excavating a stretch of slopingground and therefore presupposes a careful selectionof the sites for the dumping or storing of excavatedmaterial, which could constitute an overload for theground, sufficient to reactivate a dormant movement.The operations to restore the profile of the terrain towhat it was before installation of the pipeline, toreduce the visual impact on the environment, can havea further destabilizing effect when new material issubstituted or the original material is returned havingbeen previously taken away and, therefore, altered.

A crucial element in the design of an onshorepipeline is that of checking the surface andunderground drainage in the area which falls withinthe strip of land along which the pipeline itself willrun and the erosive effects associated with the flow ofwater. Careful planning of specific works capable ofcontrolling the hydraulic regime is generally effectivein avoiding the appearance of serious erosivephenomena which could expose the pipeline to theexternal environment or initiate processes ofinstability. Works of this type consist of diversionchannels, gabions, dykes and drains.

FaultsFaults are fractures in the rock-mass associated

with the different movements of the two parts incontact with each other. Deforming movements are notrestricted merely to sliding along one or more fracturesurfaces but can also be accompanied by distortion,breakage and fragmentation of the rock at the contactsurfaces generating fault breccias called mylonites.The movements can occur suddenly, following to anearthquake, or build-up gradually over time andconstitute a serious threat to the integrity of a pipelinewhich crosses a fault. The length of the fracture andthe extent of the movement depend on the magnitudeof the seismic event and the depth at which it takesplace, while the classification of the various types offaults is based on the geometric characteristics of the

sliding (Bonilla, 1970). From the point of view of theinteraction with a pipeline, crossing a fault should beavoided inasmuch as it can cause intolerableconditions of stress for the structural integrity and theefficient operation of the pipeline itself (Fig. 8). Tocreate an effective design for a pipeline which will becapable of resisting the deformation to which it mightbe subjected in crossing a fault, it is necessary to knowthe geometry and typology of the fault, and the size ofthe area in question, to know if it involves a movementcaused by an earthquake or by a stress build-up overtime (creep) and, obviously, to examine thecharacteristics of the terrain. Certainly, the mostimportant factor, is represented by the type and extentof the sliding which can be assessed on the basis of thecharacteristics of the associated seismic event.

Seismic activityOne of the consequences of a seismic event is

instability of inclines and slopes. During anearthquake, a system of acceleration waves passesthrough the ground, propagating from the point oforigin in the sub-soil towards the surface. The transientdynamic load which instantly follows alters thetensional regime which establishes the nature of thebalance of a slope, simultaneously causing an increasein the acting shear force and a diminution of theresistance capacity of the ground, as a result of thesudden increase of interstitial pressures. Other factorswhich influence the response of a slope during aseismic event are the magnitude of the event, itsduration, the resistance characteristics underconditions of dynamic stress of the material of whichit is composed, and the dimensions of the slope. Thereare a variety of methods for analysing the stability of aslope under seismic conditions. The most common arethe pseudostatic limit-equilibrium method and thesliding block analysis method perfected by Nathan M.Newmark (Kramer, 1996). The first involvesmodifying the conventional limit-equilibrium analysisadding to the forces involved, a component derivedfrom the seismic activity which is assumed to be a fraction of the weight of the potential landmassinvolved in the landslide multiplied by the acceleration. Newmark’s method is based on the movements of an embankment during seismic activity. It consists of a combination ofconventional pseudostatic procedures with a background of dynamic considerations on the movement of the ground.

Seismic activity does not affect only the stability ofslopes. The reduction of the shear resistance of aterrain combined with the increase in interstitialpressures can cause fluidization of the soil, especiallyin the case of loose, saturated sands, and the



Fig. 8. Interaction betweena fault and the pipelinewhich crosses it.

development of large and permanent deformationscapable of causing serious damage to any type ofstructure resting on the affected terrain. It is possible toidentify three types of movement or breakage of theterrain associated with the process of fluidization(Youd, 1978): widespread lateral deformation,gravitational flow and reduction in the ground’s bearingcapacity. Other effects are subsidence, and above all theraising of an originally buried pipeline caused by itsfloating in the temporarily fluidized groundmass.Widespread lateral deformation relates to thehorizontal shifting of the upper beds of the terrainresulting from fluidization of the underlying ground. Itis a phenomenon that occurs in areas with gentleslopes, and the associated sliding are measured in tensof centimetres. Such movements can have a verydestructive effect on pipelines, even if the level ofdamage depends on the extent of the movement and onthe characteristics of the pipeline itself. From the pointof view of planning analyses, the study of the pipeline,with respect to widespread superficial deformations,presents similar problems to those resulting fromfaults. The deformations are concentrated in the slidingarea, as happens with the movements of a normal fault.At the base of the mass, instead, a compression takesplace similar to that of a reverse fault.

Gravitational flows relate to the movement offluidized masses, occasionally containing rocksboulders which slide down steep slopes. However,many of these occurrences are more frequent in a subsea environment. The reduction of the ground’sbearing capacity can cause serious sinking in astructure built on it, such as, for example, anembankment, and therefore, can induce tractive forcesof traction on a pipeline which crosses it orcompressive forces in the adjoining areas. To preventthe buried pipeline from floating in the fluidizedgroundmass, systems for anchoring and weighting thepipeline are used.

Sub sea pipelines

The seabed can be very morphologically irregularand laying a pipeline across it involves significanttechnological effort. Deciding on the route cannot becarried-out using direct vision from the air, nor fromexamining aerial photographs or visual inspection as ispossible with onshore pipelines. The reconnaissancephase is totally instrumental, and entrusted to the useof highly sophisticated technology. For example, thecollection of samples for characterizing thestratigraphic profile of the sea floor is always difficult,and often altogether impossible (very deep water, verysteep sub sea slopes). Therefore, the morphological,geotechnical and physical characterization of the subsea area is usually derived from the interpretation ofgeophysical measurements (Fig. 9) and frommeasurements of the ground’s resistance to thepenetration of tools designed for this purpose(penetrometric tests). The environmental difficultiesand the need to use sophisticated technology to obtainthe project data make route selection a very importantand critical phase of a sub sea pipeline project, and thebasis for a realistic technical-economic analysis forcarrying out the work (Palmer and King, 2004). A lessthan careful initial choice can cause a series ofunforeseen problems during the construction phase,the solutions for which will always have considerablefinancial implications. For example, crossing irregularsea floors is not always possible from a technical andeconomic point of view; therefore it is important toidentify a route which avoids them.

The salient phases of sub sea route selection can besummarized as follows: • Optimization of the pipeline route with definition

of the alignments which represent the bestcompromise between the shortest distance, theslightest stimulus acting on the pipeline during theinstallation phase or when active, and the smallestnumber of pipeline lengths which are suspendedabove the seabed because of its irregularities.

• Identification of the extent and type of remedialand preparatory work needed on the seabed toresolve the problems relating to overstressing ofthe pipeline and the formation of unsupportedspans in the areas of irregular seafloors.

• Definition of the construction methods and sizingof the work-site for the associated cost andplanning of the work.The experience garnered over the years on sub sea

pipeline construction has led to some fundamentalconsiderations. As far as the preparation of the seabedis concerned, with the exception of the minimumrequirements necessary for making landfall,excavating is not an appropriate solution from an



Fig. 9. Image of the profile of the sea floor from geophysical surveys (courtesy of R. Bruschi).

economical point of view; in fact this would have to bevery extensive given the level of precision achievableduring laying operations, which it occasionally slowsdown. The laying schedule is dependent on the seabedpreparation and the route selected for the pipelinemust be such as not to induce stresses which mightcompromise its structural integrity. The layout of thepipeline across irregular seabeds can be either retainedor modified as required using appropriatemethodologies, in such a way as to guarantee meetingthe design criteria during the hydraulic and functionaltests. In deep water, correction of the spans is a morereliable approach than preparation of the seabed. Inevery case, the selection of the project solution bestable to resolve the various problems of the pipelineand its interaction with the marine floor is made on thebasis of simulation models calibrated on the basis ofthe experience from various projects.

From an operational point of view, the first phaseconsists of gathering and processing the data relatingto the area under consideration. As already stated, thechoice of a route for the sealine requires a profoundand detailed knowledge of the morphology andlithology of the seafloor. Acquisition of the data isachieved through a programme of geomorphologicaland geologico-geotechnical research which leads tothe definition of the characteristics of the seabed. Thefirst approach is based on a study of conventional

bathymetric charts which become more and moredetailed with the increase in volume of the results ofresearch conducted with surface and submarinevessels (Fig. 10). The more accurate the survey of theseafloor’s characteristics, the more reliable the resultsof the route analysis and of its subsequent simulations.To ensure the necessary level of detail in the survey ofareas of highly irregular morphology, mini-submarinesare used. The data are gathered, processed andrecorded on appropriate peripheral support unitslocated on board the surface vessels and thesubmarines, and contribute to the construction of apreliminary survey database. When interpreting thegathered data, use is also made of visual imagesrelayed from the sea floor. The output from this phaseis specific bathometric charts which constitute thepoint of reference for the subsequent analysis of thepath and the process of selecting the route. With thehelp of integrated software-hardware systems capableof handling the data in real-time, engineers cananalyse alternative routes and, on the basis oftechnical-economic evaluations, make a choice of thepreliminary corridor. The morphological profile of theselected corridor is then developed along its own axis(see again Fig. 10) and parallel profiles are alsoexamined, in order to characterize the area of thecorridor, and also to highlight the lateral variations inthe marine profile in the event that the pipeline’s axis



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Fig. 10. Depiction of a route typical of an undersea pipeline.

is shifted from that of the corridor. The profile of thesea floor is the point of reference for the routeselection process, since the structural reliability of thepipeline, the number of open spans and the extent ofthe remedial work are all correlated to it.

The following phase involves the use ofmathematical models for an evaluation of the qualityof the route. The selected route defines thecharacteristics of the best profile for the elasticequilibrium of the pipeline laid on an irregular seabed,so as to reduce to a minimum the preparatory work onthe sea floor and the adjustment of the configurationnecessary to guarantee stable and safe conditions forthe pipeline during the course of its working life. Ingeneral, the key factors influencing the choice of routefor a sealine and its design are linked to politicalconsiderations and environmental problems, but alsoto specific factors such as connection to platforms andrisers (vertical collectors for floating platforms) or thecrossing of existing pipelines or cables. Areas withanchorage points for ships and those where there is ahigh risk of objects falling from ships should beavoided as should minefields and areas wheredredging is carried out or where there are wrecks.Specialized studies are carried out to analyse theeffect on fishing operations and to assess the risks forthe pipeline’s integrity in the event of any impact.From a geotechnical point of view, the elementshaving the greatest influence on a sealine project arethe crossing of rocky areas or zones in which theterrain is too soft and where there are boulders,

depressions caused by leakage of subterranean gasand of furrows created by drifting icebergs. Thecoastal landing is, without doubt, a critical aspect ofthe project: an incorrect choice of landing area canresult in excessive costs and legal disputes. In additionto the geology of the seabed, the designer must knowand understand the geomorphological factors whichcharacterize the coast and foresee the environmentalimplications associated with installation of thepipeline, such as the refraction and breaking of thewaves and the movement of sediment along the coast.Marine geotechnology is a very complex science andthe geomorphological and topographical profiles areas varied as those found on land. The ideal situationfor sealine laying is, obviously, a flat sea floor (so thatopen spans do not form) composed of fairly firm clay;laying onto this type of sea floor gives the pipelinegreater stability. If the sea floor is irregular and rocky,many open spans can form and the pipeline crossesthe depressed areas like a bridge. Often, the length ofthe span is such as to require a support, while thestress is concentrated on the pipeline at the supportpoints with the potential for damage to the externalcovering (Fig. 11). Hard ground is difficult andexpensive to excavate, while with a surface that is too soft, the pipeline can sink completely making it very difficult to carry-out subsequent operationssuch as joining to the next section of pipeline,inspection or repair.

Some seabeds present problems of mobilitybecause they are in a state of constant evolution from



Fig. 11. Image of a pipeline laid on an uneven sea floor with resulting free spans and construction of support work (courtesy of R. Bruschi).

the action of waves and currents which cause rippledand undulating surfaces (Komar, 1976; Sleath, 1984).Sandbars on the seabed are constantly moving duringthe working life of a pipeline, so that a pipeline thatwas resting on a ridge of accumulated sand at the timewhen it was laid, can end up suspended when theaccumulation shifts. The movements are irregular anddifficult to predict with sufficient certainty: for thisreason, it is preferable to avoid crossing areas wherethe seabed gives evidence of constantly changingundulations. When this is not possible, provisionshould be made to excavate a trench which lies belowthe level of the troughs of the sandy undulations. Thissolution has often been adopted for pipelines laid inthe North Sea, where shallow water, strong currents,wave action and loose sands which move easily createcomplex and constantly changing conditions on theseabed. In areas where there is a high rate ofsedimentation, the accumulation of deposited materialcan overload the subsoil and cause submarinelandslides on slopes that are not particularly steep. Aseismic event can also cause a landslide on a partiallystable slope. When a pipeline is struck sideways by alandslide, it can be shifted enough to create a force oftraction sufficient to break it. If the landslide strikesthe pipeline longitudinally, the situation is less seriousas the stresses induced are of a minor level.

Some areas (such as the Norwegian sector of theNorth Sea) are characterized by the presence of largeboulders on the sea floor, which can also be partially(or completely) buried in the clay of the seabed. Theseboulders were transported by icebergs and fell onto thesea floor as it melted; they can be a metre or more inlength and present serious obstacles for trench-diggingequipment. In the same marine areas, the leakage ofunderground gas creates typical depressions in the seafloor. In tropical zones, coral formations can havepeaks which may even reach as high as 15 metres.Coral is very hard and difficult to cut; in addition it isan element of nature of great ecological interest and itsconservation must be guaranteed and protected.Tropical areas are generally characterized by carbonatesediments which, as a result of the diageneticprocesses which take place over a period of time, tendto become harder and therefore make trench diggingmore difficult.

In arctic regions, the planning of a sub sea pipelinefaces additional problems. During the spring season,the thawing of water courses which reach the stillfrozen sea, create streams of fluvial water which flowacross the marine ice. When there are cavities orfissures in the ice, the fluvial water seeps in creatingvortices and jets towards the bottom that can dig deepholes in the sea floor (Palmer, 2000). Large icebergsmove through shallow water blown by the wind and

pushed by other icebergs, ploughing up the seabed andmaking ditches as much as 10 metres deep and 100 metres long. Avoiding the risk of damage from theaction of moving icebergs is one of the primaryobjectives in planning a sealine in arctic zones(Woodworth-Lynas et al., 1996; Palmer, 2000).

Hydrodynamic factors also influence the choice ofthe route. It is preferable to avoid areas in which thereare a very strong current capable of moving thepipeline laterally and complicating the laying. Strongsea currents can occur in shallow water, and nearestuaries and straits. It is often preferable to choose alonger route with weaker currents rather than crossinga strait just because it is shorter. Areas where waveaction is particularly strong should be avoided bothbecause of problems of stability of the pipeline andbecause of the difficulties that they create in the layingoperations (Komar, 1976). Since the effects of waveaction are greater in shallow water, it is best to keepthese stretches of the route as short as possible, infavour of laying in deeper water. However, thedifference in the specific gravity of the various depthsof a column of water can cause strong bottom currents.

Marine meteorological environment

Marine meteorological conditions are highlysignificant because of the implications they can haveon different aspects of a pipeline’s functionality suchas its ease of installation, stability and integrity overtime. The aspects of marine meteorology of directinterest are the dynamics associated with themovement of water masses: currents and wave action.The distinction between these is justified both by theirdifferent scales of space and time and by their different



waveshear stress current

shear stress

current speedprofile

orbital speedprofile (wave)

Fig. 12. Simultaneous wave and current actionon a sandy sea floor.

physical characteristics. While the capacity of waveaction to transport material is modest and it is mainlythe energy of the wave action itself that is propagatedover long distances, it is especially in currents that themovement of masses, that is of water, can be seen(Fig. 12). However, other characteristics of the marineweather environment cannot be overlooked, such aswind and atmospheric pressure, the variations of sealevel (tide) and chemico-physical characteristics of thewater (Csanady, 1982).

In order to arrive at an adequate characterization ofthe area in which the sealine is to be laid, a reliableassessment of the project’s marine meteorologicalparameters is needed. The analysis is carried out using amethodological approach which makes use ofexperimental data, of statistical processing proceduresand of mathematical simulations of the dynamicprocesses; modified each time in line with theconditions specific to the site being studied and thenature of the project, such analyses consist of: a) analyses of conditions with the aim of identifying themarine meteorological characteristics relevant for theproject; b) gathering of available historic data; c) carrying out of oceanographical meteorologicalcampaigns of supplementary measurements; d) dataanalyses and determination of the statistical propertiesof the parameters influencing the project; e) reconstruction using mathematical models of thephysical events relevant to the project; f ) calculation ofthe acceptable limits of the oceanographicalmeteorological parameters affecting structural stabilityand determining of the project parameters; g) analysisof the effect that the work will have on the environment,for the Environmental Impact Assessment.

Marine meteorological analysis providesinformation on two categories of conditions: normaland extreme marine meteorological conditions. Theinformation on the ‘normal’ marine meteorologicalconditions, which is needed to confirm the pipeline’sdurability over time, for the choice of barges and of theconstruction period, for the estimate of the functioningof the pipeline, etc., is derived from the statisticaldistribution of the oceanographical meteorologicalparameters and their values. The information on‘extreme’ marine meteorological conditions, which isneeded for sizing and confirming the stability of thework, is based on the estimates of the maximum values;in other words, values associated with rare events,which the different oceanographical meteorologicalparameters could reach during the lifetime of the workand the risk of those values being exceeded. Startingwith this information, and on the basis of the projectcriteria adopted, and which are generally set byregulations in force, the project’s environmentalparameters are defined, and form the basis on which the

work will be planned (Herbich, 1990). Moreover, themarine meteorological analysis provides a set ofinformation which is of assistance to the engineers inselecting the type of project to adopt, or forincorporating into the documentation to be presented tothe competent authorities for obtaining project approval,such as, the evaluation of the dispersal of sediment putinto suspension during the digging operations forburying of the pipeline at the coastal landing.

Wind and atmospheric pressureEven though meteorological elements do not have a

direct bearing on the planning of the pipeline, otherthan for such aspects as the ability to operate the laybarges, an understanding of the wind and atmosphericpressure conditions is of fundamental importance foran accurate assessment of the waves and currentswhich are generated by the wind itself. In fact, thespace-time scale and the intensity of marinehydrodynamic activity are correlated to the scale andthe intensity of the meteorological activity whichbrings them about, and therefore a deep understandingof meteorological features makes it possible toreconstruct the hydrodynamic activity usingmathematical or empirical models. This capability is ofparticular importance when, as often happens, directmeasurements of wave action and currents are notavailable, or when they do not reflect the variability inspace and/or time of the activity. The atmosphere, likeevery other turbulent fluid, presents a variety ofmotions on all the possible scales of time-dimension.Some of these motions are repeated with characteristicswhich are fairly similar, enough to be considered asbelonging to families of dynamic systems with well-defined characteristics and space-time scales. Almostall of the atmospheric space-time scales are of interestin planning sub sea pipelines. Ignoring the small scaleactivities (timescale of less than 1 hour, dimension ofless than 1 km), which are of marginal interest, realinterest lies in the range from medium scale activities(timescale of less than 1 day, dimension less than 100 km) up to planetary scale activities (timescale over1 month, dimension greater than 1,000 km).

CurrentsMarine currents operate at various levels on the

integrity of a pipeline laid on the sea floor: theyoperate as lateral and vertical forces, with implicationsfor the pipeline’s stability; they cause lateral andvertical vibrations on sections of the pipeline notresting on the seabed, with consequent problems offatigue; they cause complex phenomena of sedimentsmovement in the area around the pipeline itself, whichcan cause erosion of the bed on which it is resting andconsequent fatigue and stability problems.



Consequently, the intensity and the direction of thecurrents and their frequency of occurrence are amongthe principal environmental factors to be borne inmind when planning a sub sea pipeline.

