Introduction to Offshore Structural Engineering

8/16/2019 Introduction to Offshore Structural Engineering

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Introduction to OffshoreStructural Engineering

1) PerIvar Lokstad, Senior Structural Engineer,Civil Engineer (Norwegian Institute ofTechnology), Technip Geoproduction (M)Sdn Bhd

2) Khairudin Ahmad, Senior Structural

Engineer, B.Sc., M.Eng. (Cornell, USA),Technip Geoproduction (M) Sdn Bhd

Synopsis

The main purpose of Offshore Platforms is tosupport facilities required for Oil & Gas Drilling,Production and Export. This paper provides anintroduction to Offshore Structural Engineeringthrough references to both fixed and floatingplatform concepts, main design requirementsand the way temporary phases, marineenvironment and soil conditions at site affectdesign.

Modern platform structural design is based ondetailed knowledge of design procedures,international codes and standards reflecting thelatest developments within research andexperience. This in addition to classical theories,extensive use of sophisticated computerprograms and creativity as frontiers arecontinuously moved. Unique challenges foroffshore platforms are mainly from:

• Designing to resist forces from naturalphenomena (waves, current, hydrostaticpressure, wind and earthquake), corrosioncaused by the marine environment,

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Designing for fatigue caused by the cyclicnature of wind and waves for a duration of25 to 50 years,

• Designing the topside (offshore plant) withinextremely confined spaces,

• Designing to maximize completion atconstruction site and to minimize offshoreconstruction work,

• Designing for temporary phases(construction, loading out, sea transport andoffshore installation).

Notations

1) FPSO Floating Production Storageand Offloading

2) GBS Gravity Base Structure3) LMU Leg Mating Unit4) MT Metric Tones5) MODU Mobile Offshore Drilling Units6) SSCV Semi Submersible Crane

Vessel7) TLP Tension Leg Platform

1. Introduction

The structural engineering profession plays animportant role in design of offshore platforms forproduction of hydrocarbons oil & gas. Rapiddevelopments have taken place the last fiftyyears since the first fixed offshore platform wasinstalled in more than 30meters (100ft) water

depth in the Gulf of Mexico. Today more than9,000 fixed platforms have been installed worldwide in water-depths up to 400m (BullwinckleJacket in Gulf of Mexico) and has reachedinstallation weight of more than 650,000 metrictones (Troll A GBS in the North Sea). Floatingproduction platforms are taking fielddevelopments into deeper waters with severalinstallations in more than 2000m water depth(Gulf of Mexico). In South-East Asia the firstdeep water TLP platform was installed at WestSeno field in Indonesia in 1,000m water depth,while the first deep water platform in Malaysianwaters is planned for installation in 2007, in

water depth of 1,330m (Kikeh Spar).

Initial field development phase includes seismicsurvey and exploration drilling using either jack-up rigs for water depth up to around 100m, orMODU (Mobile Offshore Drilling Units) whichare jack-ups, semi submersibles and drill shipsfor deeper water depths. To assess if furtherdevelopments are technically and commercialviable, decisions that affect structural designmust be made on:

• Selection of platform concept (fixed orfloating platform),

• Extent of processing to be done offshore

(none, partial or full),• Type of drilling facilities (MODU, tender

assisted or platform rig),

• Staffed or un-staffed,

• Export of oil & gas (riser, shuttle-tanker,power-plant, re-injection).

Figure-1 shows a typical offshore fixed platformtopside supported by a jacket (sub-structure).

Figure 1 – Fixed Platform, Malaysia Water.



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The jacket itself is usually a tower like bracedtubular structure that has two main functions;firstly to support for the production facility(topside) to keep it stable and clear of the wavecrests, and secondly to provide lateral supportsand protect well conductors and pipelines.

The main functions of typical platform topside

are:(a) drilling and well control,(b) separation of oil, gas and byproducts (water,

sand, wax),(c) support for pumps and compressors

required for export of the producedproducts,

(d) power generation and(e) staff accommodation.