Currents are brought about by various forces:primary forces, which generate and maintain themovement of a body of water, and secondary forces,which merely alter already existing movements. Theprimary forces are, in turn, divided into internal andexternal forces. The internal forces are caused byvariations in internal pressure within the body ofwater; typical examples are the force generated by theaccumulation of water near the coast, caused by thedragging action of the wind, or the forces arising fromdifferences in the specific gravities of bodies of water.Examples of external forces are the tangential forcesof the wind on the surface, the tidal forces, variationsin atmospheric pressure, etc. The principal force is thetangential force of the wind. It produces driftingcurrents on the surface of the sea and influences theinternal forces of pressure inasmuch as it causes theaccumulation of water near the coast. The tidal forcesgenerate periodic movements and do not cause large-scale movements of the body of water. The secondaryforces do not generate any movement but modifyexisting motions. The secondary forces are the force offriction, which reduces the velocity of the current, andthe Coriolis force, caused by the Earth’s rotation,which causes a variation in direction of the current.

The motion of the fluid in the sea is governed bythe physical laws of conservation and by the equationof state (Neumann and Pierson, 1966). These physicallaws are translated mathematically into a system ofequations with unknowns comprising the componentsof velocity, pressure, specific gravity, salinity andtemperature. The equations of motion are resolvednumerically by means of the application of numericalmodels which reproduce, with any necessarysimplifications and approximations, the bathymetricand marine meteorological conditions of theenvironment which it is desired to examine. Theapplication of numerical models, calibrated previouslyby means of measurements carried out in the sea,makes it possible to extend the normal range obtainedfrom the measurements themselves, which must belimited by cost and time, and therefore to calculate themost extreme conditions of the current from which the‘project current’ for the sealine is derived.

WavesThe sea’s wave motion is made up of the

oscillations (waves) generated by the action of thewind on the surface of the sea, which can maintain andpropagate themselves even when the wind hasstopped. Apart from the force of friction (of the wind

on the surface of the sea, of the seabed on the motionof the wave itself), the only force acting on the wave isgravity, which is the derivation of the name ‘gravitywave’. In the generation phase when the wind transfersenergy to the surface of the sea and makes it grow insize, the waves have complex and unstable forms,characterized by fragmentation and non-linearprocesses. In the absence of wind, the waves tend tobecome more regular and more easily described withmathematical models (Goda, 1985). Marine waveshave identifiable crests and troughs; the height of thewave is taken as the vertical distance between thetrough and the crest, the length of the wave (l) as thedistance between two successive crests, the period ofthe wave as the interval of time which passes betweenthe passage of two crests at a fixed point. Otherparameters of interest in wave motion are thepropagation velocity of the wave and the speed of theparticles of water during the wave’s passage. Theformer is generally greater than the latter; only in thebreaking phase does the velocity of the particles of thewater exceed that of propagation and the wavecollapses. The velocity of the water particles is theparameter of greatest interest in planning a pipelinebecause it defines the dynamic load conditions as thewave passes. On the surface, over a wave period, theparticles of water follow an almost circular orbit, thediameter of which is equal to the height of the wave(z0), so that the orbital velocity is greater in a short buthigh wave compared to one that is long but low. As thedepth increases, the orbits of the particles decrease onan exponential basis (aekz0, where a and k are twoconstants) ruling the corresponding radius, and flatten-out as they near the sea floor where the orbital motionbecomes completely horizontal (Fig. 13).

There are two types of surface waves: wavesproduced by the wind and residual waves. The wavesproduced by wind have dimensions and periods whichare dependent on the strength of the wind, on itsduration and on the area of open sea on which it isacting. They have a very irregular form and can beconsidered the superimposition of a large number ofelementary waves (sinusoidal), each having its ownheight, period and direction of propagation. These



shallow water

intermediate water

deep water

l/2

z0

a

aekz0

Fig. 13. Circular wave motions in relation to depth.

elementary waves form the ‘wave spectrum’, thebandwidth of which is an indication of the extent towhich the elementary components differ. The wavemotion remains even in the absence of wind and ispropagated by the area of generation in other areas ofthe sea. In the process of propagation, the waves whichare the lowest and have the shortest periods weakenmore rapidly than the higher waves with longerperiods; a more regular wave motion develops with areduced spectral dimension and more focusseddirectionally. A characteristic of wave motion is itscapability to propagate itself over long distances withthe minimum dissipation of energy. One thinks, forexample, that along the whole of the coast of westAfrica, an area in which the equatorial calm results inlow conditions for the generation of waves by the wind,the waves are produced by the wave motion comingfrom the area of the so-called roaring forties of thesouth Atlantic, several thousand kilometres away.

The interaction between the seabed and waveaction is of particular importance in pipeline designand it becomes significant when the water depth isless than half the wave length. In fact, in shallowwaters the wave motion has a dynamic effect near theseabed and acts directly as a load on the pipeline.Such effects grow as the depth decreases and reachtheir greatest intensity in the surf zone, the area ofwhat is called the shore approach, where it is usuallynecessary to bury the pipeline in order to protect itfrom hydrodynamic loads of excessive intensity(Tucker, 1991). Among the principal effects of theinteraction between the marine floor and wave motion,there are: refraction (rotation of the wave fronts whichtend to align themselves with the isobaths), shoaling(variation of the concentration of energy or steepnessof the wave), diffraction (generation of semi-circularwave fronts corresponding to submerged obstacles),reflection of the wave, absorption (loss of energyresulting from action from the seabed or of anobstacle) and finally, breaking. The last mentionedoccupies a role of primary importance in sub seapipeline planning, inasmuch as the surf zone iscertainly the most dynamic area along the course of apipeline. The surf zone is characterized by highlyturbulent conditions, intense coastal currents causedby the breaking of the waves and violent impacts ofbreaking bodies of water; such dynamic activitycauses extreme conditions in terms of bothhydrodynamic loads and movement of sediment. Itfollows that, in the planning phase, the study of thedynamic characteristics of the surf zone should befocussed on determining the extent of the length ofpipeline to be buried and on the temporary loadconditions in the time interval between the laying ofthe pipeline and its being buried.

Chemico-physical properties of marine waterThe chemico-physical properties of marine water

of greatest interest in the planning of a sub seapipeline are its composition (essentially its salinity, thedissolved oxygen content and the pH factor) whichaffects its corrosive action, the temperature whichregulates the thermal exchange between the pipelineand the marine environment, with implications for itscorrosive action, the hydraulic characteristics of thefluid within the pipeline and the specific gravity whichinfluences all the dynamic forces of interactionbetween the marine environment and the pipeline itself(Neumann and Pierson, 1966).

Almost all known substances are present in seawater, at least in traces; the elements present in thegreatest quantities are chlorine and sodium. Sea wateris slightly alkaline (pH�8.1-8.2) and it behaves like abuffer solution, maintaining its pH unalteredindependently of the substances poured into it.Moreover, because of the significant quantity of ionspresent, sea water is an electrolyte with a fairly highconductance which increases in direct proportion tothe salinity. Another characteristic of sea water is theconstant ratios between the concentrations of theprincipal substances dissolved in it. Thanks to thispeculiarity, it is possible to express the total salinity,that is the percentage by weight of the salts dissolvedin the sea water, as a function of the concentration ofthe chlorine or chlorinity. While the proportions of theprincipal substances are practically constant, theabsolute concentrations vary both in space and time,following the variations in salinity. In the open oceanssalinity varies from 34‰ to 38‰; the average is closeto 35‰. The water of enclosed seas, having a limitedexchange of water with the open ocean, have a lowersalinity in regions where there is heavy rainfall andsignificant fluvial influxes. Typical examples are theinternal parts of the Baltic Sea and certain fjords,where the salinity can fall to values in the order of 0.5-1‰. In regions where evaporation exceeds the rainfall,the water of internal seas exhibits values of salinitywhich are much higher, such as in the Red Sea wheresalinity reaches 43-45‰. Since the variations insalinity are essentially linked to the variations in thedifference between rainfall and evaporation, there is avariability in salinity which is linked to both thelatitude and the season. The seasonal variability isgreater in the internal seas because of the limitedexchange and the contribution of the fluvial influxes.

The temperature of the water has a powerful effecton the majority of the physical, chemical and biologicalprocesses which take place in the sea. In the oceans, thetemperature varies from about �2°C to about 30°C. The lower limit is determined by the formation of ice,while the upper limit is regulated by the processes of



radiation and thermal exchange with the atmosphere. Ininternal seas with limited exchange the temperature canbe higher, but rarely is so in the open ocean. Thetemperature of the water at the bottom of the oceans isalways relatively low, varying between about �1°C andabout 4°C. The temperature’s variation in space andtime is influenced by different factors, even thoughcertain general characteristics can be identified. As faras geographical distribution is concerned, the highesttemperature values are recorded slightly north of theequator; in addition the surface temperatures of thesouthern hemisphere are slightly lower, atcorresponding latitudes, than those of the northern

hemisphere. This can be attributed to differences in thecharacteristics of the atmospheric circulation in the twohemispheres, to the effect of the smaller area of landsabove sea level in the northern hemisphere and thepresence of the continent of Antarctica. The temperatureof the water also exhibits an annual variability which isdependent on a number of factors, mainly on thevariations in incident solar radiation and on the regimeof the currents and winds. The periodic oscillations intemperature are also present in the deepest layers, eventhough they tend to lessen rapidly with depth.

The specific gravity of sea water is dependent ontemperature and salinity and also, because of the lowcompressibility of water, on pressure. However, thislast effect is very weak and becomes significant onlyat great depths. The specific gravity increases as thesalinity increases and as the temperature falls. Thespecific gravity of sea water ranges from 1,005 kg/m3

to 1,030 kg/m3. The geographical distribution of thespecific gravity of ocean water exhibits two principalcharacteristics: vertically the density stratification isgenerally stable; horizontally differences in relativelystable stratification of specific gravities are possiblewhen there are currents.

Geo-risks for sub sea pipelines

As already indicated the seabeds are asmorphologically varied and complex as dry landsurfaces (Fig. 14) and the physical processes whichaffect them are often complicated by wave action andcurrents as well as the impossibility of studying themdirectly (Poulos, 1988). In particular, if the coastalenvironment is in a state of constant and rapidevolution, being subjected to wave action and currents(Sumer and Fredsoe, 2002), in deep waters slopes andcontinental margins are characterized by very complexgeological conditions and present problems of



dome/diapir

gaschimney

retrogressivesliding

debrisflow

seismicaction

potentialfailure

line

mudvolcano

wavegeneration

tsunamiFig. 15. Diagram of the various types of geo-morphological-seismic risk that characterise theundersea environment.

Fig. 14. Three-dimensional image of the sea bed: very irregular escarpments and continental shelves with the presence of steep slopes and canyons favourable for the development of gravitational flows(courtesy of R. Bruschi).

instability of various kinds, the nature of which, attimes, is very difficult to identify.

Some of the geomorphoseismic problems whichregulate the stability of seabeds are very similar tothose typical of the terrestrial environment, both intheir type and their analytical treatment. These types ofissues are handled in the same way as comparableproblems relating to onland pipelines. Specificfeatures of the marine environment are submarinelandslides, certain aspects of seismic activity and waveaction (Fig. 15).

Submarine landslidesA submarine slope, as in the case of terrestrial

conditions, is subject to the force of gravity whoseeffect is to pull bodies downwards. Therefore, itsstability is closely linked to the gradient, to the weightand to the resistance of the ground. A study of thestability of an area is based on a knowledge of itsbathymetry (in particular of all the forms of featuressuch as canyons, depressions, etc.), its stratigraphy andthe mechanical properties of the sediments. Submarinelandslides represent one of the principal risksassociated with the sub-aquaeous environment. Thecauses which generate them can be manifold and aredivided into two fundamental types: causes whoseeffect is to increase the forces acting on the systembeing studied, and causes whose effect is to reduce theground’s capacity of resistance.

Among the causes increasing the forces in theterrain are the deposits and accumulations upstream(or erosion at the foot) of a slope. On a large scale, ingeneral, the factors involved take place gradually andtherefore do not interfere with the operational life of ahydrocarbon transport pipeline. Nevertheless, they canhave a significant influence on the general stability ofthe area and should be taken into considerationespecially in the case of a rapid increase in dip or theformation of channels caused by erosion.

Leakages of underground gas and fluids causedepressions in the sea floor which often are associatedwith presence of faults. The deep of these depressionscan be very steep giving rise to moderate instabilities.The intrusive actions of salt or mud, as also thepresence of little volcanoes of mud, exercise a stronglateral pressure in the surrounding terrain which, iflocated upstream of a slope, can induce seriousdeformations with the ejection of material which couldspread over any pipeline situated downstream,damaging or covering it.

Earthquakes affect the stability of sub sea slopes inthe same way as those on land. The action of glaciersand icebergs can, instead, be very significant for theseabed and can be seen in the existence of highlyconsolidated ground and deep furrows. The objectives

of a study of the stability of an area should include aconsideration of the effects of the installation of thepipeline and possible subsidence following theexploitation of sub sea reservoirs causing compactionassociated with the depletion of pore spaces.

Among the causes capable of reducing theresistance capacity of the terrain, should be includedall those factors which cause variations in the overallstate of tension in a non-drained or only partially-drained terrain, that is to say all those very rapidlyoccurring variations of load relating to thepermeability of the terrain, so as not to allow thesimultaneous escape of the water in the pores andtherefore cause an increase in interstitial pressures. Anexcessively rapid depositing of sediment (a typicaloccurrence at the mouths of rivers) causes theformation of poorly-consolidated material, thebehaviour of which, in terms of stability, should beanalysed from time to time using suitable models.Some terrains display a fragile or contracting form ofbehaviour (Poulos, 1988), that is, they undergo astrong compaction when they are subjected to shearingactions greater than the limit (or peak) of the material.The load due to a seismic shock subjects the ground toa series of cyclical variations of force (Kramer, 1996)which can cause compaction of normally consolidatedor mildly over-consolidated material, with an increasein the interstitial pressure and a reduction inresistance. Soft and normally consolidated clays areparticularly sensitive to this action and can undergo anotable reworking with serious consequences for theirmechanical behaviour, something which it isfundamentally important that those carrying outstudies or modelling should take into account. Thereduction in effective tension, and therefore ofresistance, can also be due to the dissolution or theexpansion of gas contained in the pores. Groundcontaining gas represents one of the most importantthemes in the study of marine floor stability. Thepresence of hydrates in the ground is another aspect ofthe same problem. In certain conditions of pressureand temperature crystalline formations tend todissolve releasing large quantities of gas and inducingan increase in interstitial pressure.

Earthquakes and seismic activitiesEarthquakes are one of the most important factors

among the natural causes capable of causing powerfulphenomena of instability. Specialized literature reportsnumerous catastrophic occurrences of sub sealandslides (Poulos, 1988). One of the effectsassociated with a seismic event capable of causingpowerful phenomena of instability is fluidization ofthe terrain. During a seismic event the loose sand andthe mud tend to compact transferring the load to the



water that fills the pores, with a consequent increase ininterstitial pressure and the creation of upward flows.This leads to the reduction of the effective tensionswhile the gradient of the upward flow can reach avalue such as to cause fluidization of the uppermostlayers. In very deep waters, the seabed is generallycomposed of clay or clay-based sediments. Themajority of cohesive terrain is not subject tofluidization, nevertheless some types of sediments(sensitive clay), characterized by low clay content, lowplasticity and high water content, can undergo a greatloss of resistance (Seed et al., 2003). Many marineclays can also undergo this loss of resistance becauseof chemical changes caused by washing out of salinesubstances and the loss of intergranular contacts.Where there is material at risk of fluidization it isnecessary to carry out a detailed analysis of theproblem, estimating the risk of the events occurring,and if necessary arranging for appropriate counter-measures (Kramer, 1996).

Even if the material deposited in deep water isgenerally not subject to fluidization, a seismic eventcould, however, cause conditions of instability throughthe accumulated downward movement resulting from acyclic reduction of resistance. The evaluation of thestability of slopes in seismic conditions is similar tothe solution of the analogous problem on land. Tounderstand the phenomena in order to design a sub seapipeline on a clay seabed, it is important to establish ifthe deformations which are generated in thegroundmass are elastic (and therefore will recoverafter the seismic event) or plastic (i.e. consisting of apermanent residual deformation). In the latter case, itis important to establish the extent of such adeformation and if the terrain is able to retain its ownmechanical properties after the seismic event or willundergo some degradation.

In specific morphological conditions (steep slopes,availability of material, existence of canyons ortransport channels) gravitational flows and turbiditycurrents can follow conditions of instability (Bughi andVenturi, 2001). The generic cases of gravitational flowsinvolve a flow of fairly lumpy material carried by afiner matrix which, from a mechanical point of view,behaves like a visco-plastic material which movesdown fairly steep slopes. The velocity of gravitationalflows are approximately of the order of tens of metresper second and the distances covered in the order ofseveral kilometres. It is relatively easy to detect thesigns of a gravitational flow that has taken place on themarine floor through a geophysical survey, but nomeans of direct measurement of movement exist.Given the mass of the material, the volume involvedand, above all, the velocity of gravitational flowsrepresent a serious threat to the safety of the sub sea

pipeline. Turbidity currents refer to a mixture of finematerial and water which, once in motion, is capable ofdeveloping and self-generating even on seabeds withvery low dips. The specific gravity of the material inthis case is little higher than that of water and theaverage velocity is around 10 m/s. The distancescovered can be over several tens or hundreds ofkilometres. Turbidity currents can also affect thestability of a pipeline on the seafloor and given thedistances that they cover, as also is true of genericgravitational flows, it is understandable that theenvironmental study for the planning of a sub seapipeline must necessarily cover a very wide area andcannot be limited to the corridor formed by its route(Bughi and Venturi, 2001). In contrast with the landenvironment, exploration and gathering ofmorphological and especially of geotechnical data formodelling and analysis of the conditions which developon the seafloor is very difficult and often impossible.The greater the uncertainties surrounding the measuredparameters, the higher will be the degree ofinterpolation of the information applicable to the area.

Fluidization caused by wave motionThe fluidization effect can also be caused by a load

other than that due to an earthquake. In particular, incoastal areas and where the depth of water issufficiently shallow for the effect of wave motion to betransmitted to the bottom, the cyclical action which itcreates can cause fluidization of the ground if itscharacteristics are such as to make it sensitive to suchan effect (Rahman and Jaber, 1986; Madsen, 1989). Ina similar way to that caused by seismic activity,fluidization resulting from wave action reduces theresistance of the material and creates ideal conditionsfor causing phenomena of instability. Especially when,due to construction or natural reasons, a pipeline isburied or partially buried in the ground, fluidization ofthe seabed can lead to it floating, creating geometricalconfigurations such as to induce unusual states oftension on the pipeline which can seriouslycompromise its integrity.

7.1.3 Transport fluid dynamics

Correct hydraulic design of a pipeline and itscomponents calls for the use of fluid dynamic modelsto predict the properties of the fluid and how they maychange with variations in temperature and pressure.Depending on the type of pipeline under consideration,land or marine, and the typology of the fluidtransported, liquid or gaseous, etc., the fluid dynamicanalyses can be carried out with various objectives in



mind (Mohitpour et al., 2000; Palmer and King, 2004).For example, they may be carried out to determine: themaximum flow-rate, once the length of the pipeline,the route followed, the delivery pressure, the outletpressure, the properties of the fluid and the diameter ofthe pipeline have been determined; or, the diameter ofthe pipeline, once the maximum flow-rate, the deliverypressure, the outlet pressure, the properties of theproduct being transported, the length of the pipelineand the route have been defined; or the delivery oroutlet pressure, once the other variables in play havebeen defined, but with other variables to be optimized.

The first example is typical of marine pipelines,the second of both onshore and marine pipelines, andthe third of onshore pipelines. These objectives mustbe pursued making sure of two things. The first is thatthe pipeline has been correctly designed in order tooptimize the construction costs (which increase withthe diameter of the pipeline) and the operating costs(which decrease as the diameter of the pipelineincreases, because of the reduction in the loss ofpressure and of the consequent reduction inperformance called for at the pumping/compressionstation). The second is that the necessary safetyrequirements with regards to the surroundingenvironment are respected.

Physical properties of the fluid

The fluid transmitted through a pipeline may be insingle phase or multiphase. A single phase flow can begaseous or liquid, without any solid particles andwithout any other type of liquid or insoluble gas.Flows of natural gas and of treated oil are examples ofa single phase flow. A multiphase flow contains atleast two separate phases, for example: a) a liquid anda solid phase; b) a gaseous and a solid phase; c) aliquid and a gaseous phase; d) two immiscible liquidphases. Multiphase flows are often present in pipelinesthat connect the hydrocarbon field with the treatmentcentre close to the field, where the producedhydrocarbons are processed before being transportedto the refinery. A multiphase fluid in general consistsof a gaseous phase, one or more liquid phases (oil andwater) and sometimes a solid phase (sand); it isincompressible if the specific gravity of each particlein the flow, be it fluid or solid, remains constant, and itis homogeneous if the specific gravity is constantalong the flow itself.

The physical properties needed for an engineeringanalysis are: a) phases present and their percentages;b) molecular weight; c) density; d) compressibility;e) viscosity; f ) heat capacity; g) thermal conductivity; h) surface tension (for multiphase flows). In the literatureof the sector there are several correlation available that

make it possible to link together the above-mentionedphysical properties. To describe the behaviour of thefluid system, equations of state are used, that link thepressure, the volume and the temperature betweenthem. To describe the behaviour of real fluids, semi-empirical relationships have been developed that linkthe various parameters, using constants developedexperimentally. Such equations are generally called bythe name of the researchers that developed them, likethe Peng-Robinson equation, the Soave-Redlich-Kwong equation, the Benidict-Web-Rubin-Starlingequation and the Chao-Seader-Grayson-Steed equation(Katz et al., 1959; Mohitpour et al., 2000). Thefundamental parameters of a fluid are its specificgravity, its viscosity and its compressibility (importantfor gaseous fluids). In the Newtonian fluids, theresistance to movement is directly proportional to thevelocity of the fluid across the dynamic viscositywhich is constant; in the non-Newtonian fluids, theviscosity varies with the variation of the cutting forceand their behaviour is notably more complex. Thephysical properties of a fluid system generally dependon the pressure and the temperature. An increase intemperature has a positive effect on pipelines thattransport liquid hydrocarbons because it reduces boththe viscosity and, as the specific gravity drops, the lossof pressure (Mohitpour et al., 2000). On the contrary,an increase in temperature has a negative effect on gaspipelines since, as the transmissibility falls, there is agreater loss of pressure. The absolute value of theviscosity of gas increases with the increase in pressureand in temperature; this increase causes an increase inthe friction along the pipeline and, therefore, a netincrease in the power needed to reach a given flow-rate(Mohitpour et al., 2000).

Hydraulic analysis

The hydraulic analysis of a pipeline can be carriedout on the hypothesis of steady state or unsteady stateconditions. In the case of steady state flow, it isassumed that the properties of the fluid such aspressure, temperature and velocity within the pipeline,do not vary with time or vary slowly (McAllister, 1988;Kern, 1990). For an unsteady state flow (transient) thebehaviour of the system is analysed in a situation inwhich some fundamental parameters vary in time suchas, for example, the delivery pressure, the velocity ofthe fluid, etc. (Mohitpour, 1991).

Generally pipeline systems are designed assumingsteady state flow conditions, which are consideredsufficient to optimize the project parameters of apipeline. However, there are situations that call formore sophisticated analyses than those conventionallyused in steady state flow conditions. These situations



include very severe operating conditions (for example,high pressure and temperature) and rapid variations offlow-rate, such as those that precede and follow thetesting of the pipeline. In this and in othercircumstances, transient hydraulic analyses (alsocalled dynamic or unsteady flow) are needed to checkthe capacity of the pumping/compression station, tochoose its auxiliary components and to ensure a givendegree of safety.

Since the beginning of the Nineteenth century agreat deal of work has been carried out to developinterpretative models with a view to accuratelypredicting the flow conditions of monophase fluids,both liquid and gas (Mohitpour et al., 2000).Excluding multiphase flows, the mathematical modelsavailable have evolved to the point where, in themajority of equations, the intrinsic errors arenegligible compared with the errors caused by theuncertainty of the input data such as the surfaceroughness of the walls, the operating temperature, etc.For example, the hydraulic analysis of a gas pipeline insteady state conditions can predict loss of pressure,with an accuracy of less than 3%; the parameters usedas input in the flow equations, such as flow-rate,temperature and operating pressure and roughness ofthe inner walls have, instead, a much greater impact onthe total error. The hydraulic models that describe thedynamics of transmission of fluids in pipelines aregenerally based on three fundamentalequations/relationships relating to the conservation ofmass, the balance of forces and the conservation ofenergy (Katz et al., 1959; Mohitpour et al., 2000).

Steady state flow conditionsGenerally, the results of the hydraulic analyses

in steady state conditions are used in the designingof a pipeline. In particular, an analysis is madeof the flow-rate and the drops in pressure to determinethe capacity, the diameter of the pipeline, the lengths of the closed circuits and the powerneeded at the compression/pumping station.

According to Bernoulli’s formula (Mohitpour etal., 2000), the trinomial

p v2z�13�13

rg 2g

assumes the same value in all the sections of a pipelinetransporting an incompressible fluid of density r in asteady state flow, in which the effects of friction arenegligible. In connection with a given section, z is thedepth referred to a reference datum, p is the pressureof the fluid, g is gravitational acceleration and v is thevelocity of the fluid.

In reality, there is friction in pipelines and hence various modifications to the Bernoulli formula have been proposed. In particular, the lossof pressure per unit of length of pipeline due to the friction exercised by the walls of the pipeline on the fluid in it is given by

frv2hf�132

2D

In this formula D is the inner diameter of thepipeline and f the friction factor, for which variousforms have been proposed (one of the most common



. . .. .0.050000.04000

0.03000

0.020000.01500

0.01000

0.100criticalzone

transitionzone

laminar flow

complete turbulence0.0900.0800.070

0.060

0.050

0.040

0.030

0.025

0.020

0.015

0.010

32 4 56 104 32 4 56 105 32 4 56 106 32 4 56 107 32 4 56 108103

0.009 0.008

0.008000.00600

0.00400

0.00200

0.001000.000800.000600.00040

0.00020

0.00010

0.00005

0.00001

rela

tive

roug

hnes

s (e

/D)

fric

tion

fac

tor

(f)

Reynolds number (Re = VD/n)

rough pipes

smooth pipes

laminarflowFig. 16. Moody diagram.

being that of Darcy-Weisbach; the Fanning formula isoften used as an alternative). To calculate the frictionfactor, it is necessary to assess the flow regime:laminar or turbulent. This is determined by calculatingthe Reynold’s number

rVD VDRe�1323�13

m v

which represents the ratio between the forces of inertia,rV 2, and the viscose forces mV

13

D, where m

is the dynamic viscosity and v �m/D the kinematicviscosity. For Reynold’s numbers below 2,000, theflow is laminar and the friction factor is equal to

64f�13

Re

For Reynold’s numbers above 2,300, f can becalculated using the empirical Colebrook-Whiteequation, in which the roughness e of the inner wallsof the pipeline comes into play:

1 2e 18.711�1.74�2log�1�111��1

f D Re�1

f

For very high Reynold’s numbers, the secondterm in the brackets becomes negligible, whichsimplifies the calculation of f noticeably. The Colebrook-White equation is used toconstruct the Moody diagram (Fig.16), that allows the friction factors to be evaluated graphically.Typical roughness values of steel tubes are: polished steel, maximum 0.005 mm; lightly corrodedsteel, circa 0.01 mm; heavily corroded steel, up to 1.0 mm. These values are indicative andgenerally depend on the method of construction, thecleaning processes that can be used and on the historyof the pipe from construction to use. During theoperational life changes can occur, for example anincrease in the friction factor due to corrosion or todeposits of wax in offshore pipelines that transportcorrosive and partially treated fluids. The velocity ofthe fluid inside a pipeline varies according to the phaseof its operational life. Typical velocities are:hydrocarbons in liquid phase, from 1.0 to 4.5 m/s;hydrocarbons in gas phase, less than 18.0 m/s(up to the noise limit); two-phase hydrocarbons(liquid and gas), greater than 3.0 m/s and less thanvelocities at which consistent scouring effects appear,equal to

122ve�111

�1

r

(Palmer and King, 2004). Localized losses of pressure are associated with

reductions in the cross-section or a change of directionof the flow in the pipeline. In transport over long

distances, this loss is not generally important, but inthe case of short pipelines, or those with manyconnections, this loss can be significant. The loss ofpressure in the connections can be determined througha combination of theoretical formulae andexperimental tests; they can be considered as loss ofpressure given by a coefficient of resistance, or anequivalent length of straight pipeline. The thermalenergy of the transmitted fluid can be considered to bepart of the total energy of the pipeline system,applying the first law of thermodynamics.Nevertheless, the transformation of mechanical energyinto thermal energy is not significant in the hydrauliccalculation. On the other hand, the effect oftemperature on the physical and chemical properties ofthe fluid transmitted is important: in fact it caninfluence the viscosity, the stability of the liquid/gasphase, corrosion or the formation of hydrates. Ingases, temperature is a fundamental variable in theflow-rate calculation; therefore it is important topredict the temperature profile along the pipeline.

Unsteady state flow conditionsTo evaluate the difference between steady state and

transient flows, the basic behaviour of compressibleand incompressible flows must be analysed. In reality,all fluids are compressible, but gases are much moreso than liquids, as reflected in the calculation of thedrops in pressure and flow-rate. To predict the drops inpressure accurately, the properties of the fluid must bedetermined as the pressure and temperature vary alongthe pipeline. In the case of liquids, the properties varylittle with pressure and temperature, therefore there isno need to calculate the properties of the fluid alongthe pipeline. In the case of gases, the compressibilityof the fluid does not modify only its own properties. Intransient flow conditions, effects linked to thevariations in pressure and compressibility of the fluidappear, such as packing of gas in the pipeline, with aconsequent slow build up of pressure, and dynamicoverpressure linked to rapid processes. These effectsmust be taken into consideration when planning apipeline.

For pipelines that transport liquids, the objectivesof dynamic analysis can be: a) economic optimizationof a pipeline, which includes the use of a dynamicoverpressure control system in relation to a higherthickness of steel; b) controlling of fluid knock and thesizing and positioning of blowoff valves foroverpressure induced by dynamic overpressure; c) design and use of a control system, which takesaccount of the planning of compression station controlvalves to reduce the increase of pressure and to protectthe pipeline from excessive dynamic overpressure; d) the effect of a rapid closure of the valves; e) the



effect of rapid fluctuations in the flow; f ) designing ofa dynamic overpressure release system; g) determiningof possible breaks and leaks along the pipeline(Mohitpour et al., 2000).

For pipelines that transport gas, the objectives ofdynamic analysis can be: a) determining possibleleakages of gas along the pipeline; b) the response ofthe pipeline to rapid changes in flow-rate orcompression; c) the opening and closing of valves; d) determining the packing condition of the pipeline;e) the pulsation generated by the internal flow;f ) optimization of the location of compression stationsalong the route of the pipeline; g) capacity planning ofthe pipeline to guarantee the required volume of gas ina complex/articulated pipeline system (Mohitpour etal., 2000).

Transmission of gas

Inner wallsPipelines are coated internally whenever the fluids

being transmitted have a corrosive or oxidizing effectwhich must be countered or controlled. The choice ofthe type of coating is made on the basis of the type offluid being transmitted (water, gas, oil, etc.), theproperties of the fluid (chemical, physical andbacteriological composition, and operatingtemperature and pressure) and the mode of operation(temporary operation or otherwise, partially filled ornot, etc.). Often, a coating is applied to the inside ofthe pipeline in order to reduce the roughness of theinner surface and hence the loss of pressure of a fewpercentage. The reduction in loss of pressure, due tothe compressibility of the gas, increases the amount ofgas transmitted as the transmission pressure increasesand hence has implications on the costs oftransmission of gas pipelines operating over longdistances (5,000-6,000 km).

The flow in a gas pipeline is generally turbulent,that is, it is characterised by a high Reynolds number(≈ 1.0·107) due to the low viscosity and high specificgravity of the gas, which is subjected to operatingpressures varying from 10 to 30 MPa (the first valuerepresents the upper limit for sub sea pipelines, thesecond that for underwater pipelines). At such highReynolds number values, the Colebrook-Whiteequation predicts a high friction factor even whenthere is minimal roughness ( ≈ 1 mm). In recent years,there has been a series of experimental and theoreticalresearch projects carried out to quantify moreaccurately the resistance of pipes with and withoutinner coatings that are used for gas transmission, andto specify inner coatings able to reduce the loss ofpressure over the whole operational life of thepipeline.

CompressorsCompression of the transmitted gas is necessary in

gas pipelines in order to overcome the loss of pressurethat occurs along the pipeline and to guarantee certainconditions of flow-rate and pressure at the arrival point(Pfleiderer and Petermann, 1985; Mukherjee, 1997).Loss of pressure is caused by expansion of the gas,friction against the inner walls of the pipeline,variations in altitude or variations in temperature.Compressors can be grouped into three differentcategories: volumetric, dynamic and injectioncompressors (Fig. 17).

Volumetric compressors trap a certain quantity ofgas within a closed volume: by reducing the volume,they increase the pressure of the enclosed gas; thecompressed gas is then released at the compressor’spoint of discharge. These compressors can, in turn, bedivided into reciprocating and rotary compressors. Inthe first, a piston reduces the volume of the gas withinthe cylinder and valves are needed for the entry andexit of the compressed gas and to prevent reversal ofthe flow. In rotary compressors, the rotors havecavities that trap the gas within a fixed or variablevolume between the cavities themselves and the fixed,external casing of the compressor. The gas movesfrom the entry point (also called the suction side) tothe discharge point. This type of compressor does notneed valves and generally is used for compressing airin plants. Dynamic compressors, also calledcontinuous, are in turn divided into two maincategories: centrifugal (or radial) and axial. In thefirst, the rotor blades increase the kinetic energy ofthe incoming gas: when the blades rotate, thecentrifugal force pushes the gas outwards and soincreases the tangential velocity of the gas. The gascompression comes about in part in the impeller andin part in the radial diffuser that surrounds the rotor,or in the compressor’s discharge diffuser. Instead, in



compressors

positivedisplacements

reciprocating rotary centrifugal(radial flow)

axial flow ejectors

sliding lobestraight lobehelical lobe

etc.

single-stagemulti-stage


multi-stagewith fixed

stator vanesand

variablestator vanes


dynamics injectors

Fig. 17. Classification of compressors.

axial compressors a rotor transfers its energy to thegas that passes through it during compression. In thistype of compressor, the flow of the gas is parallel tothe shaft of the motor. Injection compressors use thekinetic energy of one flow of fluid to compressanother fluid. This type of compressor is not used ingas transmission systems.

At times, to obtain the discharge pressure requiredto transmit the gas, a number of compressors may beused in series (where the discharge of each compressoris connected to the suction side of the next one) toovercome the compression ratio limitation of anindividual gas compressor. The main limitations are:• The compression ratio; for safety reasons, the

compression ratio is generally less than 6 for gascompressors. High compression ratios significantlyincrease the forces applied to the shaft and othermechanical elements of the compressor, making itsdesign complicated and expensive and in certainconditions the compressor’s operation may beunsafe. Especially at high pressures and flow-rates,typical of long pipelines with large diameters, thecompression ratios range from 1.2 to 2.0.

• Temperature; compressor manufacturersrecommend a maximum discharge temperature ofabout 100°C.

Cooling systemsSystems for cooling are widely used in the gas

transport industry and can be employed as pre-cooling(at the suction side/intake of a compression station) oras inter-cooling (between compressors in series)systems in order to protect the system from excessiveheating. They can also be used as post-cooling systems(at the discharge of a compressor station) to protect thepipeline’s external anti-corrosion coating from beingdamaged by high temperatures. Cooling the gas at thedischarge of compressor stations also reduces the dropin pressure along the pipeline, as the gas is transmittedat a lower temperature. Post-cooling systems alsoreduce the power needed at the next compressionstation, which receives the gas at a lower inlettemperature.

There are two types of gas cooling systems: air-cooled heat exchangers and water-cooled heatexchangers. Depending on the climatic andgeographical conditions, either type (or a combinationof both) can be used to achieve the cooling required.Operating costs of water-cooled heat exchangers aremuch higher than those of air-cooled heat exchangers.If the environmental conditions permit, especially inremote areas, air-cooled heat exchangers are used ongas pipeline transmission systems. The principalcomponents of this type of exchanger are: a) thecooling fans; b) the motors connected to the cooling

fans; c) the fan speed control system; d) the supportstructure; e) collectors for the gas to be cooled.

Mechanical driversCompressors are generally used in conjunction

with mechanical drivers connected to the compressorshaft, such as gas turbines, electric motors and steamturbines (Pfleiderer and Petermann, 1985; Wilson,1991; Mohitpour et al., 2000; Palmer and King, 2004).

Gas turbines are the most commonly used driversin remote areas, especially for gas transmissionsystems, and they are generally coupled to centrifugalgas compressors. In these, the turbine is connecteddirectly to the shaft of the pipeline compressor. Gasturbines are relatively compact, have a highpower/weight ratio and are well suited to the highspeeds required by centrifugal compressors. Generally,they are very flexible and adapt well to the operatingconditions of the compressors.

Transport of liquids

Drag reducersA principal requirement for the transport of

hydrocarbons in liquid phase is that the pressure at thepumping station should be sufficient to guarantee therequired arrival pressure and fluid flow-rate. Anotherimportant factor is that there should not be any pointsalong the pipeline in which the inside pressure dropsbelow the vapour pressure of the liquid. The behaviourof a liquid is totally dependent on specific gravity andviscosity. The viscosity of all liquids varies withtemperature; for example, in short lines the inlettemperature is high and can have a powerful influenceon loss of pressure.

To increase the capacity of a pipeline system fortransporting liquids, it is possible to add parallelsections beside those sections where there is a bottleneck and/or to increase the power of the pumpingsystem. At times, a good alternative could be to injectan additive, called a drag reducer, which reduces thefriction. The additive is a polymer with a highmolecular weight and a long molecular chain. Theeffect of a drag reducer is to suppress the vorticeswhich can form in proximity to the walls of thepipeline and which dissipate energy; hence dragreducers are effective only in fully developed turbulentflow conditions. The drag reducer’s effect diminishesin the direction of the flow because of the gradualbreaking of the molecular bonds of the polymer’s longchains. The polymer is destroyed in auxiliary stationswith centrifugal pumps, and therefore it is necessary toinject new additive to maintain the effect downstreamof the station (volumetric pumps are less damaging tothe additive). Drag reducers by their nature are very



viscous liquids and must be injected downstream ofthe pumps by means of a small volumetric pump.Their effectiveness and the quantity required can becalculated once the characteristics of the polymer andthe liquid hydrocarbon being transported are known,but current predictions are not always reliable. Aninitial indication (Palmer and King, 2004) of therequired quantity is: 4-10 g/m3 for an increase in flow-rate of 5%; 8-25 g/m3 for an increase in flow-rate of10%; 13-40 g/m3 for an increase in flow-rate of 15%.

Drag reducers are generally more effective at highvelocities (�1.8 m/s) and low kinematic viscosity(�10 cSt) and along short stretches of pipeline (�75 km). The effectiveness diminishes as the watercontent percentage increases, since drag reducers aresoluble only in the oil phase. Drag reducers areeffective with hydrocarbon liquids containing wax at atemperature below its pour point.

PumpsA compression phase is needed in pipelines

transporting liquid hydrocarbons, in order to overcomethe loss of pressure that takes place along the pipelineand to guarantee certain conditions of flow-rate andpressure at the arrival point. Loss of pressure is due tothe friction on the inner walls of the pipeline and to thevariations in altitude.

Pumps can be grouped into two different categories:volumetric pumps and kinetic pumps (Fig. 18).

Volumetric pumps are, in their turn, classified asrotary pumps and reciprocating pumps (Karassik,1976). Rotary pumps consist of a fixed casingcontaining gears, screws, pistons, vanes or similar

elements driven by the rotation of the motor shaft;reciprocating pumps in their turn are divided intodirect action pumps and pumps which are driven by acrank, both driven by steam turbines.

The primary kinetic pumps are the centrifugal typewhich, in turn, are divided into radial, mixed and axialflow pumps (Pfleiderer and Petermann, 1985;Lobanoff and Ross, 1987). In radial flow pumps, thepressure is developed principally from the action ofcentrifugal force: the liquid enters the impeller close tothe hub and is moved radially towards the outside ofthe pump; in mixed flow pumps the pressure isdeveloped partially from centrifugal force and partiallyfrom the force of the vanes on the liquid; the liquidenters the impeller close to the hub and is dischargedboth along the axis and radially; axial flow pumps,sometimes also called helical pumps, develop themajority of their pressure through the rotation andthrust of the vanes on the liquid: the liquid enters theimpeller axially and is discharged axially.