Platforms crew access can be by boat thatrequires a boat landing connected to the jacketat sea level, by helicopter that requires ahelicopter landing deck or bridge linked to

nearby platform. For material handling, a fixedplatform crane is required for lifting to and fromsupply boats.

2. Platform Types

Fixed platforms are mainly found for relativeshallow waters, with installations up to around400m water depth for jackets, 300m for GBSand 600m for Compliant Towers (see below fordescription of GBS and Compliant Towers). Floating platforms are utilized when water depthis rendering fixed platform concepts

uneconomical or technically unfeasible.Maximum water depth possible with presenttechnologies is around 3000m for floatingplatforms.

In addition to water depth, the main drivers forselection of platform type are topside weight,soil conditions, weather limits, chosen exportmethod (local storage or through pipelines),selection of dry or wet trees (main valves),availability and capability of fabrication yard, andavailable barges/vessels for transport andoffshore installation.

2.1 Fixed Platform

More than 90% of offshore platforms around theworld are supported by steel framed structures(Jackets). These platforms are typicallyanchored to the ocean floor using steel piles tocarry both lateral and vertical loads and arefabricated from tubular sections (Figure-1).

Several concrete Gravity Base Structureplatforms (GBS) have been installed mainly inthe North Sea, and provide advantages throughoil storage capacity, utilizing traditional civil

engineering technology, are mainly self-installedand prove low maintenance costs. The GBSstructure uses gravity combined with suctionestablished between the base and the seabedsoil to remain stable under loading from waves,wind and current.

Compliant towers are slender and non-battered jacket structures connected to the seabed by

means of piles and optional guy lines. Theplatform fundamental natural period around 30seconds, is about twice the storm wave periodensuring the required compliancy or de-amplification in response to wave loading. Thiscontrary to traditional fixed platform types thatare having an upper practical limit of around 5 to6 seconds only for the fundamental naturalperiod to avoid excessive dynamic amplificationand fatigue from wave loading.

Jack-ups (Figure-2) are self-elevated platformsthat have been in use since 1955 mainly fordrilling in water depths generally less than100m. In recent years, this platform type hasbeen introduced also as a permanent productionfacility either by converting existing platforms ornew builds. Jack-up platforms are usuallyhaving three or four vertical legs that support theplatform using special footings (spud cans) orone large single mat. This platform type ismobile as legs can be elevated allowing theplatform to relocate to new location (self-

installed).

Figure 2 – Jack-up (TPG500, ELGIN), North Sea. ©Technip.



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2.2 Floating Platforms

Semi-submersible production platforms (Figure-

3) are moored to the seabed using platforminertia and catenary mooring lines (or slackmooring) to restrict extreme lateral motionsduring a storm. The hull form is derived from atraditional drilling semi. By taking advantage ofbeing a stationary platform, a ring pontoonconfiguration can be utilized to accommodatehigher deck loading. Most commonly four or sixcolumns are connecting the pontoons to thedeck structure. Due to large heave motions, thekey factor is the seabed well completion (wettree) driving up costs, compared to on-platformwell completion (dry tree).

Floating Production Storage and Offloading(FPSO) (Figure-4) are ship-shaped vessels(mono-hulls) similar to trading tankers. The mainadvantage of FPSO is storage capacity atlocations having none or limited possibility forconnection to export pipelines. Similar to semisthe FPSO is connected to seabed using slackmooring and can only utilize wet tree due tolarge heave motions.

A Tension Leg Platform (TLP) is using a hullshape similar to semi-submersibles but ismoored using vertical tubular tendonsconnected to bottom of columns and anchoredto the seabed. For larger platforms, either threeor four columns are connecting the pontoons tothe deck structure, while for a smaller ormedium size platform a single column can beused as shown in Figure-5. The TLP is designedso that the tendons always stays in tension alsoduring a storm condition resulting in very small

vertical and angular motions with the advantagethat wellhead can be utilized, similar to a fixedor a compliant tower platform. For lateralmotions, the main resistance to wave loading isthrough inertia similar to semi-submersibles andFPSO.