Centrifugal pumps can have a single stage, wherethe total pressure is developed in a single impeller, orthey may be multi-stage, having two or more impellerswhich operate in series within the casing of thediffuser. The pumps most commonly used incompression stations for pipelines transporting liquidhydrocarbons are the centrifugal and the volumetric.Centrifugal pumps operate at high revolutions,connected through speed multipliers to an internalcombustion engine or an electric motor. In largepumping stations they are connected in series, so thateach pump handles the entire flow, increasing thepressure of the liquid to be transmitted.



pumps

positivedisplacement

reciprocating

pistonplunger

single actiondouble action

single bodydouble bodytriple bodymulti-body

diaphragm

single bodymulti-body

camsgearsscrew

impeller

radialflow

mixedflow

axialflow

singleintake

single intakeand double intake

one stagemulti-stage

one stagemulti-stage

self-fangingnon self-fanging

jetelectromagnetic

etc.

rotary centrifugal special

kinetic

Fig. 18. Classification of pumps.

Centrifugal pumps offer a number of advantages:first of all the fact that there is no significantpulsation in the flow of liquid coming out of thepump; if they are installed and operated correctly,they are not subjected to any significant vibration;they can operate in the open air or in small buildings;they require only light foundations and can be easilycleaned. Moreover, they are inexpensive and simpleboth to construct and to operate, as well as requiringrelatively little space.

Multiphase transport

RegimesIn a single-phase flow, the average velocity is the

ratio between the volumetric flow-rate and the area ofthe inner cross-section of the pipeline. If there aremore phases present, the velocity of each of them isexpressed as the velocity corresponding to theseparation surfaces between the phases, whichrepresents the velocity which each phase would have ifit were the only one being transported inside thepipeline. It should be noted that this velocity does notexpress the velocity at which the phase moves withinthe pipeline, but rather represents the volumetric flow-rate relating to that phase.

The hydrodynamics of a multiphase flow are muchmore complicated than those of a single-phase flow,because the different phases have very differentspecific gravities and mechanical properties andbecause different flow regimes can be generated insidethe pipeline. The multiphase flow has been, and still is,the subject of specialised studies. Fig. 19 shows thedifferent regimes for a twophase flow of gas andliquid, where the x-axis represents the velocity inrelation to the surface of the gas phase and the y-axisrepresents that of the liquid phase. The extreme lower

left corresponds to transmission of both phases at lowvelocity; the extreme lower right corresponds with agas which is moving quickly or in high volumes and toa liquid which is moving very slowly or in smallquantities. The illustration in the middle of the graphshows a short section of the pipeline and of the flowregime that has developed inside it.

To better understand the different regimes presentin the case of a twophase transport it is sufficient toconsider the example of a twophase flow that we seevery frequently, for example water which flows slowlythrough the neck of a bottle. In this case, the waterrepresents the liquid phase and the air the gas phase;the specific weight of the water is some eighty timesgreater than that of the air and the flow is stratifiedbecause, as the velocity of both flows relative to thesurface of the two phases is slow, the heavier phase,the water, occupies the lower part of the bottle neckand the lighter fluid, the air, occupies the upper part(see again Fig. 19). If the surface velocity of the gasincreases, the gas will move more quickly than thewater. That happens when the wind blows across thesurface of the sea: unless the relative velocity is verylow, waves form on the surface and the stratified flowbecomes undulated. When the velocity of the gasincreases further, the effect can be compared to a verystrong wind blowing across the surface of the sea: thewind catches the water and nebulizes it. This is amixed regime in which the gas phase is continuous andthe liquid is transported in the form of drops: if thequantity of the liquid increases, the walls of the pipecan become completely covered with the liquid beingtransported through the gas pipeline and a mixedannular flow takes place. Another significant situationarises when the surface velocity of the liquid is highwhile that of the gas is low. The gas forms individualbubbles in a continuous liquid flow. The bubbles tendto move along the upper part of the pipe, since they arelighter than the liquid.

The final situation to consider consists of a flowthat starts as an undulated, stratified flow; however,when the surface velocities increase and the wavesgrow in height until they completely fill the crosssection of the pipeline, the flow becomes anintermittent flow (slug flow) in which pockets of liquid(slugs) form along the pipeline, separated by stretchesof gas. Slug flows can be very dangerous since thetreatment process at the point of the flow’s arrival mustbe able to handle this irregular arrival of high volumesof liquid with little gas and of high volumes of gaswith little liquid. The existence of slug flows generallyrequires the presence of special devices at the end ofthe flow up stream of the arrival station.

Currently there are several models availablewhich are theoretical semi-empirical, or based on



superficial gas velocity

bubble

slug annular-mist

fluctuating

stratified mistsupe

rfic

ial f

luid

vel

ocit

y

Fig. 19. Flow regimes for biphase systems.

equations in which coefficients have been calibratedthrough reduced scale experimental tests and withprototypes of pipelines that transport biphase flows.Hydraulic analysis of multiphase flows iscomplicated by a number of factors whosesignificance is much greater than it is in the analysisof single phase flows. For example, the variations inaltitude have a small effect in single-phase flows,whereas they have much greater consequences instratified flows (with the denser liquid on the bottomof the pipe and the lighter gas above the liquid). Theflow regime in a stretch of pipeline on a downwardslope with a fall of 2m/km, can be completelydifferent from the flow of a stretch on an upwardslope at the same angle. In the former, the force ofgravity will tend to favour the flow of the liquid and,when the pumps at the pumping station which aredriving the fluid are stopped, the liquid will continueto flow for a certain period of time. In the latter caseof a rising pipeline, gravity tends to slow the liquiddown and the liquid may be pulled onward solely bythe dragging effect of the gas flow that is movingmore quickly than the liquid. The liquid, which ismoving more slowly, tends to accumulate and formslugs which fill the entire cross section of the pipe.The dynamic pressure that is generated when the gasis isolated from the slugs pushes the slugs forwardpowerfully. Another complication arises from the factthat, as the pressure and temperature vary, so thecomposition of the phases varies. In conditions inwhich variations in the flow take place very rapidly,the different phases present may not reach a state ofequilibrium.

DevicesWhen a twophase flow reaches the treatment

station, it must pass through a separator (called a slugcatcher, see Chapter 5.4), which must have a volumegreater than that of the biggest slug that might form inthe pipeline during the transport. The separator isdesigned in such a way that the gas continues to flow,while the liquid is trapped and drained towards aspecial discharge system before the next slug arrives;otherwise consideration would have to be given to useof a larger trap.

Generally, there are two types of separators in use:vessel and tube. The first are pressurised vesselscapable of withstanding the same pressures as thepipeline; hence the walls may need to be very thick.For this reason, tube separators are often used, madeup of short and long lengths of tube with a negativeslope (that is, opposite to the slope of the flow), with asingle inlet and two outlets: one for gas and one for theliquid. Tube separators generally take up more spacethan a vessel separator.

7.1.4 Materials

Selecting materials

The choice of materials for the pipes used fortransport of hydrocarbons passes through a decision-making process that involves several types ofanalyses.

Analysis of the chemical composition of the transported product

The requirements that lead to the selection of thematerial best suited to the transmission of the specificproduct, are defined right from the preliminary phasesof a project, in relation to the project parametersforeseeable at the time. The experience gained over thelast two decades, taken as a whole from the point ofview of the technological development of materials,makes it possible to classify pipelines on the basis ofthe materials used.

Carbon steels. Carbon steels are used for thetransport of treated gas, acidic gases and oil. For the lasttwo, specifications have been developed for theproperties of chemical reactivity, and also corrosionforecasting techniques for defining the extra thicknessof steel required for the predicted working life at thestated operational pressure (Palmer and King, 2004).

High resistance carbon steels. Steels of this type areobtained through strictly controlled thermo-mechanicalprocesses of lamination and cooling. The use of thesematerials is born of the need to reduce the investmentcosts associated with the choice of wall thickness of thepipeline, both for transmission at very high operationalpressures, and for very difficult installation conditions(pipelines on very deep seabeds).

Special steels. Such materials (stainless steel,duplex, super duplex, 13% chrome) are used for thetransport of corrosive products (Corrosion ResistantAlloys, CRAs) and in particularly severe operatingconditions. In some cases specific claddings have beendeveloped, along with associated manufacturing andwelding techniques, in which a thin layer of corrosionresistant steel is used in contact with the product beingtransported, to line a pipe in traditional steel, whosewalls are sized according to the mechanical resistanceneeded for the operating and environmentalconditions.

Flexible pipes. Flexible pipes are often used fortransporting particularly caustic fluids in smalldiameter pipes over short distances (Fig. 20). From astructural point of view, flexible pipes are made usinga concentric sequence of metal sheets andthermoplastic polymers applied from the insidetowards the outside of the pipe, according to thespecific use of the final product and the product



transported. The flexible pipes are then rolled ontoreels ready for installation. These types of pipes costsabout 5 or 6 times more than a normal steel pipe,although this cost is partially offset by reducedinstallation times and cost, as well as by greateroperational flexibility. The equipment generally usedfor installing flexible pipes is less sophisticated, andtherefore less costly than that used for installing atraditional pipeline. In some cases, speeds ofinstallation can be achieved (500 m/h) which areunimaginable in the case of rigid pipes, a pointcertainly not to be ignored when one considers that inthe majority of operational projects the costs related toinstallation of the pipeline are comparable to thoseincurred in the purchasing of the materials. To thisshould be added the possibility of recovering thepipeline for inspection, maintenance or even for re-usein the future. The design and installation of flexiblepipes are generally carried out differently from atraditional pipeline (Palmer and King, 2004), in thatthe manufacturer takes care of the detailed design ofthe pipe and is often even responsible for itsinstallation. Instead, in the case of steel pipes, thefuture operator generally takes care of the designphase and the manufacturer is rarely involved in theinstallation.

Analysis of pipeline useThe growing demand for hydrocarbons on the

part of industry and the general conviction that theuse of pipelines for transport is an economical and, atthe same time, strategic solution, have given impetusto new projects in which the environments traversedare increasingly difficult, often being extremelyvulnerable to any incidents that could result in theloss of the product into the environment. Thesescenarios are made all the more critical since theyoften feature complex solutions that require extremeconditions of usage of the materials. Added to this, isthe need to minimize the quantity of steel and thenumber of interventions and, therefore, theinvestment costs.

Market research indicates that there is a highincidence of both the transmission of gas at highpressure over long distances, and the crossing of waterbasins having depths that had never been reachedbefore (over 2,000 m). Industry is, however, makingprogress in developing on the one hand, thetechnologies and methodologies that will make itpossible to transmit gas at much higher pressures thanthose used at present (�100 bar), for example 200 barwhich is standard in seb sea transmission and, on theother hand, the technologies that enable the



outer protective sheath

rough bore flexible pipe

rough bore flexible riser

smooth bore flexible pipe

longitudinal stress retaining layers

hoop stressretaining layer

thermoplastic pipe liner

carcass



hoop stressretaining layer




hoop stress retaining layer

secondary thermoplastic liner


carcass

Fig. 20. Flexible pipes for the transmission of fluids at high pressure.

manufacture of high-quality pipelines with highthicknesses for installation on very deep seabeds, e.g.in the Black Sea (more than 2,000 m) or in the AtlanticOcean (up to 3,000 m).

Analysis of manufacturing processesIn recent years, because of the number of projects

in progress, but also because of the need for largediameter thick-walled pipelines with mechanical andgeometric properties defined on the basis of veryrestrictive specifications, attention has been turnedtowards what is attainable in terms of mechanicalproperties and geometric tolerances in relation to thedesign criteria and the sizing of the lines. In particulardiscussion has centred on: a) how much can beobtained in the light of a manufacturing process thatforesees (usually for diameters above 16-18 inches) orotherwise (usually for diameters below 16-18 inches)longitudinal welding (see below); b) the geometrictolerances specifiable and obtainable in the two cases;c) the uniformity of the mechanical properties insideboth an individual pipe, and the various pipesproduced; d) the necessity, in the two cases, ofhydraulic testing (see Section 7.1.6) in themanufacturing phase and the subsequent conditioningof the statistical distribution of the mechanicalproperties used in the design stage; e) the possibilityof using pipes that have been welded helicoidally intraditional operating conditions (reduction in materialcosts) or for special applications, e.g. the externalcasing of insulated pipes.

Particular attention has, moreover, been paid to theproblems associated with checking of the chemicalcomposition and the production of blooms/billets toobtain products with particular performancecharacteristics (for example, to reduce the fall inmechanical resistance at high temperatures, somethingthat is particularly critical in later constructionprocesses, which can generally produce a fall inresistance in temperatures below 100°C). Significantemphasis has also been placed on the study of howmuch the construction process can affect themechanical resistance of the pipe in specificconditions. A typical example, in the case of seb seapipelines at great depths, is found in UOE technology(U press, O press and cold Expansion), normally usedto produce large diameter (20-30 inches) and highthickness (30-40 mm) pipes, where there is noprovision for heat treatment after the expansion phase.In particular, the levels of deformation of thecircumference reached during the expansion phase(circa 1-2%) cause a loss in circumferential pressureresistance, because of the Bauschinger effect, which issufficient to jeopardize its resistance to collapse underexternal pressure.

Analysis of types of breakage attributable to the material

The attention given by society to the consequencesof industrial accidents has generated the need toanalyse the principal causes that can lead to the failureof a pipeline and, in particular, what level of safety canbe attained by an appropriate and specific choice ofmaterials. In particular, it has been possible to verifythe extent to which accident statistics have beenaffected by events resulting from:• External interaction with mechanical equipment

working in the vicinity of the pipeline (excavatorsin the case of onshore pipelines, and fishingequipment in the case of sub sea pipelines).

• Internal and external corrosion (for example, fromcathodic protection, damage to the passiveprotection and the usage factor, or where there issynergy between the external environment,material and usage factor).

• Material defects or welding defects (in fact suchevents have become very rare following the use ofnew materials, or rather the use of modern testingtechniques that do not destroy the welds).

• Extreme environmental conditions such as storms,in the case of sub sea pipelines and, in the case ofonshore pipelines, earthquakes or rainfallconditions severe enough to cause hydrogeologicalinstability and as a consequence, landslides andsubsidence.Consequently, because of the interaction of the

pipeline with environments that are increasinglysubjected to human activity, and therefore topotential causes of external interference, particularattention has been given to the study of the modes offailure relating to superficial defects, that is dentswith or without the presence of defects, with a viewto defining those specifications of resistance andductility of the material that allow it to handle suchsituations successfully. The propagation ofductile/fragile breakage has been investigatedthoroughly, in particular for those applicationswhere loss of the product and bursting of thepipeline have a major impact on the externalenvironment concerned, because they interrupt thesupply of the product, which is indispensable to thecommunity.

Analysis of the mechanical properties of the chosen material

At present, the industry is proposing design criteriabased on a limit states approach (see Section 7.1.5) asan alternative to the traditional sizing based onallowable stress. This is highlighted in recent standards(Germanischer Lloyd, 1995, and DNV, 2000, for subsea pipelines and CSA, 2002, for onshore pipelines).



In this approach, an attempt is made to correlate thesizing of the pipeline with the actual ways that it mightbreak, the whole expressed in a ratio that links theeffects of the load with the pipeline’s resistance to thefailure mode being analysed (this ratio is the so-calledfunction of limit state), and to associate the effects ofthe load and the resistance to partial safety factors,that enable a predetermined level of safety, associatedwith the consequences of exceeding that limit state, tobe respected. In this analysis it is very important to beable to establish, with a degree of certainty, how muchthe material is capable of resisting within a specifiedfield of variation and in relation to the manufacturingtechnology, but also how much the manufacturingtechnology is able to guarantee in terms of geometrictolerances.

By way of example, the DNV (2000) standards forsub sea pipelines propose inside pressure related tocircumferential usage factors which vary according tothe availability of information on the mechanicalcharacteristics of the chosen material. It is also worthyof note that developments in recent years have beenprincipally concerned with sub sea pipelines, for whichtransmission at high pressure and great thickness ofsteel are often implicit; a refining of the projectapproach could, therefore, have significant financialimplications. Consequently, the limit states investigatedhave been those typical of sub sea applications and theeffects of the loads imposed have been analysed in thecontext of high thicknesses and the submarineenvironment. Extension to onland pipelines must surelyinvolve specific studies, in which their typicalthicknesses and the environments they traverse canestablish totally new situations compared with thosefaced previously. In particular the use of new materialsand of high resistance steel brings with it fairlyimportant implications in the development of a limitstates approach, in relation to the difference of thecurve that links stress to deformation for extreme usesand therefore to its substantial involvement in the wayof initiation of the failure. For example, in the field ofsteels for traditional usages or even in the field of highresistance steels, attention is focused on the ratiobetween yield stress and breakage stress, so that, atpresent, some project guidelines limit the applicationof the proposed criteria to materials for which suchratios do not exceed certain values (0.85 in the Dutchand European standards, 0.80 in the Germanstandards).

Analysis of problems related to tests The above leads to a reconsideration of the choice

and the specification of the material in relation to themanufacturing process, and of the design and sizingon the basis of criteria that permit an adequate safety

level to be guaranteed and, finally, of the constructionand the subsequent tests carried out to guaranteeacceptable product quality. On the other hand, facedwith the need to reduce the costs imposed by testing,the effectiveness of which is often not demonstrable,the industry is fighting, from one side, the problem ofbetter qualification of resistance of the materials andproducts and, from the other, ways of testing theproducts traditionally carried out on all pipelines. Inparticular, it is trying to quantify the influence thattesting has on design and particularly on the processesused to qualify the structural integrity of a pipeline. Insummary, testing permits the statistical distribution ofthe characteristic resistance to be restricted, insofar asthe passing of the test would allow it to be said that,up to the level provided for in the test, the pipelinehas met the specifications. But it is also evident thatthe introduction of new materials focuses a certaindegree of attention on this theme, which goes beyondwhat might have been thought in the light ofexperience gained on traditional materials andapplications.

Traditional materials

In the majority of the countries of the world, thespecifications of materials for construction ofpipelines for the transport of hydrocarbons refer to theAPI 5L standard (American Petroleum Institute;Palmer and King, 2004). In its original formulation,the API 5L standard identified the grade of a steel, i.e.its mechanical resistance by means of its yieldstrength, for example X52 or X60, where the numberidentified the yield strength of the material expressedin thousands of pounds per square inch (kpsi). Thismeans that the symbol X52 identifies a material with ayield strength of 52,000 pounds per square inch, equalto 358 MPa. The most recent up-date of the API 5Lstandard conforms to the ISO standard adopting themeasurement units of the International System, eventhough in reality it is still common practice to use theold denomination. Although the first version of theAPI 5L goes back to 1920, that document was adoptedas a reference specification only from 1948 onwards.At that time, the highest grade material included in thespecification was an X42, while today the standardincludes steels up to grade X80. In 1999 the API 5Lwas converted to the international specification ISO3183 that deals with the choice and use of materialsfor pipeline construction with and without longitudinalwelding and spiral tubes.

Evolution of the API 5L over time towards everhigher grades of material has gone hand-in-hand withthe technological development of metallurgical andthermo-mechanical processes aimed at improving the



performance of the materials. At the beginning of thesecond half of the twentieth century, pipes for oil andgas transport were produced with the UOE and SAW(Submerged Arc Weld) processes starting with lowalloyed steel sheets, which were hot laminated andthen subjected to a thermic normalization treatment.The resulting microstructure consisted of polygonalferrite and perlite, often arranged in bands. Themechanical characteristics could reach the grade APIX60 and the low temperature tenacity was not alwayssatisfactory. In time, the growing demands of themarket generated the need to investigate the possibilityof increasing the mechanical resistance withoutincreasing the content of alloying elements, in order toavoid the negative effects on tenacity, weldability andon costs in general. Also taking into consideration thespecification of the products to be transported, to theserequirements was added the need to guarantee aneffective resistance to attack by hydrogen (hydrogendamage). A certain number of hydrogen atoms can, infact, be present in the fluid inside the pipes (acidambient), but also result from chemical reactions inthe pipeline’s external environment or from errors indesign of the pipeline’s cathodic protection of buriedor sub sea pipelines (cathodic overprotection). Thesehydrogen atoms change into molecular hydrogenwithin the micropores of the material, particularly inthe presence of manganese sulphide. Thetransformation into molecular hydrogen inside theintergranular spaces generates high interstitialoverpressures that lead to cracking of the material tosuch an extent as to jeopardise its mechanicalperformance (Nicodemi, 1986).

It is therefore easy to see how, faced with all theserequirements, it has become necessary to lay down atechnological process that combines a precisechemical composition with a specific laminationprocess. The main objective was to produce steels oflow carbon content with a mixed ferritic-bainiticmicrostructure, or even completely bainitic, that wouldallow the problem of hydrogen damage to beovercome. In the latter case it was imperative to reducethe sulphur content to very low levels, to eliminatepossible reactions caused by the formation ofmanganese sulphide, which is extremely deformable,to control the form of the inclusions through treatmentwith calcium (CAB), zirconium or rare earths and toreduce the level of carbon and of manganese.

Later on, the introduction of the process of thermo-mechanical lamination and accelerated cooling(TMCP, Thermo-Mechanical Control Process)facilitated the production of ferritic-bainitic steels,without the need for substantial additions of alloyingelements, with beneficial effects both on costs andweldability. At the end of the production cycle a mixed

ferritic-bainitic, or totally bainitic microstructure isobtained with variable micro-structural characteristics(dislocation bands within the ferrite grains,substructures, precipitates) according to the chemicalcomposition and the TMCP parameters adopted. Theeffectiveness of the TMCP process depends on thecareful choice of the micro-alloying elements inrelation to the variables related to the laminationprocess and on the significant reduction of the centralliquation process. Since steels produced with theTMCP process require an alloying elements contentless than, or at the most equal to, that of firstgeneration thermo-mechanically laminated steels,particularly as far as carbon content is concerned, theyhave the same carbon values (CE�0.42%) whichguarantee good weldability and acceptable values ofhardness in the zones affected by heat (HAZs, HeatAffected Zones) in the circumferential welds.

The development of steels with high mechanicalcharacteristics (API grades X70, X80, X100) for theconstruction of pipelines has, therefore, undergone anotable acceleration since the 1970s with theintroduction of the controlled lamination process andof accelerated in line cooling. Numerous manifacturershave carried out intense research activity on theformulation of a suitable chemical composition that,combined with the definition of the variables in thecontrolled lamination process, has made the productionof pipes in low carbon content steel (�0.07-0.09%)possible, with three fundamental requisites: highmechanical characteristics, fundamental for copingwith the high levels of stress imposed, above all, in thecase of pipelines for transmission at very highpressures; excellent low temperature tenacity, anindispensable property for controlling the propagationof fragile fractures; and good weldability in the field.Today, pipes in API X70 grade steel are an establishedproduct for which qualification and installation criteriaare available: in European countries there are manyinstallations in this type of steel. The production of APIX80 grade pipe steel has undergone a significantacceleration since 1980. In that connection there are anumber of publications that contain the resultsachieved in the refinement of experimental products(Dillinger Huttenwerke SG, Hoesch, Sumitomo MetalIndustries, Nippon Steel Corporation, Stelco, ILVAILP) and in the construction of the first transportpipelines with this material.

The requirements for high resistance steels arespecified in several existing standards up to gradeX80. At present there are no specifications relative tograde API X100 and, therefore, the requirements forpipeline material in this type of steel have to be agreedwith the manufacturer in the product definition phase.Even though pipes in API X100 grade steel are not yet



available commercially, many internationalmanufacturers (Nippon Steel, NKK, Kawasaki,Europipe) have produced experimental pipelineproducts, with characteristics that seem to satisfy therequirements for high pressure natural gastransmission. The production of pipes in API X100grade steel is an evolution from the API X80 grade:the mechanical resistance characteristics of theAPI X100 grade are obtained starting from thechemical composition of grade X80 with targetedadditives of micro alloying elements (manganese,niobium, titanium) and by a process of controlledlamination and accelerated in line cooling to theutmost limit of the most modern laminating plants’capabilities. The velocity of accelerated coolingemployed in the production of API X100 grade plate isgreater than 20-25°C/s, compared with the 15°C/scharacteristic of the production of API X80 and withthe 5-10°C/s typical of medium-low grades. The pipesproduced on an experimental scale displaycharacteristics in line with grade API X100, measuredon cylindrical test-pieces because of the highBauschinger effect. The tenacity values of the basematerial appear satisfactory, both from the point ofview of the CharpyV energy (200-300 J) and from thatof the ductile/fragile transition. It is held that aCharpyV energy value of 300 J constitutes an upperlimit for thermo-mechanically laminated steels. On thebasis of the experimental production of pipes inAPI X100 grade steel it can be assumed that these canbe produced with a chemical composition that satisfiesthe 0.45% maximum equivalent carbon requirement.

The process of lamination adopted (very lowtemperature at the end of lamination and at the end of cooling) ensures that the pipes in API X100 gradesteel display very high values of the yield/failureratio (>0.90).

The welds

The construction of gas pipelines and oil pipelinestakes place through the welding together of individualpipes, about 12 m long, that go to make up thepipeline, carried out by welding the pipes one after theother and advancing progressively along the routedesignated in the design phase (Fig. 21). Today’stechnology makes many welding techniques available,so it is necessary to make a choice to select the mostsuitable: the parameters that guide such a choice aregenerally the diameter and the thickness of the pipe,but also the characteristics of the place chosen for thelaydown and the working conditions. The speed ofprogress during the laydown is governed by thecircumferential welding between one pipe and the next; consequently, the greater the timeneeded to do this welding the longer will be the timeneeded to complete the pipeline. For these reasonstechnological research tends to concentrate on thedevelopment of new methods of welding that helpspeed up the installation and reduce the overallcosts of completing the projects.

Classification of weldingsWelding is used to join the edges of separate bodies

that, at the end the process, become integral parts of asingle structure. There are many welding techniques,but the ones most often used in the pipeline field areforms of arc welding. The common characteristic of thistype of technique is that the two edges are joinedtogether by fusing the material of which they are madeup, through heating up to an adequate temperature. Theheat needed for this purpose is generated by making anelectrical arc jump between the base material of the twoedges and an electrode. Depending on the weldingtechnique employed, the electrode can act as a filling



Fig. 21. Coupling of two pipes by means of an internal mandrel before the welding phase (courtesy of R. Bruschi).

material, since by melting it is mixed in the moltenpool, otherwise the filler is introduced separately in theform of wires. At times, however, the process can besuch as not to need the addition of filler material intothe pool. Another fundamental aspect of the weldingprocess is the need to protect the molten material fromthe gases present in the air, e.g. oxygen and nitrogen,which are harmful to the mechanical characteristics ofthe join.

In welding technology, weldings are oftenclassified on the basis of acronyms. The mostfrequently used acronyms are listed below.

SMAW (Shielded Metal Arc Welding). Arc weldingwith coated electrodes was among the first to beemployed in the field of welding and it is carried outmanually. It provides for the use of electrodes in theform of metallic rods coated with a cellulose-basedmaterial. During the welding process, thedecomposition of the electrode on the one handgenerates a large amount of gas that protects theaffected area, while on the other hand it also causesthe melting of the metallic material of the rod whichends up in the molten pool.

SAW (Submerged Arc Welding). Submerged arcwelding is generally used for the longitudinal weldingof pipes that are created by bending a sheet of steeluntil it takes on a circular form, bringing together itstwo edges. In this case, the uncoated electrode alsobecomes the filler material; the protection of the arcand the molten pool is entrusted to a blanket ofgranular material that covers the join, separating itfrom the air. This procedure is completely automated.

GMAW (Gas Metal Arc Welding). In continuous wirearc welding in a protective atmosphere, the arc jumpsbetween a metallic wire wrapped around a spool and thebase material. The protection of the molten pool isentrusted to a gas mixture introduced externally whichcontinuously fills the affected zone. The sequence of thedifferent operations has been embodied in a singleappliance called a torch, which gives the process greatflexibility of usage without the need for repeatedinterruptions to renew the filler material.

GTAW (Gas Tungsten Arc Welding). In weldingwith a tungsten electrode, the heat needed to bring thematerial to fusion is produced by a tungsten electrodewhich, because of its own high melting temperature, isnot consumed during the welding process. Theprotection of the molten pool is entrusted to a gasmixture, while the filler material can be introducedexternally.

The welding processThe pipes used in the construction of pipelines

have thicknesses that seldom allow the welding to becompleted in a single run; consequently it is necessary

to make a series of runs until a complete join along itswhole length is obtained. Before welding, the two endsof the pipes to be joined must be prepared. Thisoperation requires the cleaning up of all impurities orresidues of any previous work and a subsequentinspection to check for the presence of any defects. Atthis point, the two ends undergo a mechanicaloperation called chamfering, which consists ofremoving material around the whole circumference ofthe pipe until the thickness takes on the right shape.Note that the angle of inclination of the chamferdepends on the type of process used, and in particularon the thickness of the electrode used during thewelding. In fact, for the fusion to reach right to thebase of the chamfer, for manual welding carried outwith coated electrodes, an angle of about 30° isadopted, while the angle is reduced down to 20° oreven 10° approximately if the welding is of semi-automatic or automatic type, in which the electrode isa wire of only a few millimetres thickness. This geometry is ideal for guaranteeing the completepenetration of the molten pool as far as the internalsurface of the pipes, whereas, the greater the angle of opening of the chamfer, the greater thequantity of material that needs to be deposited, with corresponding consequences in terms of time and costs.

After chamfering, the two pipes have to be placedclose to each other and aligned. This operation iscarried out with the help of coupling tools which,depending on the dimensions of the pipes, can beinternal or external. Typical values of maximumdeviation allowed in the alignment of the pipes cannotbe more than one millimetre.

Before being welded, the two edges are preheated upso as to prevent the first seam laid down from coolingoff too quickly, otherwise it could be damaged. Thepreheating, from this point of view, leaves a greater timespan for the operator to carry out the subsequentwelding runs. Finally, the preheating dries out the twoedges thereby preventing the formation of hydrogenduring the welding which, as already mentioned, cancause cold fracturing of the join. The completion of thefirst welding run is the most critical phase of theprocess. Conventionally it is carried out starting at thetop of the pipe and finishing at the bottom, with twowelders working together. In the case of semi-automaticwelding on large diameter pipes, three or even fourtorches can be used working simultaneously around thecircumference, thus drastically reducing the timerequired for completing the join. The second weldingrun has to be carried out as soon as possible, becauseevery uncontrolled movement, however small, couldstress the first weld just completed and cause it tobreak. The process ends by carrying out the filling



welds that completely cover the space that separated theends of the two pipes. To do this, the torch must bemoved in an oscillating manner, moving it from oneside of the join to the other, at the same time cleaningaway the slag from the welds as the work proceeds. Thefinal welding run forms a cap on the weld, which isthick enough to rise above the external surface of thepipe by some millimetres.

Metallurgy of the weldDuring welding, the electric arc causes the base

metal to melt, in this way joining the two ends to beunited. In this zone, the temperature is very high(about 1,600°C), while the adjacent part of the pipe isat a much lower temperature. This very high variationof temperature causes the formation of severalmetallurgical components in a limited zone close tothe weld: the HAZ. In particular, in those points wherethe heat has been sufficiently high to allow the basemetal to crystallize, the grains of the metallic matrixundergo a change, forming finer grains, whichimproves the mechanical characteristics of thematerial; in contrast, where the heat has not been sohigh, the grains of the metallic matrix increase in size,causing a worsening of the mechanical characteristicsof the material. This zone proves to be particularlysensitive to corrosion and characterized by poortenacity. Its dimensions depend on the thickness of thepipe, on the preheating carried out, and on the quantityof material deposited during the welding, in proportionto the amount of heat that develops during the process(Lancaster, 1993).

Another problem related to the welding isassociated with the porosity that is created if themolecules of gas dissolved within the weld justcompleted remain entrapped there. In fact, while themetal is still in a molten state, the gases can dissolvein the liquid phase but also escape from it, whereaswhen it solidifies it is unlikely that these gases will beable to get out from the solid phase. This problemmust not be underestimated, because about one half ofthe failure of weldings is connected with the effects ofporosity. Hydrogen is certainly the most dangerous gasfrom this point of view, because its high degree ofsolubility in the molten pool can allow it to penetrateup to the HAZ and induce stresses in a zone madefragile by its thermically altered microstructure.Finally, particular attention must be given to thewelding of steels with a high sulphur content. Thesemetals, in fact, can give rise to the production ofsulphides which, at the end of the welding, are the lastto solidify, remaining at the centre of the grains of themetal. These sulphur-based compounds are muchweaker than the base metal and therefore contribute, toa reduction in the mechanical resistance of the join.

Definition of weldabilityThe term weldability indicates the ease with which

the metal can be welded in such a way as to satisfy thequality standards expected of the welding. Low carboncontent steels, for example, have a good weldability,while stainless steels have a significantly lowerweldability. The carbon content of steels is one of theparameters typically used to classify this characteristic; italso depends, however, on the micro-alloying elementscontained which are to be found dispersed in themetallic matrix. Consequently, to unequivocally definethe metal under examination, recourse is made to afictitious quantity of carbon called Carbon Equivalent(CE). The formulae used to calculate the CE are allbased on the same principle: to the percentage ofcarbon contained are added the quantities of the otheralloy components on the basis of different weightsdepending on the type of component. The higher theCE value, the less weldable the material.

Inspection techniques for weldsAll the welds in high pressure pipelines are

subjected to several types of inspection, with a view toensuring that the results obtained in the weldingprocess satisfy the standards required. The mostcommon type of inspection technique is radiography.Some portable instruments have been developed suchthat, once the welding is finished, they are installed onthe pipe and make X-ray photographs of the weldings:any defects present on it are easily identifiable in theimages recorded on film. This technique is veryspeedy and enables good results to be obtained interms of reliable identification of defects; against this,it requires large amounts of energy for its operationand the operators have to be protected carefully fromthe source of the radiation.

Another technique, the use of which in recent yearshas seen an overall increase, is ultrasonic examination.This is based on the propagation through the thicknessof the pipe of an ultrasonic wave, which is reflectedwhen it encounters a sharp variation in density on itspath such as, for example, the internal surface of thepipe or a defect. The advantage of this technique liesin the fact that it provides information on the threedimensions of the defect, defining both its completegeometry and its orientation within the weld.Radiography, instead, provides only a two-dimensionalimage of the defect, but it is definitely more accuratethan the ultrasonic technique. For revealing superficialdefects, the inspection technique using ferromagneticpowders is very effective. These powders, mixed in aliquid, are spread over the external surface of the weld,and then immersed into a magnetic field. In normalconditions, the particles tend to position themselvesalong the lines of flow of the magnetic field, while any



irregularities present on the surface, in other words,the defects, create an irregularity in the magnetic field,which is made visible by the orientation assumed bythe iron particles.

Another technique used both on magneticmaterials and on non-magnetic, is one which usespenetrating fluids. These fluids are generallyfluorescent, so as to be easily seen under ultravioletlight. They are spread over the surface of the weld andallowed to penetrate by capillarity into any defectspresent. The surface is then cleaned and sprinkled withan absorbent powder that takes up the liquid that haspenetrated into the defects, making their positionvisible. This technique is easy to apply and usablewherever the inspection takes place; it is definitelyeconomical, but takes a rather long time.

Once the presence of the defects has beenestablished, it can be decided whether to leave them orto remove them by repairing the weld. There are twotypes of criteria adopted for this purpose: the firstdefine the limits of acceptability of the degree of thedefects, on the basis of what can be expected to befound in a weld correctly performed by a good welder.These criteria are purely empirical and are identifiedby the acronym WMS (WorkManship Standard). Thesecond type of acceptability criteria is based on thefracture mechanics, which enables it to be establishedwhether or not a structure containing a defect can stillbe safety used. Every repair made to a weld takes along time to carry out and consequently imposes highcosts, but most importantly, it is still not possible toguarantee that the result of the repair will be betterthan the initial defective weld.

7.1.5 Mechanical resistance

A pipeline is made up of various components: a) asteel pipe capable of withstanding all the mechanicalstresses to which the pipeline is subjected; b) an anti-corrosion system usually consisting of a covering,made of asphalt, polyethylene, polypropylene or epoxyresin (passive protection), and of sacrificial anodes ofzinc or aluminium (cathodic protection); c) an internallining, generally of epoxy resin, which has the task ofreducing the friction between the fluid beingtransmitted and the steel walls; d) an external coveringof cement (reinforced by a steel mesh) which gives thepipeline the weight needed for its stability in the layingbed, as well as being a mechanical protection againstexternal interference.

The primary objective of the designer is to definethe diameter of the pipe necessary to transmit a givenflow (or quantity) of product (principally oil or gas ora mix of the two) from one location to another in a

certain unit of time. During this phase, in consideringvarious engineering parameters, such as losses of loadalong the pipeline, the inside pressure necessary forthe transport is also defined. Once the diameter andpressure have been defined, it is necessary to decideon the type of material to use, and this will dependprimarily on the fluid to be transported (gas or liquid,corrosive or not, etc.). It is therefore necessary todefine the mechanical resistance required of the pipein relation to the loads to be applied. Pipeline for thetransport of petroleum products, such as oil andnatural gas, must be strong enough to withstand thestresses resulting from the loads which will be appliedto it both during the construction phase and during itsworking life (Bruschi et al., 1982a).

During construction, depending on the methodadopted, the pipe will be subjected to bending, axial andtorsional loads (the latter is generally negligiblecompared with the first two). These loads are relevantfor both onshore and offshore pipes. In the case of subsea pipelines another relevant construction load ishydrostatic pressure generated by the column of waterdetermined by the chosen site: based on the depth of themarine site the external pressure can reach very highvalues (Torselletti et al., 2003b). A major differencebetween onshore and sub sea pipelines is that theexternal loads applied during the construction phase aregenerally much greater for sub sea pipelines. On theother hand, when onshore pipeline has to cross terrainwhich is particularly undulating, in order to adapt thepipe to the ground profile, bended sections of pipes areconstructed in the field by applying a high level of coldplastic bending strain (Bruschi et al., 1995).

During the operating phase, the pipe is subjected toloads which result from the action caused by internalpressure and axial forces caused by a constrantingthermal expansion. These are both linked totransmission of the internal fluid (gas or oil). Furtherdesign criteria, which take account of the other loads onthe pipeline such as its own weight, thermal loads,traffic, ground movement, wind, waves, externalinterference, etc., generally have little influence on thechoice of wall-thickness, but define the measures takento deal with these load conditions, which are generallyconsidered secondary, other than in exceptional cases.Among such cases, it is worth mentioning the testswhich are peculiar to sub sea pipelines, where theexternal pressure or the installation conditions canchange a sizing of the steel wall which had been basedexclusively on containing internal pressure.

Sizing on internal pressureThe thickness of a pipeline’s steel wall is the most

important factor regarding the pipeline’s capacity towithstand the loads imposed by installation and the



operating conditions. Given that the wall’s thicknesshas a considerable impact on project costs, it is veryimportant to use optimized design criteria whichbalance the requirements of both safety and costs(Bruschi et al., 1997). The traditional design criteria,codified in all the industrial regulations and practices,are generally based on the capacity of the pipelinerequired to contain the internal pressure underoperating conditions. In considering the internalpressure load, the stress it induces has to be less thanthe material’s characteristic resistance reduced by autilization factor, often generically called the safetyfactor. Obviously this utilization factor is intended tocover the uncertainties of both the applied loadingsand the resistance of the pipe. The formulae proposedin the regulations for calculating the stress at thecircumference, sh, acting in a pipeline with an internalpressure of Dp, are derived from Mariotte’s formulafor pipes with thin walls (i.e. pipes with adiameter/thickness ratio, D/t, greater than 10):

Dp�Dsapplied �sh�

1112

2t

The values attributed to the diameter D and to thethickness t to be used in the calculation change fromregulation to regulation (Fig. 22). In the case ofthickpipes D/t�10, more sophisticated formulae needto be used.

The applied stress must be within the so-calledallowable stress. The allowable stress is defined inrelation to the possible types of failure ormalfunctioning to be avoided. The ‘limit states’ methodis derived from the analysis of failures which have takenplace in existing pipelines, or which can occur indifferent project scenarios and under different operatingconditions. A ‘limit state’ constitutes the limit betweenan acceptable condition and an unacceptable condition,expressed by a functional link between the parametersof the pipeline’s resistance and the loading effects, foreach mode of failure. In this way the resistance capacityis characterized through its resistance in the face of eacheffective failure mechanism (Bruschi et al., 1997). Thetype of failure or malfunction can be classified inaccordance with two categories.