Figure 3 – Semi-Submersible (EDP). ©Technip.

Figure 4 – FPSO (AKPO). ©Technip.

Figure 5 – TLP. © Atlantia Offshore Limited.



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The Spar platform (Figure-6) is essentially adeep draft cylinder with rigid risers protectedinside the central part of the cylinder. The Sparis gaining very small motions from having asmall water plane area and large mass with theadvantage that taut steel catenary moorings canbe utilized for all water depths.

Depending of required functionalities, topside(or deck) structures can be found havingfootprints more than 100x100m and total weightin excess of 50,000MT with structural steelusually making up around 50% of the totalweight. For substructures, the structural buildingmaterials usually make up around 90-95% of thetotal weight.

3. Design Criteria

For a specific platform project, structural design

has to comply with specific offshore structuraldesign procedures (codes), local governmentalregulations and developer (owner)specifications. Design procedures arenecessary to provide acceptable standard ofsafety by defining minimum requirements for thestructural design during pre-service phases(construction, load out, sea transportation andinstallation), in-service and during eventualremoval. These procedures will also serve asguidelines and technical reference documents incontractual matters.

For steel platforms utilizing large diametertubular structures (typically Jackets), designprocedures were originally developed by the oilindustry in the Gulf of Mexico. The AmericanPetroleum Institute (API) RecommendedPractice (RP) 2A was first issued in the late1960s, and has been updated regularly to reflectimprovements in knowledge and technology.

The current trend is towards internationalstandardization mainly through the InternationalStandards Organization (ISO) incorporatingupgraded design procedures compared to latestedition of API. The principal ISO standard forboth fixed and floating offshore platforms is ISO19900 (Petroleum and natural gas industries).

Governmental regulations also require thatrecognized bodies certify primary aspects of thestructural design with the consequence thatstructural design also must comply with designprocedures issued by relevant classificationbody. The major certification bodies are

American Bureau of Shipping (ABS), DetNorske Veritas (DnV), Lloyds Register ofShipping, Bureau Veritas and GermanisherLloyds.

Insurance companies covering temporaryphases also require the structural integrity to bereviewed and accepted by Warranty Surveyorswith the possibility that additional requirementsmust be considered in design.

4. Design Loads

For offshore platforms, it is essential toestimate, report and monitor platform weight(mass) with high accuracy throughout designand construction process, as operations duringtemporary phases and in-service conditions areusually sensitive even to small changes inweight and centre of gravity. Particularly fortopside structures weighing is usually requiredprior to load out using jacks and load cells thatcan provide weighing inaccuracy less than 1%.Thorough weight estimation and reporting willtypically achieve weights within 3% of the actualweighing results.

4.1 Gravity Loads

For structural design, the gravity loads, bothpermanent and variable loads need to beestimated. Permanent loads can best bedescribed as loads that will not vary inmagnitude, position or direction during the timeperiod considered. Examples are:(a) Topside dead weight (structural steel,

equipments, piping and buildings),(b) Sub-structure dead weight (structural steel,

anodes and timber mud-mats),(c) Hydrostatic pressure and

Figure 6 – Truss Spar platform. ©Technip



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(d) Permanent ballast.

Variable loads originates from normal operationof the platform and vary in magnitude anddirection during the period considered.Examples are:(a) Liquids in equipments, tanks and piping,(b) Drilling and crane operations,(c) Crew and helicopter and

(d) Variable ballast.

4.2 Environmental Loads

A major challenge in structural design ofoffshore platforms is to consider the often-dominating effect of the environmental loadingimposed on the platform by natural phenomenalike wind, waves, current and earthquake. Insome regions even snow, ice build-up andicebergs must be considered. Waves and tidalvariations will also cause loading effects fromchanges in hydrostatic pressure and buoyancy.Marine growth and possible seabed subsidence

will also cause increase in loadings for bothfixed and heave compensated platforms.

For platform in-service condition, the magnitudeof environmental loads is determined fromstatistical analyses setting annual probability ofexceedance typically equal to 10

-2. For a storm

wave condition, this is equal to a wave that inaverage will be exceeded every hundred year(100year wave). When theoretical predictions ofenvironmental loads are subjected to significantuncertainties, hydrodynamic model tests, windmodel tests or observations from existingstructures can be used to support theoretical

predictions.

During temporary phases, the magnitude ofenvironmental loads is usually limited byoperational requirements. Too strict limitationson wave conditions for a sea transport maycause unnecessary delays and cost increase ifweather routing is necessary to satisfy the wavecriteria.

4.3 Deformation

Loads originating from deformations imposed onthe structure by foundation settlements, duringfabrication, during in-service and fromtemperature changes must be assessed andconsidered in the design. Deflections will alsobe imposed during load out, sea transport andlifting and may cause extra offshore work ifpiping must be disconnected from equipmentflanges to avoid overstress during temporaryphases.

4.4 Accidental Loads

Another common design requirement foroffshore platforms is the ability to sustain

accidental loading typically caused byhydrocarbon jet and pool fires, hydrocarbon gasexplosion (blast), dropped objects, helicoptercrash, boat impacts and accidental flooding ofcompartments. Magnitude and extent ofaccidental loads is normally determined fromrisk analyses based on annual exceedanceprobability of typically 10

-4 per installation.

5. Computer Modelling and Analyses

The global response of most types of platformscan be determined from a linear elastic finiteelement analysis (FEA) model of the platformand it is usually sufficient to use a relativecoarse mesh made from 2-noded beamelements to describe the three-dimensionalgeometry of the platform. Of main interest is toextract internal member forces and moments toconfirm structural integrity against coderequirements.

The main criteria that usually can be confirmedusing automatic computer program proceduresare (a) check for acceptable stress level, (b)check for global and local instability (buckling) ofmembers, (c) check for buckling of plates,stiffeners and stiffened panels and (d) check forpunching shear of tubular connections. Inaddition, check for unusual connectionconfigurations, lifting joints or other local designfeatures usually must be verified throughmanual procedures or FEA using shell orvolume elements.

Displacements and vibrations must be extracted

and verified against given limits, and supportreactions must be extracted and verified againstrequired foundation capacity or used infoundation design. For temporary phasessupport reactions are of importance to (a)confirm capacities of temporary supportstructures during construction, (b) verification ofskid ways, (c) design of barge grillage and seafastening and (d) verification of transport bargelocal strength.

Although floating platform (TLP and semi-submersibles) in the early phases of adevelopment can be analyzed effectively usingbeam models, relative coarse mesh plate/shellelement models are more commonly used forfinal global design. For floating platforms, allhydrodynamic loading effects onto the floatingbody and at interfaces (mooring, risers, tethers)are calculated using Naval Architecturalprograms and transferred to the structuralsoftware. As the body to be analyzed is floating,only soft springs are provided to restrain againstrigid body movements. This is causing negligibledummy reactions as the combined loads arehaving zero load balance at any time step.



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Locations of particular interest can have refinedmodeling to determine load pattern in the caseof complicated geometry and discontinuities.Typical examples are interface between pilesleeves and legs for jackets and interfacebetween pontoons and columns for TLP, semi-submersibles and Spars.

Static analysis is appropriate when dynamicbehavior is negligible, while Quasi-staticanalysis is sufficient when dynamic effects canbe assumed small and uniform throughout theplatform so that a static analysis in combinationof a small dynamic correction factor issatisfactory.