Serviceability Limit State (SLS). ServiceabilityLimit States denote the inability to carry out therequired function; as such they do not involve the lossof the transmitted product and generally require lesssevere safety coefficients. Reaching the yield point,excessive ovalization of a section of pipe, and Eulerianinstability (provided that it does not burst, collapse, orcause a breakage, which would classify it as a ULS)are typical of an SLS.

Ultimate Limit State (ULS). Ultimate Limit Statesdenote a break in the wall and consequent loss of the

product; for this reason they generally require a highersafety factor. ULS relate to the pipeline’s ability towithstand both the loads created by containing thepressure, and secondary loads (breakage, localcollapse, fatigue).

The stress at the circumference generated by theinside pressure, as the pressure increases, reaches theso-called limit of elasticity (yield stress). If this limit isexceeded, residual plastic deformations develop. Theseplastic deformations are such that they increase thediameter of the pipeline and consequently reduce thethickness of the walls, as well as increasing the appliedstress. As the internal pressure continues to increase,the wall-thickness will be reduced to a level at whichits resistance capacities are no longer strong enough,leading to the bursting of the pipe itself. The allowablestress is linked to the resistance characteristics of thematerial through the safety coefficients defined on thebasis of a precise safety objective.

Safety criteriaIn rationalizing the project criteria a quantitative

definition of the safety level to be pursued is required.To define this, the concepts of structural reliability andprobability of failure have been introduced. Therequired safety level is defined as the maximumannual probability that the effects of the load willexceed the resistance capacity of the pipeline, inrelation to the mode of failure generated in thespecific scenario, and by the loads in action. Thecertifying bodies, having to define the safetyobjectives in non-traditional circumstances, usuallycarry out a series of preliminary activities. The aim ofthese activities is to analyse what has already beendefined in comparable circumstances, and todetermine the safety level implicit in the generallyrecognized regulations, giving due consideration to thelevels of uncertainty relating to the period in which the



t

D

sh

Dp

sh

Dp

Fig. 22. Illustrative diagram of the tensions developedin a pipe subjected to internal pressure.

regulation was issued. The final definition of theannual probability of failure usually aims to ensure anacceptable level of risk taking into account both thetype of failure and the consequences, considered interms of risk to personal health and safety, damage tothe environment and economic loss. The risk is derivedfrom the product of the probability that an event willoccur and its consequences. Therefore, once anacceptable level of risk is known, it is possible toidentify the required level of safety based on theconsequences associated with a potential accident.

The parameters generally used to evaluate theconsequences of a failure are: a) the phase in the life ofthe pipeline (the construction phase, temporary oroperational); b) the type of fluid being transported; c) the characteristics of the zone being traversed; d) thetype of failure. Other, generally negligible, parameterswhich can have an impact on the consequences of afailure, are the diameter and the pressure of thepipeline under consideration. This is a problem whichbecomes relevant when there is the possibility ofcarrying out high pressure gas transmission through anonshore pipeline. This application can be of strategicimportance in a situation where production is remotefrom the users. The type of fluid transported isclassified on the basis of how dangerous it is; the zoneis generally classified according to the populationdensity; both, through a matrix chart, enable thedefinition of the grades of safety which can beassociated with a given transported fluid and the zonetraversed. A maximum allowed probability of failure(exceeding of the limit state) is associated with eachgrade of safety (ISO 2004). It is generally believed thatan annual probability of 10�4 per km is acceptable forstretches of pipeline in operational conditions throughremote areas. This value can fall by several orders ofmagnitude when the potential consequences to persons,property and the environment, associated with a givenmode of failure, rise significantly, for example whenthe population density of the zone traversed increasesand/or the type of gas transported is particularlydangerous. In general, an acceptable range for themaximum annual probability of failure per kilometre is10�3 to 10�7, according to the population levels of thezone traversed and the type of fluid being transported(gas, oil, water, etc.). The most suitable safety valueand the related documentation have been a subject ofheated debate in many countries. For some years inEurope new legislation has been in the process ofdevelopment, which requires documentation that apipeline has been adequately designed for its purposefor the whole of its operating life, and that theconsequences of potential failures, in terms of threat tohuman life, to the environment and to property havebeen reduced to the lowest reasonably practicable

value. Leaving aside at this point any elaboration onwhat is meant by ‘lowest reasonably practicable’, it isimportant to underline the extent to which this hasforced, first those companies operating in the NorthSea, and now also those operating in the MediterraneanSea, to update and revise their approach to safety. Withthis regulation it has now become the responsibility ofthe company operating the pipeline to define thepracticable level of risk, and to document how this riskhas been reduced to the minimum practicable. Eventhough it is possible to refer to the planning practicescodified in the regulations regarding well-knownaspects, the level of reliability implicit in theregulations traditionally is not quantified. To defineand document a probability of failure which isconsistent and uniform for all modes of failure has,therefore, become a primary objective. A dueconsideration regards the difficulty of quantifying andrationalizing the phenomena of failure linked to humanactivities such as excavations and onshore well-drilling,or the anchoring of ships in areas of the sea lacking thefacilities. Even though these activities can be includedamong the loads that can cause pipeline to fail, it seemsmuch more appropriate and rational, where analysis ofthe frequency of occurrence demands it, to identifyprotective measures and/or warning and surveillanceprocedures which prevent such activities taking placein the vicinity of a pipeline (ISO 1999).

To complete the process of establishing thethickness of the pipeline necessary to contain theinternal pressure, the limit state method allows, forexample, the definition of a project format whichguarantees a safety objective with respect to bothbursting and yielding. This is:

ga�sapplied�sallowable�min{hg�sy; hu�su}

where hg is the partial safety factor associated with theSLS relating to exceeding the yield tension, hu is thepartial safety factor associated with the ULS relating toexceeding the ultimate tension, ga is the partial safetyfactor associated with the intrinsic uncertainty in thecalculation of the applied tension and the parameterswhich are involved in the formulation used. Thematerial’s yield tension, sy, is generally defined as thetension at which the structure shows no significantresidual deformations, i.e. those present once thestructure itself has been emptied. The material’sultimate breaking tension, su, represents the value of thestress beyond which there is no longer any capacity ofresistance (in the case of containing internal pressure,exceeding it will cause the pipeline to burst).

The behaviour of the material just described is alsohighlighted in the preceding planning equation. Infact, the closer that its ultimate breaking tension, su, isto the yield tension, sy the closer the material will be



to bursting: the closer the ratio of the yield tension tothe breakage tension (indicated as Y/T) is to one, themore likely the pipeline is to burst. The precedingplanning equation can be simplified by reducing thesafety coefficients down to just one:

sapplied�sallowable�h�s

This format is the most common among theregulations in force internationally (Table 1). In thisregard, it is possible to see how, from the point of viewof current regulations, different interpretations of thesame structural problem (that of a pipe’s containment ofinternal pressure) have led to different project criteria,which also have widely differing results. If we consider,for example, the sizing criteria proposed in some

international regulations, and reported in Table 1, it canbe seen that, among those most commonly used todayin pipeline engineering, the same concept of containingthe tension at the circumference is expressed in differentways. In fact, in the Mariotte’s formula reported above,each regulation assigns a precise and specific meaningto the various terms it contains. These differences canresult in variations of as much as 10% in the wall-thickness necessary for a given internal pressure and agiven nominal diameter. The maximum operatingpressure is defined directly by the ASME B31regulation of 1958 as the pressure at which thecorresponding tension at the circumference is notgreater than 72% of the material’s yield tension. In otherwords it defines a safety coefficient, h, equal to 0.72.The safety coefficient has been revised during recentyears for the different regulations, and is currently ashigh as 0.87 under operating conditions.

Onshore pipeline feature relatively low internalpressures (in Italy and Europe the maximum pressuregenerally used is around 70-90 bar, that is 7-9 MPa)when compared with sub sea pipelines where, foroperational reasons, the pressures used fluctuatebetween 200 and 300 bar (20-30 MPa). Consideringthat the safety coefficients used are similar, the onshorepipes have relatively low thicknesses (they vary from 12 mm to 20 mm depending on the diameter) whencompared with undersea pipework, in which thethicknesses vary between 15 mm and 35 mm dependingon the internal pressure and the diameter.

It should be remembered that, in defining thecriteria for the project and hence for the safetycoefficients, a certain margin, generally around 10% ofthe thickness, is allowed to cover the possibility of localdefects (such as defects caused by corrosion, jags or



Fig. 23. Effect of the longitudinal propagation of a fracture following a local burst (courtesy of R. Bruschi).

Table 1. Regulations for design of sub sea or overland pipelines

Regulation Country Format

DNV OS-F101 Norway limit state

ASME B31.4 USA allowable tensionand B31.8

BS 8010 United Kingdom allowable tension

NEN 3650 Holland allowable tensionlimit state

C.S.a.R. 2.06.05.85 Former USSR allowable tension

CSA Z662 Canada allowable tensionlimit state

ISO Europe allowable tensionlimit state

DM 24-11-84 Italy allowable tensionof the Ministryof the Interior



ovalization) which could reduce the pipe’s resistancecapacity. Onshore pipelines for the transport of oilfeature lower pressures and higher temperatures thanthose for gas. In fact, in order to prevent thecondensation of waxes which could cause blockages,pipelines that transport oil operate at relatively hightemperatures (even exceeding 100°C). This means that,in defining the thickness of the pipe, account must betaken of the reduction in the material’s resistancecapacity caused by high temperatures. In offshorepipelines for gas transmission, the high pressuresneeded to effect the transmission are often accompaniedby high temperatures resulting from the process ofcompressing the gas itself. In onshore pipelines for gastransmission, a very dangerous type of failure consistsin a burst caused by an accidental event, which is thenfollowed by the propagation of a split along the lengthof the pipe. This can have extreme consequences bothfor human life and the environment, and also in terms ofcosts (Fig. 23). This effect does not occur in pipelines foroil transport because of the practically instant drop ininternal pressure in the event of a breakage.

Secondary effectsIn many situations, along with the internal pressure

there are other loads involved, generally calledsecondary, which can have significant effects (Bruschiet al., 1982a). These are: bending actions due todifferential subsidence or where there are ondulations inthe ground’s profile where the pipeline is lying; axialactions, generally due to an obstructed thermalexpansion or to ground sliding (in some cases thesemovements can cause instances of Eulerian instabilitywhich lead to the development of uncontrolled bendingactions); local actions, due to impacts or to outcrops ofhard matter on which by chance the pipeline comes torest, which generate very localized deformations of thepipe wall (dents, gouging, etc.). These secondaryactions can cause malfunctions or failures.

Local pipeline collapse. This can be associatedwith local conditions of instability, caused by theexceeding of the resistance capacity of a section ofpipe where there are longitudinal compression stresses(both axial and circumferential).

Opening of a defect in a circumferential weld. Thisis due to having reached the resistance capacity in asection of pipe where there are very high or fluctuatinglongitudinal tensile strains (Bruschi et al., 1984) withconsequent problems of structural fatigue (Celant etal., 1982). These openings can cause a burst, or maybe limited to causing a gas leakage. This mode offailure is relevant for all phases of a pipeline’s life(from installation to operation), and there are specificresistance criteria for defining the acceptable levels ofwelding defects on the basis of the applied loads or,

for given defects, for determining allowable loads toavoid the defects themselves opening up and loosingthe internal fluid. A specific regulation appliedinternationally is BS7910-1999 of the BritishStandards Institution.

Instant failure from local or incipient damage. Thisis linked to the propagation of initially local damagethrough the entire thickness of the wall, due to fatigueor corrosion associated with a combination of load andenvironment.

To prevent each mode of failure linked to secondaryactions there are specific design criteria, like thosediscussed for the sizing of the pipe walls (see againTable 1). The objective is to keep the applied stressbelow a carefully determined allowable stress level. It isevident that the design must seek to prevent suchfailures being generated, by identifying the necessaryprecautions. These often involve measures to protect thepipe by reducing the extent of the external loads ratherthan by changing the thickness of the pipe itself.However, in environments characterized by changinghydrogeological features (terrain prone to landslides), orthat might be subject to significant seismic activity, thesecondary actions can become critical. During theplanning phase, the engineer has little in the way oftools for determining the nature and extent of thesecondary actions which might develop. In these cases,the company operating the pipeline generally makes useof a series of instrument-based surveillance andchecking activities to keep the potentially critical areasunder observation, and to initiate any suitable mitigationoperations to prevent the development of conditionswhich could lead to failure of the pipeline (Bruschi etal., 1995). The scarce attention paid to secondary loadconditions, in practice limited to checks at an equivalentstress level which does not really correlate to actualfailure conditions, is a short-coming in currentregulations. Only a few regulations, in force in countrieswhich have particularly difficult environments, forexample Japan, where the ground is subjected to verysevere thermal and seismic conditions, provide forchecks formulated specifically for secondary loads.Among the secondary effects not specifically linked toexternal loads, can be included the possible corrosion ofthe pipe’s steel walls, resulting from two causes:• The external environment (currents induced by the

pipeline on land, corrosive terrain, sea water withhigh levels of chlorine, etc.); for this type ofdamage, as already mentioned, the design providesfor total coating of the pipe with insulating plasticmaterial such as polyethylene or polypropylene(passive protection), and the installation of anodes(for example of zinc or manganese) on which toproduce any corrosive activity in place of the pipe’ssteel walls which might accidentally be in contact

with the external corrosive environment (activeprotection).

• Transported fluid which might be corrosive; forthis type of corrosion, material which is resistant tocorrosion (stainless steel) is generally used, or thefluid to be transported is pre-treated before beingpumped into the pipeline to minimize the presenceof corrosive agents such as carbon dioxide (CO2),hydrogen sulphide (H2S), etc.Onshore pipelines are generally buried so as to

avoid any possible interference from other humanactivities, such as agriculture or road-building.Nevertheless, some types of secondary loads arepossible (Bruschi et al., 1995). For example: static anddynamic loads in buried pipeline due to movement ofthe ground (landslides, soil creeps and earthquakes),types of load which generate both bending of thepipeline and axial traction and compression (Figs. 24and 25); accidental loads linked to interference fromhuman activity, of the sort that cause local damage suchas surface dents and gouging on the pipe wall made bythe impact from the bucket of a mechanical excavator,typical of construction activity, or from a plough duringagricultural activity.

Sub sea pipelines, unlike those on land, aresubjected to significant loads during construction. This

means that wall-thickness can be defined in view ofthe fact that the pipeline must withstand the loadscaused by construction (Torselletti et al., 2003b).Moreover, because of the high costs of burial resultingfrom having to work at great depths (�500 m), thepipeline is often laid on the seabed. This means thatthe pipe will remain exposed to the unevenness of theseabed, to interference from human activity, tohydrodynamic actions linked to currents and seawaves, and to accidental loads caused by earthquakes(landslides, turbidity currents, etc.). Sub sea pipelinesare subjected to the following secondary loads: a) loads caused by the hydrostatic pressure of thesurrounding water, a particularly important type ofload for pipeline constructed at great (�1,000 m) andvery great (�2,000 m) depths, for which the thicknessof the pipe walls is defined; b) loads which combinepressure (internal or external), bending (linked tofunctional loads such as the weight of the pipelineitself and of the fluid being transported, and to theunevenness of the sea floor, etc.), and axial loads(linked to functional loads such as the operatingtemperature which has the effect of expanding thepipe); c) dynamic loads caused by surface waves andmarine currents which can cause lateral instabilitiesand the vibration of the pipeline when it is in asuspended state, spanning two support points (Bruschiet al., 1982b); d) static and dynamic loads in buriedpipelines caused by movements of the ground (such aslandslides, soil creeps and earthquakes; Bruschi et al.,1995), loads of the sort that generate both bending ofthe pipeline and axial traction and compression; e) accidental loads linked to interference from humanactivity, of the sort that cause local damage such assuperficial dents and gouging on the pipe wall fromthe impact of fishermen’s dragnets and ships’ anchors.

The loads on onshore pipelines caused byinstallation are relatively low because the technologyused makes it possible to prepare an ideal path for thepipeline. The opposite is true for the installation of subsea pipelines, which calls for very sophisticatedanalyses to predict the applied loads and the structuralresponse of the pipe. Thereafter the design criteria,which are used to guarantee the integrity of the pipelineduring the installation phase, are drawn up.

Resistance of sub sea pipelines to external pressureThe installation of sub sea pipelines often foresees

the presence of air at atmospheric pressure inside thepipe (Torselletti et al., 2003b). The high externalhydrostatic pressure which is frequently present duringthe installation of sub sea pipes, tends to ovalize thecross-section of the pipe (not perfectly circular evenwhen produced in the steelworks) until it reaches theultimate limit state of collapse (complete flattening).



Fig. 24. Landslide parallel to the axis of the pipe.

Fig. 25. Landslide across the axis of the pipe.

The instability of the cross-section of the pipesubjected to external pressure is reached when thispressure is equal to the pressure of collapse, pcollapse,defined in the following formula:

pcollapse pcollapse pcollapse D0�111�1��111�2

�1��f011113

pel, d py, d py, d t

where

Dmax�Dminf0�111123

D0

2E t tpel, d�

11 �13�3

py, d�2sy13

1�v2 D0 D0

D0 is the outer diameter, t the thickness of the steel, f0the initial ovalization of the cross-section, and Dmaxand Dmin are the maximum and minimum outerdiameters respectively. In the case of pipes beinginstalled at great depths (�1,200 m), the externalpressure load can also have a significant bearing onthe specification of the pipe’s thickness in place of thecriteria for the containment of inside pressure. Thesafety criteria is similar to that used for internalpressure, and is expressed as:

Pexternal � hp�pcollapse

where hp is a coefficient which establishes the level ofsafety, that is the improbability of the pipe’s cross-section being collapsed.

A local collapse can spread along the pipeline if theexternal pressure exceeds a certain critical value (whichcan be the collapse pressure or a lower value dependingon local defects such as dents, or on a combination ofloads which amplify the effects of the external pressure).In this case the collapsed section of pipe can cause the

pipeline of the adjacent sections to be collapsed(Torselletti et al., 2003a). This knock-on-effect can onlybe stopped if the external pressure is lower than amaximum value capable of providing the necessaryforce to sustain the propogation, or if the section of pipeis strong enough to withstand the external pressure, inother words the thickness is greater than that at which apipe without local defects would collapse.

The first instance should be avoided in order not torisk the collapse of long stretches of pipework. Thesecond instance is achieved by increasing the thicknessof all the pipes (a very costly solution), or by insertingthicker sections of pipe (4-12 m in length) at regularintervals to block any propagation of the flattening.

Sub sea pipelines are subjected to fairly high loadsduring construction in comparison with onshorepipelines. In fact the pipe is subjected simultaneously tobending, axial, and external pressure loads. In addition,during pipeline construction at sea, because of the needto support the weight of the span being installed, the piperests on supports at intervals (rollers) which can applyfairly high transverse forces. In onshore pipelineconstruction all four of the above-mentioned stresses areabsent. In particular cases it is necessary to install curvescold-formed on site, in order to obtain a curve with awell-defined radius. In any case, these types of load areapplied sporadically in a very controlled way, so as toavoid any possible failures of or damage to sectionsduring the operational phase.

In the case of the installation of offshore pipes, thecombined loads can cause a mode of failure linked tothe instability of the cross-section and its eventualcollapse (Fig. 26). To express this failure modeanalytically the various regulations use differentequations (Torselletti et al., 2003b). There are twoequations which are used most frequently. The first



Fig. 26. Formation of a localised dent caused by a concentrated external force and collapse of the crosssection of the pipe (courtesy of R. Bruschi).

uses the external loads applied to the pipe’s steel wallsdirectly through the following inequality:

Mapplied Napplied pexternal�11123�a

��11123�b

��111�g

�gMcritical Ncritical pcollapse

where M is the bending moment, N is the axial forceand pexternal is the external pressure. The denominatorsare the critical values of failure/instability of theindividual loads taken separately, and g is the safetycoefficient with a value less than one. The collapsepressure, pcollapse, is a function of the pipe’s geometry(which includes imperfections such as a cross-sectionthat is not perfectly circular, in other wordsovalization, or the presence of localized dents), and ofthe material’s characteristics.