Dynamic behavior is likely to be of importance ifthe fundamental natural period is relative closeto the period of excitation. Fixed platforms withfundamental natural periods greater than 2.5-3seconds are usually sensitive to dynamic effectsfrom wave loads, and the practical upper limitfor the fundamental natural period is around 5seconds. To determine the platform dynamicsensitivity a natural vibration analysis must becarried out based on a linear elastic modelmodified by converting static loads into a massmodel and by linearization of the pile-soilinteraction. If a fixed platform is proven dynamicsensitive, linear dynamic analysis are usuallycarried out for in-service storm wave and fatigueconditions.

Dynamic analysis may also be required to provesufficiently small vibration amplitudes duringoperation for rotating equipments, particularly ifun-balanced forces and moments as in the caseof reciprocating machinery.

Non-linear analysis may be required in order tosimulate non-linear effects from loading,geometry or material properties. Common

examples are (1) Global response of a jack-upplatform considering non-linear geometry (P-Delta) effects, (2) Accidental fire scenarioconsidering non-linear material properties(Figure-7) and (3) Accidental boat impactscenario considering non-linear materialproperties.

To simulate boundary conditions for fixed

platforms it is common to model the non-linearinteraction between piles and the soil toconsider that soil stiffness vary with deflectionlevel for both axial and lateral loading. Datamust be site specific to provide reliable results.Particular care must be taken in modeling therelative thin layers near seabed as the lateralsoil stiffness change rapidly. This is significantfor bending moments imposed on the piles atseabed level.

Offshore platforms are susceptible to fatiguefailure during the platform design life due to thecumulative effect of cyclic loads induced by wind

and waves. Wind fatigue is of major importancefor flare and vent structures typically attached tothe topside, while fatigue from waves need to beconsidered for both fixed and floating platformssubstructures and topsides.

6. Foundation design

Essential for foundation design is availability ofreliable and site-specific soil investigation datathat consider type and size of platform,foundation method, uniformity of soil, type of soildeposits and seabed conditions. The soil

investigation must also consider requirementsapplicable for temporary stability during platforminstallation.

Platform foundation design must considernecessary requirements to carry static, cyclicand transient loads without failure, excessivedeformations or vibrations. To ensure load-deflection compatibility between the platformstructure and the foundation/soil system, it isnecessary to perform an integrated analysistaking into account the non-linear properties ofthe soil, or alternatively the linearized soilresponse for a specified load level.

The most common type of foundation for fixedoffshore steel platforms are open-ended steelpiles driven into the seabed using impacthammers. For problem soils such as carbonatematerials resulting in very low friction resistance,pile capacity may benefit significantly frominstallation by drilling and grouting method.Open-ended steel piles can have diameter morethan 2m and penetration depth in excess of100m. The piles can be driven either throughthe jacket main legs using a hammer operatingabove sea level, or underwater driving through

Figure 7 – Plastic utilization from a topside fire analysis scenario.



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pile sleeves connected to the jacket legs at sea-floor level. The underwater sleeves can begrouped in clusters around the jacket legs eitheras vertically sleeves or as inclined sleeves. Thelatter arrangement will provide a stifferfoundation due to increased base at the bottomof the piles at the cost of added steel weight andmore complicated pile driving.

Design pile shear loads are determined fromplatform maximum base shear (BS), whiledesign axial loads are determined from platformoverturning moment (OTM) combined with deadweight and operational loads like drilling. Baseshear is determined mainly from environmentalloads like waves, current, wind and earthquake.Due to large OTM, piles commonly must bedesigned for uplift forces as well.

Pile penetration depth is calculated frommaximum axial force in the pile balanced withresistance from pile skin friction and tipresistance. Soil for the upper strata near seabed

can be very soft with the implication thatmaximum pile moments caused by platformbase shear can be located typically more than5-10 times pile diameter below sea floor, andwill be the controlling factor for pile wallthickness around seabed level. Pile design mustalso consider maximum stresses during pilingoperation and possible fatigue from both piledriving and platform in-service.

7. Fabrication

The main goal with onshore fabrication of

offshore platforms is to complete as much workas possible on land under conditions that are farmore productive, produce higher quality andinvolve less risk than offshore works. Some ofthe principles that are followed to reduceoffshore work are (a) Fabricate as largecomponents as possible, (b) Simplification andreduction of number of Offshore (Hook-up)interfaces.

In order to reduce fabrication lead times it isimportant that design consider (a) Concurrentfabrication of subassemblies, (b) Simplifyplatform configuration and use standardizeddetails, (c) Optimization of welding andinspection, and (d) Reduce number of materialgrades and sizes.

Offshore platforms are usually fabricated at siteswith easy access to sea or river for loading outthe structure after completion of fabrication. Seaaccess is also essential for receivingconstruction materials, equipments and pre-fabricated sub-assemblies completed at othersites. Of major importance for larger structuresis setting out fabrication supports locations andheight to accommodate chosen arrangement for

load out, selected transport barge depth, quaywater-depth and tidal variations.

Fixed platform jacket structures can be built indifferent ways depending of size and weight.Smaller jackets can be built standing vertical,while larger jackets are assembled on their side.Complete panels can be built on the ground androlled up by lifting. Figure-8 shows section rollupfor a 6,500MT float-over jacket for Malaysianwaters.

Dry-docks are used for both topsides andsubstructures. A topside structure can befabricated inside a dry dock supported ontotemporary supports allowing for transfer(mating) onto a barge before load out. For aconcrete gravity base structure (GBS), the

bottom part of the substructure can befabricated inside the dry-dock and completion ofthe fabrication outside the dry-dock (in shelteredwaters) allowing for completion of very largestructures.

Structural design must consider loads andsupport conditions that are imposed duringfabrication and assembly. Of particularimportance is to consider effects from jacksused during assembly of sections as thesemechanically imposed forces will be built inpermanently. Foundation settlements must bemonitored to avoid building in permanent

deflections and to prevent overstress of eitherthe structure or the foundation.

8. Load Out and Transportation

After completion of fabrication of the structure, aload out operation will take place to move thestructure from fabrication site onto a transportbarge or a self-propelled vessel (ship) fortransport to offshore site.

Figure 8 – Lifting of jacket structure section at fabrication site.



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8.1 Load Out

The by far most common method for all size ofstructures is skidding the structure along two (ormore) skid-ways connected with the barge skid

beams. The structure is supported onto skid-shoes (or launch runners) resting on the skidbeams during fabrication, and pulled byhydraulic strand-jacks anchored at the end ofeach skid-way on the barge. To achieve lowfriction, top of skid beams is usually steel platecovered with teflon, while bottom of the skidshoe is covered by timber or teflon. Whilecontinuous skidding can show friction as low as5-8%, the entire system must be designed forbreak-off friction as high as 20-25% usinghydraulic break-off jacks to assist. The mostcritical phase is when the skid-shoes areentering the barge, as increase of loading onto

the barge from the skid shoes will reduce thebarge freeboard, which eventually must becompensated by de-ballasting. Temporarily theskid shoe may drop as much as 25-50mm whenentering the barge and this scenario must beconsidered in structural design. Figure-9 showsa skidded load out of a 17,000 MT float-overtopside for Malaysian waters. The topside isskidded along two parallel skid ways onto a 42mwide launch barge.

An alternative system in use is skid-shoessupporting the structure onto vertical jacks forload control, and eventually equipped with

individual hydraulic jacks for pulling of eachskid-shoe instead of strand-jacks. The vertical jacks will automatically change stroke to keepsupport reactions constant during the entire loadout operation.

While skidding can achieve speed of typical 5-10meters/hour, much faster operation can beachieved using self-propelled trailers with thespeed limited by barge ballast capacity (Figure-10). Trailers also provide load control ashydraulic jacks as each axle are changingstroke to keep loading constant during the entire

operation. Local undulations in the ground,sloping ramps or small changes in barge draftwill not cause additional loading into thestructure. Trailers have been used for load outof topsides structures up to around 11,000MT.