In the second equation the tensions resulting fromthe external loads are used and applied through thedefinition of the so-called equivalent tensions. Theequivalent tension applied for a allowable state,which can be defined both in relation to the yield (sy)and in relation to the failure (su), is defined from oneor more safety coefficients (Bruschi et al., 1982a;DNV, 2000).

The objective of the criteria described above is thatof installing a pipeline capable of withstanding theoperating and environmental loads which will assail itduring its working life. With reference to these criteria,the irregularity of the seabed is particularly prone toinduce unacceptable external bending loads on apipeline laid on it. In this situation either the seabedmust be modified through appropriate interventions, or,if this is too costly, the route followed by the pipe mustbe changed so as to reduce the effect of the appliedloads (equivalent tension and sectional instabilitycriteria) below the allowable tension. Some types ofoffshore pipe installations enable very carefulmonitoring of the level of deformation induced in thepipe walls. In these instances the design criteria permitthe use of less restrictive safety coefficients or,alternatively, introduce the concept of allowable

deformation for defining the limit of instability/collapseof the pipe’s cross-section. The criteria is similar to thatbased on the external loads mentioned above, but themoment M and the axial force N are substituted by thecorresponding longitudinal deformation.

For all the criteria described above there are designequations based on limit states, as for the containmentof internal pressure. In the case of installation loads ofoffshore pipes the greatest uncertainties are linked toexternal loads generated by the environment. Inparticular, during construction of the pipeline, thework barge can be subjected to sea storms with veryhigh waves which can have catastrophic effects on thestructural integrity of the span being installed and leadto the pipe being lost in the sea. It is clear, therefore,that compared with the operating conditions, the safetycoefficients adopted for the construction phase aregenerally lower.

Resistance to environmental loads and external interference

Inside pressure is not the only load acting onoffshore and onshore pipelines. The precedingparagraphs described some typical situations involvingthe interaction of several loads on a pipeline (Bruschiet al., 1982a). An analysis of the pipeline’s resistanceto operational loads (its own weight, pressure,temperature, etc.) and environmental loads (waves andmarine currents, landslides, etc.) can be sub-dividedinto two phases: the analysis of the pipe’s response,that is the calculation of the applied stresses, and thecomparison of these applied stresses with a tensionlimit through a resistance criterion.

The balance configuration of a pipeline laid on anirregular seabed generally results in a series ofsuspended pipeline sections, separated by stretches ofdifferent lengths along which the pipe lies on theseabed (Fig. 27). This situation is typical of sub seapipelines. Onshore pipelines, however, are generallylaid in a trench that follows fairly regular ground



nominal bending / bending limit

configuration after works configuration before works: not acceptable

local weightingsartificial supports

fillings with gravel

2

1

0

�1

�2

mFig. 27. Equilibriumconfiguration of a pipelinelaid down on an unevenseabed.

chosen specifically to avoid excessive bending loads.The discontinuity of the points of support inducesbending moments which can be unacceptable from astatic (i.e. under operating loads) and a dynamic (i.e.under operating and environmental loads) point ofview. In particular, the unevenness of the seabed mustbe within limits which allow the pipe to be laidwithout jeopardizing its resistance.

Once the pipeline has been laid, it should beconsidered that: the balance configuration under staticactions may not be acceptable, for example during thehydraulic testing phase (see Section 7.1.6) or in thesubsequent operational phase, if the overall weight ofthe pipeline and the fluid within it, the inside pressureand the external pressure, put a stress on the pipeline’ssupport points causing bending moments that exceedthe allowable values; the balance configuration could beinadmissible from a dynamic point of view, if thebottom marine currents, acting transversally to thepipeline, are such as to induce self-enhancing hydro-elastic oscillations of an amplitude which prejudices thefatigue resistance of the pipeline during its operatinglife (Bruschi et al., 1982b; Celant et al., 1982).

The two load-based situations described abovemust be compared with the structural criteria able toguarantee the pipe’s integrity for the whole of itsworking life (Vitali et al., 2003). In particular, thebending moment limit combined with the action of theinside pressure and axial forces, above which thepipeline loses all capacity of resistance (this involves

criteria similar to those described earlier, whendiscussing installation loads); exceeding of the fatigueresistance under cyclical loads linked both to thefluctuations of inside pressure and to dynamic loadscaused by the external environment (resistance criteriaspecific to this limit state are described in theregulations in Table 1).

When tension or bending moment values are abovethe allowable levels it is necessary to carry outremedial work to modify the balance configuration ofthe pipeline. This can involve, for example, levellingany unevenness along the route of the pipeline (byexcavating trenches or using rocks and excavatedmaterial to fill in depressions), as shown in Fig. 27.

The introduction of the limit states method hasnecessitated detailed analyses of the pipe’s structuralresponse under the combined action of bendingmoment, inside pressure and axial force. The existingregulations often make reference to design criteriabased on experimental tests on a 1:1 scale, and on theapplication of numerical models (FEA, FiniteElements Analysis), as shown in Figs. 28 and 29.

Pipelines for the transport of fluids which aredangerous for people or for the environment (such asoil or natural gas which could catch fire or explode,either way causing pollution), must also be designed towithstand accidental loads; loads whose probability ofoccurrence is remote but not insignificant consideringthe risks involved. In fact the objective is to design astructure that will be safe even in the event ofaccidental loads, which although highly unlikely, couldhave serious consequences from the point of view ofthe environment and loss of human life, as well asfinancially (see above).

Onshore pipelines, passing through areas ofconsiderable human activity (housing, industry,farming, etc.), must be capable of withstanding evensignificant accidental loads. It should be rememberedthat typical accidental loads are those associated withthe environment (earthquakes, landslides, etc.) or withhuman activity, which would call for very thick steelpipelines. As already mentioned, to restrict the thicknessto what would be acceptable economically, protectivemeasures are adopted, such as burying the pipe intrenches of a suitable depth. However, there are humanactivities which can still interfere with the pipeline (asalready mentioned, interference from the bucket of anexcavator, the blade of a plough, etc.). This means thatthe steel pipe must be thick enough to withstand certainaccidental loads which occur frequently or have seriousconsequences. Nevertheless, this thickness may still notbe sufficient. In fact some instances of damage canbecome very dangerous over a period of time, even ifthey are not immediately so. It is therefore offundamental importance to plan inspection and



Fig. 29. Determination of the flexing moment limit by means of numerical finite elements analyses (courtesy of R. Bruschi).

Fig. 28. Determination of the flexing moment limit by meansof experimental trials (courtesy of R. Bruschi).

subsequent monitoring, and/or the repair of any damagefound in the pipeline (local dents and/or gouging on thesteel wall, and local or general corrosion). One of themain monitoring activities consists in checking for anycorroding of the steel wall due to deterioration/damageto the passive protection (anti-corrosion coating), and inthe consequent activation of the active protection(cathodic protection).

Another problem, generally caused by accidentalexternal loads which over time reduce the mechanicalresistance of the pipe wall, is the so-called ductilepropagation of a longitudinal fracture which often hasdisastrous consequences (loss of human life andsubstantial economic damage); this problem is typicalof pipeline for gas transmission. Generally the designcriterion is linked to the choice of a material which istough enough to prevent such propagation. Even if thetrigger may be linked to local interference from humanactivity, the monitoring activity to prevent localdamage remains of fundamental importance.

7.1.6 Construction

The construction of a pipeline is carried out bywelding together pipes which have been properlyprepared and laying them along a predefined route.Depending on the terrain traversed, the pipelines caneither be onshore or offshore; this is the first and mostimportant distinction in constructing the pipelines,given that the two different environments requiredifferent planning and construction technologies. Ingeneral, sub sea pipelines require a high level oftechnology both during construction, in terms of themeans employed, and thereafter, in the managementand maintenance of the lines.

Onshore pipelinesThe current trend in onshore pipeline construction

(Institution of Gas Engineers, 1984) is to bury thepipelines, with very few exceptions in specialcircumstances; this both increases safety and reducesimpact on the environment. The activities forconstructing an onshore pipeline can be outlined asfollows: logistics and work route; trenching andcrossing of particular points; mechanical assembly;restoration and commissioning.

LogisticsThe logistic activities include:

• Transfer of personnel, machinery and tools to thework-sites located along the pipeline, for a distancewhich depends on the characteristics of the area inquestion and the presence of infrastructure andservices (transport, water, electricity, etc.).

• Preparation of the route (Gray, 2004); a track isprepared along the entire course of the pipeline toallow the movement of mechanized equipment forcarrying and laying the pipeline. Generally, as thediameter of the pipe increases, the size of themechanized equipment and the width of the trenchalso increase. Hence a track is prepared whichincreases in width in proportion to the diameter ofthe pipeline, up to a width of about 30 m for a pipediameter of 1.2 m. In difficult situations or to reducethe environmental impact to a minimum, it ispossible to restrict the width of the track, howeverthis increases the complexity of the assembly work.After clearing the vegetation and removing the layerof humus from the area along the route, the groundis levelled to facilitate the excavation work and thelaying of the pipeline. The humus is normally storedso that later it can be put back in place.

• Transport and storage of the pipes; the pipes forthe pipeline (which vary in length from 10 to 17 m)are often produced in steelworks a long way fromwhere they are used and, for major operations, bymore than one supplier. The first part of thejourney from the steelworks is usually made byship or train; thereafter lorries are used to reachthe gathering area near the work-site. Sometimesit is therefore necessary to construct access roads.The costs associated with overland transport arevery significant, and are generally determined bybalancing the advantages and disadvantagesoffered by long pipes (produced up to a length of17 m), which reduce the number of welds andalso the cost of construction, but involve highertransport costs (generally for lengths above 13 mit is necessary to organize escorted transport). Awidely employed compromise is to foresee theuse of pipes of approximately 12 m which arewelded in pairs on work site, thereaftertransporting the length of pipe produced (doublejoint) along the track to the last weld point alongthe pipeline. In this way, doubling up the weldingphases enables a major increase in productivity.During the phases of moving, transporting andstoring the pipes, it is important not to damageeither the pipe or the anti-corrosive coatingapplied to the exterior of the steel. Although thecoating can be repaired without difficulty,damage to the steel pipe can be repaired only insome less serious instances.

• Careful movement of the pipe until its installation inthe trench; in fact it is important not to bring metaltools into direct contact with the pipe’s coating, toavoid laying the pipes being transported along theline directly onto the ground, and to handle thepipeline (especially when assembled) in such a way



as to avoid plastic deformation, ovalization anddenting.

• Deployment of the pipes along the route; this is thefinal part of the transport: the pipes are lined upalong the route beside the trench so as to be readyfor connecting. The sections where there are curvesare identified in the planning phase and theappropriate curved sections are prepared inadvance at the steelworks and transported to thesite. Other curves are made in the field as part ofthe mechanical assembly.

Excavations and crossingsPipelines are generally buried, with a covering

layer thick enough to prevent interference from surfaceactivities, especially agricultural, and to ensureprotection from the passage of mechanized equipment;the covering must also be able to cope with erosionover time resulting either from natural events or fromhuman activity.

The excavating activity involves preparing a trenchof a shape, depth and width suitable for the pipelineand for the various problems associated with the typesof ground traversed. The trench is dug using varioustypes of excavators, in conjunction with explosiveswhere there is tight rock.

The bottom of the trench is formed in such a waythat the whole length of the pipeline rests on it.Moreover, the bottom and the sides of the trench arefinished in such a way as to avoid unevenness thatcould damage the pipeline or its coating. For the samereason, the bottom of the trench is checked to ensurethat there are no foreign bodies, boulders orprotectionof rock. When necessary, a safe laying bed is created,the area around the pipeline being covered withselected material (sand), often taken from theexcavation itself.

In the field of pipeline construction, crossingobstacles, whether natural (water courses, ridges androcky slopes) or artificial (railways, roads andmotorways), has always represented a peculiarity interms of both design and construction. There is a widevariation in the scale of the operations requireddepending on the difficulties presented by thecrossing. Water courses and their infrastructures arecrossed using small work units, which operate inadvance of the line. In this context, it should beremembered that in certain instances, such as in verymountainous areas, tunnels may also be built.

The available construction methods can beclassified in numerous ways, based on the drillingcriteria, the type of excavation or the type ofmachinery being used; the two principal methods arethe open air type and the underground technologytype, also called trenchless (Vescovo and Lazzarini,

2002). The requirement for a more careful protectionof the environment in the area of the pipeline and thesignificant technological developments of recentyears, have led to the increasing adoption of trenchlesstechnologies as alternatives to open air excavations.

The most often used trenchless methods are themicro-tunnel and Horizontal Directional Drilling(HDD). Micro-tunnel technology consists in theprogressive advancement of a cylindrical cutting headpositioned in front of a string of lining pipes. Thesimultaneous advancement of the head and the trailingpipes is achieved using of hydraulic jacks positioned atthe rear, at the drive station. The micro-tunnel iscarried out using sophisticated control systems,sometimes remote controlled, which make it possibleto follow an irregular course with great precision andsafety. Once the micro-tunnel is completed, thepipeline is installed inside it using winches and cables.The dimensions of micro-tunnels vary depending onthe pipeline to be laid; their diameter may be from 1 to3.5 m, except in special cases.

HDD is a system of boring derived from thedirectional drilling methods used in oil wells. Duringan initial phase a pilot borehole of small diameter isdrilled along the predetermined project profile,generally curved, using a high-pressure jet cuttinghead – or alternatively a mud drive – connected to thefront of drill pipes. The cutting head carries out boththe mechanical action of cutting the ground and thechanges of direction needed to follow the course of theproject. The second phase is that of reaming the pilotbore up to the diameter required for installing thepipeline. The number of reaming runs depends onvarious factors (the nature of the terrain, the diameterof the pipeline, the available pulling force, etc.).

Finally, the pipeline is ‘pulled’ using the drill pipes,on the end of which a length of the pipeline itself, withits joints already welded on the surface, is attachedwith an appropriate connection.

Mechanical assemblyQualification of the construction procedures. The

principal construction procedures are welding, anti-corrosive coating of the welded joints, non-destructivechecks and repairing of the welds. All these operationsare carried out in the field. These procedures are alsotested to ensure that the personnel, the equipment andthe materials used guarantee the success of the work inaccordance with the specifications. The qualification iscarried out directly on-site at the start of the work, if nounusual difficulties arise. The welding procedures arequalified some time in advance of the start ofconstruction, to allow for the carrying out of theprescribed destructive tests and the adoption of anycorrective measures needed in the event that the



requirements are not satisfied. If non-destructive checksusing automated ultrasound systems are foreseen, oncethe welding proceedures have been qualified,construction of the calibration blocks follows.

Connecting the pipes through welding the jointsand non-destructive checks. Welding the joints is theprincipal construction activity on a pipeline. Manual,semi-automatic and automatic techniques areemployed. For major pipelines semi-automatictechniques, which guarantee the greatest productivity,are the most frequently used, although a great deal ofeffort is being focused on the development ofautomatic machines which will deliver even higherproductivity. However, manual weldings are still alwaysmade for the off-line connections of pipeline strings(tie-in). Manual welding is also used for connectingspecialized items (valves, branches, prefabricatedcurves and traps), or for pipes with walls of differentthicknesses; on minor pipelines manual welding is stillpreferred because of its greater simplicity.

There are various technologies, but generallyelectric arc welding techniques are used (see Section7.1.4) and they are differentiated by the type ofshielding of the molten pool: submerged arc welding(with a continuous wire, of both automatic and semi-automatic types, often used on double joins), and gasprotection welding (with a continuous wire, generallyof the semi-automatic type, used on pipeline joins).The completion of the welding is carried out throughsuccessive runs, also using different methods forindividual runs.

As already mentioned, the typical procedureforesees mechanical coupling of the joints to bewelded using an appropriate clamping device whichtravels along the inside of the pipeline (see again Fig. 21; in some cases an external coupling system isalso used), which has the task of holding the two edgessecurely. A check is then carried out to see if there isany misalignment, the edges to be joined are pre-heated and the welding work is carried out withintermediate temperature checks between the runs.

All the welds are checked to identify any defectswhich might compromise the integrity of the pipeline.In past years, as already pointed out, radiographicequipment supported by manual ultrasonic equipmentwas used. Currently the tendency is to use automatedultrasonic systems which guarantee a betteridentification of the most hazardous defects, andsupply more accurate information on the dimensionsof the defects.

Coating welded joints with protective material andchecking the integrity of the external coating.Normally the pipes arrive on-site already coated,whereas coating of the welded joints is of necessitycarried out on the line. To this end, the welded joint is

prepared through cleaning, sandblasting and heating,followed by the application of the protective material,which is generally of the same type as that used toprotect the rest of the pipework (layers of polyethyleneor polypropylene on epoxy resin). The electricalinsulation of the joint and the rest of the pipeline ischecked using high voltage instruments (5-25 kV)called leak-detectors.

Laying the pipe strings in the trench andconnecting them. After the pipes have been weldedtogether on the track (rarely in the trench), stringshundreds of metres long are laid in the trench. This isachieved using a series of lifting and laying machineswith lateral arms and counterweights (sidebooms),which operate in unison to lower the pipeline into thetrench avoiding excessive stresses and bending (S-bends), (Fig. 30). Once laid, the strings are weldedtogether manually inside the trench (tie-in). Once thelaydown phase is complete, on major pipelines a fibre-optic cable is laid in the same trench for thetransmission of working data and commands to thevalves and other equipment operating on the pipeline.

Equipment for completing the pipeline. Completingthe pipeline consists in the installation of section points;stations (‘traps’) for launching and recoveringequipment (‘pigs’) able to travel through the wholelength of the pipeline. These ‘line’ stations are equippedwith valves, by-pass systems, systems for emptying andblowdown of sectioned lengths of the pipeline, andinstruments for checking the transmission of data on theworking of the pipeline. They are generally built insmall enclosed areas with a cabin, inside which arelocated panels with the control functions.

Some of these stations are also equipped withpower supplies for the cathodic protection, the controland monitoring equipment and the data transmissionsystems. These stations are made accessible from theexisting roadways and are sometimes equipped withindependent electricity generators.

Installation of electrical (cathodic) protection. Inaddition to the external coating, the pipeline is alsoprotected against corrosion by an active (cathodic)protection system. This consists of a system of electriccurrents generated by equipment located along theline, which give the metal of the pipeline a negativecharge compared with the surrounding electrolyte(ground, water, etc.). Monitoring points are installedalong the pipeline in order to confirm the effectivenessof this system.

Restoration and start-upThe trench is re-filled and the landscape restored

after the mechanical assembly and laying work havebeen completed. Refilling is carried out using theoriginal material removed from the escavation (except



in special cases where the geological conditions areunsuitable): the pipeline is indicated by a marking stripand at specific points protection slabs or sand-bagbarriers may be laid. Particular attention is paid to thetop layer of the refill in order to restore themorphology and use of the ground prior to the work.

In some specific points when the work is finished,restoration work carried out must target geotechnicalissues (stability of steep slapes, hydraulic check onriver crossings, mitigation of the morphology of thelandscape) and environmental issues, with operationsto reconstruct the original vegetational cover and toregenerate the land’s original fertility.

For each stretch of the pipeline which has beencompletely connected and laid, a hydraulic check iscarried out by filling the pipeline with water (in casesof particular difficulty in obtaining sufficient water itis possible to use air or gas), and pressurizing it to alevel above the operating pressure. The phases of thehydraulic test which involve filling and draining waterare carried out using appropriate tools with hydraulicseal, commonly known as ‘pigs’, which are also used

for pipeline cleaning and commissioning operations,and for checking dimensions. For this purpose,temporary pieces of equipment called ‘test plates’ areinstalled, welded onto the end of the completed stretch.Once the hydraulic test is complete, the water isexpelled mechanically using the pig, driven bycompressed air. All water remaining in the pipelinemust be eliminated, particularly at its lowest points.

When the residual water has been cleared, a certainamount of water inevitably remains on the innersurface of the pipeline in the form of a film. Thepipeline must then be washed and dried so that whenthe fluid is transmitted it will not be contaminated bywater. The pipeline is dried using a variety of methods(using nitrogen, dry air or vacuum).

Construction and maintenance of a pipeline areusually legitimized by rights which apart from theagricultural use of the surface traversed, limitconstruction within the non-building strip (servitude)or either side of the pipeline (non aedificandiservitude). The construction limits are connected tothe category of work and the relative regulations.