Lifting is a common load out method utilizingyard cranes, mobile/crawler cranes or bargecranes for smaller structures. Occasionally alsooffshore crane vessels can be used for load out,sea transport and offshore installation if proveneconomical. Lifting arrangements vary withavailable equipments and type of structure.Spreader beams are often used to minimizelateral load components into the structure or dueto limitations in sling arrangement.

For large fixed platforms or floating platforms,inshore mating of topside and hull structure isoften required. Typically for semi-submersiblesand TLP, the hull structure will be fabricated atlocation different from the topside and will arrivetowed as free floating or on a heavy lift vessel(ship). At the same time, the topside will beloaded out on a barge. The hull structure will beput in a ballast condition with top of columnsonly a few meter above water level, and thebarge with the topside on top will be positionedbetween the columns and ballasted for contactand full load transfer onto the hull columns.

After completion of the load out operation onto

the barge, the structure will be secured for thesea transport by adding sea fastening. Onlyforces caused by barge motions need to beconsidered as the sea-fastening is welded inafter barge is ballasted to sailing condition.

Figure 9 – Skidded Load Out of 17,000MT topside for Malaysian waters.

Figure 10 – Trailer Load Out of 3,500MT topside.



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8.2 Transportation

For sea transport, the most common method isa flat top barge that needs additional tugboats to

pull the barge. Tow speed is limited to maximum5-6knots. Flat top barges are usually required iflongitudinal load out or longitudinal offshorelaunch is essential (Figure-11). If longer seatransport duration and ballast flexibility isimportant a self-propelled vessels (ships) can beused. Speed of 12-14knots can easily beachieved and motions can be reduced throughflexible tank arrangements using double bottomand deck tanks and option for weather routingdue to advantages in operability compared to aflat top barge.

During sea transport on a barge or ship, motions

caused by waves and wind can cause the bargeto roll more than 15˚ and pitch more than 5˚.The consequence is that the platform often mustbe to be designed for lateral dynamic loads inexcess of 0.5 times gravity acceleration (g) andvertical dynamic loads in excess of 0.3g causingsignificant amount of material to be added forsea fastening to secure the cargo.

For floating platforms, the final transport to siteis usually by wet tow using tugboats. It must be

emphasized that early selection and booking oftransport barge or ship is critical due to limitedworldwide availability for transport of largertopsides or substructures.

9. Offshore Installation

Offshore installation phase is critical due to the

marine operational costs and risks involved.Lifting is the most common method for smallerand medium size platform topsides andsubstructures. Crane vessels can be of mono-hull type with a single crane or a semisubmersible crane vessel (SSCV) equipped withtwo cranes. For practical purposes, SSCVutilizing tandem lifts are currently limited to onlytwo vessels world-wide capable to liftapproximately 11,000MT as proven for topsidestructures in the North sea. For lifts in thisrange, sling diameter is around 350-400mm withsafe braking loads of around 2,500MT, andarranged as double-double slings connected to

pad ears using pins and sheaves. Design loadfor a single pad ear can typically be around7,000MT. A number of jackets in the North Seain the range of 7,000MT have been directly liftedand upended (when required) using SSCV.Direct lifting is beneficial for structural weight asadditional buoyancy tanks are not required,which is often the case if the jacket must beupended while floating in the water. Mobilizationcosts are a major hindrance for the large SSCVto be considered for instance in Asia and Asia-Pacific region where the currently operating lift-vessels are having maximum capacity ofapproximately 3,000MT.

For jackets, it is common to launch thesubstructure conventionally from the stern of thebarge using tilt beams to reduce reaction intothe jacket structure. In the case of complianttowers, the structure can also be side launchedto avoid large bending of the structure. Thelaunch operation requires the barge to beballasted with a trim up to 5˚ at the stern.Hydraulic skid units along the barge skid wayswill then push the jacket and initiate the

Figure 11 – Jacket on transportation barge, Malaysian Water.

Figure 12 – Launch of jacket in Malaysian waters.



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launching motion. Jackets are usually launchedwith the top of the platform first into the water. Alternatively, a reverse launch operation will putthe jacket bottom first into the water (Figure-12). After launching the platform will be free-floatingin the water, or eventually self-upend to up-rightposition if this is planned for by free floodingspecific compartments. Due to water depthlimitations, the launching operation may take

place away from the actual platform site with theconsequence that the structure must be towedfree floating to the actual location. At the actualsite, the jacket must be upended to uprightposition by controlled flooding of compartments,by crane hook assistance or by combination ofboth until final position is reached at seabedlevel. In the case of a self upended jacket,flooding is only required to set platform at thefinal location.

A launched jacket usually requires separatebuoyancy tanks to ensure sufficient clearance toseabed during the launching operation and

additional steelwork (launch truss) is required tosupport the jacket onto the skid ways andlaunch runners. Combined this add significanttemporary weight to the jacket compared to a liftinstalled platform.

Before piling operation take place the jacketsubmerged weight is usually supported by mudmats made from either steel plates or timberand located at jacket seabed level to ensurestability for overturning or sliding when exposedto waves, current, wind and eventual weightsfrom piles and hammer during pile installation.Skirts can be added to the mud mats to

penetrate into the upper soil layer to provideadditional stability before the piles are installed.To reduce amount of materials only to be usedfor a short temporary phase, a number of pilescan be stabbed after set-down of the jacket.These piles will self-penetrate into the soil fromself-weight and provide additional slidingresistance and some overturning resistance ifinclined.

Pile installation itself is the most time-consuminginstallation job and is heavily relying oninevitable uncertainties in soil data. This phaseis also critical for jacket stability as any weight ofpiles and eventual hammer weight addingweight to the jacket can be the governing casefor platform stability until the piles are able tosecure the platform. Pile drivability can besimulated in advance using special computerprogram considering the passage of stresswave down the pile (wave equation theory) foran idealized model of the entire hammer-pile-soil system. After driving the piles to targetpenetration, top of the piles will be fixed to the jacket by either welding or grouting.

For relative shallow water depths, a self-installed platform or jack-up platform can takeaway the requirement for a heavy lift vessel.The platform can be transported on a barge ortowed to the site, and installed by loweringplatform legs down to seabed. The legs can besupported using spud-cans or large tanks thatcan be flooded and act as gravity foundation.

Offshore installation of topsides using float-overmethod (Figure-12) have been done for topsidesweights up to 17,000MT, and even largertopsides of 25,000MT are under constructionplanned for offshore float-over installation. Thetopside will be transported to site on a flat-topbarge (or self propelled heavy lift vessel) andinstalled by lowering (de-ballasting) the bargeuntil complete load transfer of the topside weightfrom the barge to the substructure that can be afixed platform (jacket, gravity base structure) or

a floating platform (Spar) in which case twobarges will be required. To achieve a smoothload transfer, leg-mating units (LMU) can belocated inside either jacket or topside legs andthese units will absorb impact loads at matinginterface induced by barge motions. LMU areusually made up of rubber designed tocompress under impact and load transfer. Forfixed platforms, hydraulic jacks can be used inaddition to de-ballasting mainly to increase thespeed, hence reduce the risk for large swellinduced motions during critical phase. Forfloating platforms, jacks cannot be usedeffectively as the platform will loose freeboard

that can only be compensated by de-ballastingof the floating platform itself.

Due to possible limitations in float-over slotdepth for jackets, the entire topside supportedon the LMU may have to be lowered to achievecontact with the substructure after removal ofthe barge. If this is the case the LMU’s cantemporarily be supported onto a thick sand layerinside the (jacket) legs, and the sand will laterbe drained out simultaneously for all legs until

Figure 12 – Topside installation, floatover method, Malaysian Water.

Introduction to Offshore Structural Engineering

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