Fig. 30. Example of the laydown of pipework. Both the trench and the equipment typically used for laydown (sidebooms) which supports the pipework can be seen (courtesy of R. Bruschi).

Once the construction and testing phases haveended, the pipeline is put into operation. The functionof co-ordinating and monitoring the activity of oil andgas transport through the pipelines is entrusted toorganizational units, which may be either centralizedor spread across the territory. The activity consists oftravelling the length of the pipeline or monitoring itfrom suitable positions in order to observe the burialconditions, the functionality and preservation ofpipeline construction and signposting, and any thirdparty actions that might affect the pipelines and therespect area. Monitoring of the pipeline can also becarried out with an aeroplane or a helicopter.

Sub sea pipelinesIn sub sea pipeline construction, the laydown

environment often requires the use of specializedvessels – lay-barges – which are very big andexpensive. These are basically floating workshopswhich accommodate a hundred or more people, whotypically work in shifts around the clock.

The work follows the same functional programmeas onshore pipelines. As many preparatory operationsas possible are carried out onshore and the phasesinvolving installation and any work on the sea floor areof particular importance (Matteelli et al., 1976). Theneed to avoid stresses and excessive deformationwhich might compromise the present and futureintegrity of the pipeline has resulted in thedevelopment of increasingly powerful technologiesand laying barges. The various systems for installing asub sea pipeline are: a) the ‘S-lay’ and the ‘J-lay’methods; b) pipeline coiled around a reel; c) on-linewelding; d) work on the sea floor; e) testing andpreparation for putting into operation.

‘S-lay’ and ‘J-lay’The most commonly used laydown method is the

‘S-lay’ system, so-called because of the shape that thepipeline typically assumes along the installation span(Fig. 31). This method uses tensioners and slidingclamps on the barge’s deck to apply a longitudinal forceto the pipeline, supporting it both where it leaves thelay-ramp and where it makes contact with the sea floor.For deep waters and high diameters, in order toguarantee the integrity of the pipeline the longitudinalforces to be applied become progressively greater; theanchoring and positioning system for the barge basedon anchors becomes ineffective, and very powerful (andreliable) motors are needed to support the pipeline.

For these reasons over recent years in very deepwaters the ‘J-lay’ method has started to be used, asystem which features an almost vertical lay-ramp. Thefirst transport line lying at a depth of over 2,000 m inthe Black Sea, and consisting of two 24 inch (610 mm)

diameter pipes, each around 350 km long, wasconstructed between 2000 and 2002 using a marineconstruction barge (Fig. 32) which had been speciallymodified and equipped with a tower for ‘J’-laying.

Pipeline coiled around a reelThere are many types of lay barges, each of which

varies according to the type of pipeline to be laid. Forpipelines of a modest diameter, up to 14-16 inches(35.6-40.6 cm), it is possible to use pipes which havealready been welded together and coiled onto a reel atan onshore worksite. During the laydown these pipesare uncoiled from the reel, straightened and loweredinto the sea. On one reel it is possible to coil from 8-20km of pipeline, depending on the diameter of the pipe.This installation system requires pipes with thickwalls, to withstand the deformation induced by thecoiling, uncoiling and straightening of the pipeline,and can leave residual deformation on the pipeline.

Pipeline welded on the linePipeline with a diameter greater than 400 mm is

never coiled onto a reel, but rather the lengths of pipeare joined together by welding which is carried out on



touchdownpoint

mudline

stinger

Fig. 31. Laydown of undersea pipework in progress.

Fig. 32. Marine construction ship adapted for pipeline laydown using a ‘J’ system (courtesy of R. Bruschi).

the installation line. Smaller diameter pipes can alsobe installed using the same system.

Various working stations are set up on the deck ofthe barge, each one specialized in one given activity.On-line welding is divided into several parts, each ofwhich is assigned its own working station. In additionthere are one or two stations for non-destructivetesting and repairing of any defects detected in thewelding. The final station is generally dedicated tocoating the welded joints. In order to line up with theworking areas on the installation deck, the pipes are allabout the same length; approximately 12.5 m.

The welding techniques are substantially the sameas those used for onshore pipelines, with the onedifference that manual welding is carried out only toconnect lengths of the line that are installed atdifferent times, or for connecting special pieces. If thespace on the lay deck permits, and the liftingequipment can handle them, double joints are used. Incertain instances, to improve productivity, evenquadruple joints are used.

Work on the seabedPreparatory work on the seabed, including that for

maintaining the laydown tolerances within as wide acorridor as possible, is very expensive and is generallyavoided. If in a certain area a pipeline has to beinstalled in a narrow corridor, generally one or moretransponders are positioned near the area and on thepipeline. Keeping within the limitations imposed by anarrow corridor can be difficult and requires a veryslow pace of laydown; it is also affected by the depthof the sea and the marine meteorological conditionsencountered.

Sub sea pipelines of a reasonable diameter, usuallystarting from 16 inches (406 mm), are laid on theseabed without any special work either preparatory toor following the laydown, except localized action forspecific reasons. For smaller diameters, decisions onwhether or not to protect the pipeline (frominterference from fishing equipment) are taken on acase by case basis. At the ‘landing places’ pipelines aregenerally laid in trenches and covered, but in otherareas this is done only when the investigation reveal theneed. Occasionally it is necessary to carry out work ona pipeline laid on the seabed. Basically the work is ofthree types: support, weighting, or excavation andcovering (see again Fig. 27). Support work is carriedout on suspended lengths using various methods: asupport can be created with gravel or sandbags or withmechanical devices. Alternatively, a mechanical devicecan be installed which provides active support (i.e. it iscapable of supporting the pipeline by using hydraulicdevices to lift it from the position where it is laiddown). Weighting work can be continuous, effected by

dumping weighting material (gravel) onto the pipe, orlocalized, effected with overweights (blanket of gravel)or with loose material (gravel). Excavation andcovering work is carried out using specializedequipment, depending on the nature of the seabed, thedepth of the sea, and the required depth of the trench.The trench is excavated under the pipeline which islying on the seabed and which then gradually settlesdown into it. Occasionally it is necessary to repeat theoperation several times to obtain the depth ofexcavation desired. The subsequent covering is usuallycarried out using the material removed from the trenchitself, or with gravel if there is a particular need. Thesystems for carrying out this work on the seabed can bevery advanced, such as when it is necessary to dig atrench under pipework at depths in excess of 1,000 m.These operations, especially when dealing with pipesalready in place, are monitored very carefully, both tobe sure that they achieve the required results, and toavoid causing damage to the pipeline.

Testing and preparation for putting into operationAfter having been laid on the seabed, and once the

prescribed remedial work has been carried out with thepipe full of air, the pipeline is filled with water. Thisoperation is also used to clean the pipe and to checkthe dimensions of its internal cross-section, usingappropriate equipment (a pig), introduced into thepipeline through traps located upstream anddownstream of the pipeline. A hydraulic test of thepipeline itself is then carried out, subjecting it to aninside pressure which at sea level ranges from 1.15 to1.25 times the project pressure, and which in verydeep waters can be significantly higher than theproject pressure. Once the hydraulic seal is confirmed,the pipeline is emptied, through a train of pigs and acompressed air station with sufficient power, and thenair-dried (Haun, 1986a, 1986b). These operationsinvolve the entire length of the pipeline, while withonshore pipes the same operation is carried out onshorter stretches, the longest distance being thatbetween two sectioning valves.

7.1.7 Inspection, maintenance and repair

Both onshore and sub sea pipelines operate in acontext that brings about deterioration over the courseof time, for reasons which are sometimes foreseeableat the design stage and sometimes are unexpected. Forthis reason, the design of a pipeline foresees for thepreparation both of equipment and of managementprogrammes based on inspections, seen as a guaranteeof the functionality of the pipeline over time.



These periodic inspections have the objective ofchecking the structural integrity of the pipeline, andplanning any maintenance operations needed toguarantee continuity of operation and to manage anyanomalous situations that might arise during theoperational life of the pipeline. The high investmentcosts demand that the degree of reliability of theoperation be kept at the highest level for the wholeforeseeable life of the system. Minimizing the risks ofdamage does not end therefore with the design andconstruction phases, but must also continue throughoutthe operational life of the transport system. This isachieved through a programme of periodic inspectionsthat make it possible to obtain all the data elementsneeded to define both the current state of the systemand the trend that may be developing. It is clear that aninvestigation into the maintenance of a pipeline is ofvalue only if it is compared with the results of a similarinvestigation carried out previously, with a view tohighlighting any variations.

In general, inspections are carried out mostfrequently in the initial period and in the final period ofthe foreseen life of the pipeline. In the beginning theinspections are very close together so as to follow theadaptation of the pipeline to its environment, whilelater they become more frequent only when there are asmany failures due to wear as there are due to accidents.

A relatively new approach (called RBI, Risk BasedInspection), still in the course of development, is toplan inspections based on the risk of a certain criticalsituation occurring. This method permits a strategy ofpipeline inspection based on principles of risk, withinwhich the role played by inspection is principallyfocused on reducing the danger of damage. The riskassociated with a component is the product of theprobability of its failure and the consequences of suchan event. The inspection plan based on the RBIapproach uses the evaluation of the risk connected toinjury to persons, to environmental damage and toeconomic damage as a basis for deciding where toinspect, what to inspect, how to inspect and when toinspect (Bjørnøy et al., 2001).

The general approach typical ofinspection/maintenance programmes consists of thefollowing main points: a) an inspection programmebased on an efficient (in terms of inspection methodsand equipment) system of data acquisition; b) anautomated system of recording and processing thesedata; c) a decision-making process for carrying out theoperations; d) a maintenance programme. For apreliminary choice of inspection methods, it isimportant to consider the available historical data onthe existing lines integrated with the statistical datarelative to the most probable causes of damage to thepipelines. In fact, an adequate understanding of the

possible mechanisms of damage relative to a pipelineand their possible consequences is fundamental inestablishing the correct form of inspection andchoosing the most suitable instruments.

An operating pipeline can deteriorate because of itsinteraction with the fluid transmitted and with theexternal environment, and the risk of failure increaseswith age. Furthermore, statistics show that for pipelineswhich transport gas and oil, the major causes ofincidents resulting in loss of fluid have been identifiedas: a) damage caused by external forces; b) defects inmaterial and construction; c) corrosion/erosion owing tothe type of fluid transmitted or the externalenvironment; d) inefficiency of the cathodic protection;e) land movement. In the case of pipelines laid along time ago, for which, however, adequatedocumentation of the project does not exist, it isnecessary to carry out an evaluation of the present stateof the network. This evaluation must identify:planimetric positions; the state and thickness of thepipeline covering; the areas made dangerous by thepossibility of landslides; the effects of soil erosion andthe presence of foreign bodies.

During the setting up of a maintenance surveyprogramme, research is always conducted to acquirequalitative data (visual data) and quantitative data(instrumental data) on the elements of the systemconsidered to be fundamental. More precisely, thegeneral characteristics of the pipeline must beevaluated in detail, together with special components,stabilization operations, spans, any damage to thepipeline, the presence of dangerous objects, and anysignificant situations. Amongst the latter, instances ofcrossing other pipelines, electricity and/or telephonecables must certainly be cited.

These data must also be supplied to the companythat carries out the work. If the data are not available,the surveys to carry out have the additional purpose ofpaving the way for any future investigations, seekingto collect all the information as accurately as possible,and paying particular attention to any that is unlikelyto change with time. On the basis of the elementsavailable, the basic route will be defined byinterpolating the data between known reference pointsor simply between the starting point and the endingpoint of the pipeline. The more precise the route, theeasier it will be to check any critical aspects, forexample the length and height of the spans, with aview to any possible maintenance actions.

External onshore inspection techniquesThe pipeline is usually traced by means of

appropriate signs fixed in the earth, correspondingwith the longitudinal axis of the pipeline. In the eventthat the signs should be lacking or unreliable, as a



result of operations subsequent to the laying of thepipeline, the pipeline must be re-located. The tracingof the exact location of the pipeline is carried out bytopographical teams provided with instruments andGPS (Global Positioning System) devices, or bycarrying out a campaign of traditional topographicalsurveys. The planimetric position and the depth of thepipeline are then defined using the above-mentionedmethods, and applying the survey data to a geodeticnetwork of appropriately defined settings.

During examination of the track, the areas ofgeological risk are highlighted, for example areassubject to landslide and soil erosion, that musttherefore be checked and included in a maintenanceand monitoring plan (Fig. 33). Any landslidemovements can in fact be a source of stress on thepipeline, even to the point of causing a failure in someinstances. Consequently, if signs appear of the possibleformation of landslides, such as for example fissuresin the surface or swelling up of the earth, or if theroute crosses hilly areas part way up a hill, it isessential to carry out periodic surveys aimed atdefining the extent of any earth movements and theirvelocity of advancement.

Soil erosion, of various forms, can reduce thethickness of the covering of the pipeline, or even causeit to be uncovered, with all the accompanying risks. Itis therefore necessary to check the pipelineperiodically, above all following flooding throughrainfall, as well as checking the torrent-like watercourses that cross it. When evidence of surface erosionactivity appears, it is possible to carry out remedialwork by making palisades, faggot barriers, gatheringand diversion channels for the surface waters, etc.When, instead, there is a need to limit the erosion ofwater courses, it is possible to use gabionades,embankments, bluffs, cement walls, etc.

No less important is the setting up of a census ofthe inhabited areas using planimetric strips to a scaleof 1:25,000, showing the built-up zones or areas ofbuilding expansion close to the pipeline. It is alsonecessary to check the forecasts of buildingdevelopment programme for an estimate of thepopulation anticipated over the next five years.Examination of the 1:25,000 scale maps, the aerialphotographs available and the census data of theinhabitants, can reveal that some tracts of the pipelinedo not conform, even if only potentially, to the legalregulations. These tracts must be highlighted with aview to deciding what operations are needed to bringthe situation back within the standards.

Surveying with pipe detectors can for exampleindicate tracts of the pipeline with a burial depth ofless than the current standard. These cases are resolvedby refilling the tract with soil or resorting tomechanical protection of the pipeline. Generally,however, it is good practice to keep the interventionactivities close to the pipeline to a minimum. In fact,excavation or earth movement, in particular anyagricultural activity going on in the area in question, ispotentially very dangerous; depending on the form ofagriculture being engaged in, earth can be ploughed todepths in the order of 60-80 cm and, in exceptionalcases, 90-100 cm. The only solution to this source ofdanger is close surveillance so that work of potentialdanger to the pipeline is not carried out or isperformed under supervision.

Techniques for external inspection of sub sea pipelines

In setting up a maintenance survey activity at seathe principal elements to consider are the depth of thewater and the distance from the coast. Consequentlytwo types of surveys can be defined: those in shallowwaters and those in open sea (offshore). Shallowwater inspections, i.e. from the water’s edge out to adepth of 15-20 m, call for small boats of shallow draftwith a type of instrumentation mainly in tow(Anselmi et al., 1990). The offshore areas, however,i.e. depths from about 15-20 m up to a maximum ofaround 800 m and usually a long way away from thecoast, need ships with specialized equipment, such asremote controlled underwater vehicles (ROV,Remotely Operated Vehicle) capable of workingcontinuously round the clock even in extrememeteorological conditions (Fig. 34).

The objectives of a sub sea pipeline maintenancesurvey are: a) to establish its planimetric position; b) define the pipe/seabed profile; c) check the state ofits covering; d) determine the external condition of itsreinforced coating (highlighting any damage), the stateof any stabilization work and the state of the cathodic



1

1�2�3 4�5�6

2 3 4 5 6

pipeline

scanner

modem computer

radio link

cable link

extensimetric sections

Fig. 33. Support system for the management and monitoring of pipelines in areas at risk of landslides.

protection; e) identify any dangerous objects close tothe pipeline.

In planning a survey at sea a decisive aspect is thecomparison of different surveys for obtaining andprocessing data carried out at different times. To beable to make such comparisons, it is necessary to havethe most information possible about the generalcharacteristics of the pipeline and the area chosen forit to be laid, as well as data on the operations of layingand stabilizing the pipeline on the seabed. Wheneversuch information is only partially available, it isnecessary to carry out an initial survey aimed atproviding the missing information which, togetherwith that already available, will constitute thecomparison data (Cherubini et al., 2001).

One of the difficulties regarding measurementstaken at sea is the need to determine precisely andrepeatably the geographical position in which themeasurement was taken. To this end, two types ofpositioning are generally adopted: one of the absolutetype and the other of a relative type. Usually in amaintenance survey it is necessary to know theparameters of importance in relation to the progressivekilometric position (relative positioning). The absoluteposition, on the other hand, is vital for all therequirements connected with the location of thestructures in a wider context, such as mapping forobtaining work permits, transfer of data to appropriate

bodies, or correlation between the measurements carriedout and the national mapping (Iovenitti et al., 1994).

Internal inspection techniquesThe internal inspection of a pipeline is carried out

using a special tool called a pig; this term refers to allinstruments that can be inserted into a pipeline andcarried forward by the thrust of the fluid beingtransmitted (Fig. 35). Pigs are used to collectinformation on the general conditions, on theconfiguration of the pipeline layout, on the presence ofdefects in the pipeline, and on its geometry.

‘Smart’ pigs are particularly suitable for use inpipelines buried under the seabed, which cannot beinspected visually or through conventional non-destructive methods. Internal inspection is a veryimportant activity when it comes to evaluatingstructural integrity and quantifying the risk of failure.It makes it possible to discover, identify, localize andsize correctly a series of defects and/or anomalies,such as indentations, deformations (buckle),ovalization, notchs, generalized areas of corrosion(internal and external), welding defects, cracks (fromstress corrosion, from hydrogen, or from fatigue) andlamination defects. In the case of lines laid a long timeago, for which adequate project documentation doesnot exist, or to verify any uncontrolled shifting of lineslaid in trenches, it is also necessary to haveinstruments available that make it possible to verifythe geometry of the line’s axis, locating curves andchanges of direction.

At the present state of technological developmentfor inspecting pipelines, it is not possible to discover,differentiate and size accurately all possible defectsusing just one type of pig. Indeed, none of the non-destructive control techniques available are suitable forall categories of defect. There are several types ofvehicles, smart or semi-smart, including: a) caliperpigs, for obtaining details of the profile of the internalwalls, including ovalization. These are recommendedfor the survey of geometric/mechanical defects;b) magnetic pigs (MFL, Magnetic Flux Leakage) andultrasonic pigs (UT, Ultrasonic Test), for metal losstypes of defect; c) pigs for revealing cracks; d) pigs foridentifying leaks and fissures; e) pigs fitted with GSMand inertial devices for surveying the geometriccharacteristics of the pipeline layout.

In areas of instable terrain where a pipeline is laidit is important to discover if and when it undergoesenough movement to induce excessive stress.Movement can be caused by seismic phenomena,subsidence, currents and erosion of the seabed. For thesafe and reliable operation of pipelines in areas ofpotential instability it is vital to check on anymovement of the line and, whenever a reduction in the



Fig. 34. Inspection of an undersea pipeline using an ROV.

Fig. 35. Pipeline inspection using a pig(courtesy of R. Bruschi).

safety margin is recorded, to evaluate its structuralintegrity with a view to defining the appropriateinterventions. Naturally it is necessary to confirm theability to inspect the pipeline, therefore checks mustbe made on its geometry (to guarantee that the pig canpass), on the existence of launching and receivingtraps, and finally on the operating conditions, toguarantee their compatibility with the requirements ofthe inspection. The choice of the type of pig for theinspection should take the peculiarities of the line intoaccount, in particular if it is characterized bysignificant variations of thickness along the path, aswell as the characteristics of the process fluids.

In the case of magnetic pigs, the high resolutiontype is certainly the most often used. The MFLconventional type pigs are in fact most suitable for aninitial inspection of pipelines on which it is expectedto find a very high number of defects. A significantvariation in thickness could create a problem if amagnetic pig is used, in that the functioning of theentire system and therefore the reliability of theinspection are linked to the ability to magnetize thewalls adequately. The use of the ultrasonic technique(UT) of inspection, however, allows for the directmeasurement of the residual thickness of the pipe andof the possible variations in thickness caused by anycorrosive action. It must be considered however thatthe UT pig is highly sensitive to the surface conditionof the wall, so it is essential to carry out carefulcleaning in advance. The choice of the type ofinstrument to use is based, therefore, on a thoroughevaluation of the possible problems of usage, to bemade also in collaboration with the companies thatcarry out this specific service (Palmer and King,2004).

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