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CONSTRUCTION

TECHNOLOGY &

maintenance

CEM 417

Stages for construction1. Building

2. Retaining walls, Drainage

3. Road, Highway, Bridges

4. Airports, Offshore/Marine structure

AIRPORT/AIRFEILDS,

OFFSHORE/MARINE

STRUCTURE

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At the end of lectures, student will be able to

:

- Identify the different types of airfields and

marine structures and their respective

functions. (CO1; CO3)

Reference:-

http://www.globalsecurity.org/military/library/policy/army/fm/5-430-00-2/Ch11.htm

http://www.tpub.com/content/engineering/14071/css/14071_80.htm

AIRFIELDS

• Road construction and airfield construction have

much in common, such as construction methods,

equipment used, and sequence of operations.

• Each road or airfield requires a subgrade, base course,

and surface course.

• The methods of cutting and falling, grading and

compacting, and surfacing are all similar. As with roads,

the responsibility for designing and laying out lies with

the same person the engineering officer.

• Again, as

previously said for roads, you can expect involveme

nt when airfield projects occur.

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RUNWAY DESIGN CRITERIARunway location, length, and alignment are the foremost

design criteria in any airfield plan. The major factors that

influence these three criteria are--

1. Type of using aircraft.

2. Local climate.

3. Prevailing winds.

4. Topography (drainage, earthwork, and clearing).

LocationSelect the site using the runway as the feature foremost in

mind. Also consider topography, prevailing wind, type of

soil, drainage characteristics. and the amount of clearing

and earthwork necessary when selecting the site

AIRFIELD DESIGN STEPSThe following is a procedural guide to complete a

comprehensive airfield design. The concepts and required

information are discussed later in this chapter.

1. Select the runway location.

2. Determine the runway length and width.

3. Calculate the approach zones.

4. Determine the runway orientation based on the wind

rose.

5. Plot the centerline on graph paper, design the vertical

alignment, and plot the newly designed airfield on the

plan and profile.

6. Design transverse slopes.

7. Design taxiways and aprons.

8. Design required drainage structures.

9. Select visual and nonvisual aids to navigation.

10. Design logistical support facilities.

11. Design aircraft protection facilities.

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Length

When determining the runway length required for any

aircraft, include the surface required for landing rolls or

takeoff runs and a reasonable allowance for variations in pilot

technique; psychological factors; wind, snow, or other

surface conditions; and unforeseen mechanical failure.

Determine runway length by applying several correction

factors and a factor of safety to the takeoff ground run (TGR)

established for the geographic and climatic conditions at the

installation. Air density, which is governed by temperature

and pressure at the site, greatly affects the ground run

required for any type aircraft. Increases in either temperature

or altitude reduce the density of air and increase the required

ground run. Therefore, the length of runway required for a

specific type of aircraft varies with the geographic location.

The length of every airfield must be computed based on the

average maximum temperature and the pressure altitude of

the site.

At the top is the Surface Course which is usually an asphalt or Portland

cement concrete material. Bound surfaces such as these provide stability and

durability for year-round traffic operations. Asphalt surfaces are from 5 to 10 cm

(2 to 4 inches) thick and concrete surfaces from 23 to 40 cm (9 to 16 inches)

thick.

The next layer is the Base Course - a high quality crushed stone or gravel

material necessary to ensure stability under high aircraft tire pressures. Bases

vary in thickness from 15 to 30 cm (6 to 12 inches).

The bottom layer is the Subbase Course which is constructed with non-frost

susceptible but lower quality granular aggregates. Subbases increase the

pavement strength and reduce the effects of frost action on the

subgrade. Subbase thicknesses are usually 30 cm (12 inches) or more.

These three (3) layers (Surface, Base and Subbase Courses) have a

combined thickness of 60 to 150 cm (2 to 5 feet) and are placed on the

subgrade - the pavement foundation.

The Subgrade is the natural in-situ soil material which has been cut to grade,

or in a fill section, is imported common material built up over the in-situ

material. The subgrade must provide a stable and uniform support for the

overlying pavement structure.

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PLANNING AN AIRFIELD

Planning for aviation facilities requires special

consideration of

1. the type of aircraft to be accommodated;

2. physical conditions of the site, including weather

conditions, terrain, soil, and

availability of construction materials;

3. safety factors, such as approach zone obstructions and

traffic control;

4. the provision for expansion;

5. and defense.

6. Under wartime conditions, tactical considerations are also

required.

All of these factors affect the number, orientation, and

dimensions of runways, taxiways, aprons, hardstands,

hangars, and other facilities.

SUBBASE AND BASE COURSE

Pavements (including the surface and underlying Courses)

may be divided into two classes—rigid and flexible.

The wearing surface of a rigid pavement is constructed of portland

cement concrete.

Its flexural strength enables it to act as abeam and allows it to

bridge over minor irregularities in the base or subgrade up on

which it rests.

All other pavements are classified as flexible.

Any distortion or displacement in the subgrade of a flexible

pavement is reflected in the base course and upward into the

surface course.

These courses tend to conform to the same shape under traffic.

Flexible

pavements are used almost exclusively in the operations for

road and airfield construction since they adapt to nearly all

situations and can be built by any construction battalion unit in the

Naval Construction Force (NCF) ate.

FLEXIBLE PAVEMENT STRUCTURE

A typical flexible pavement is constructed as shown below, which

also defines the parts or layers of pavement.

All layers shown in the figure are not presenting every flexible

pavement.

For example, a two-layer structure consists of a compacted

subgrade and a base course only.

Figure shows a typical flexible pavement using stabilized layers.

(The word pavement, when used by itself, refers only to the

leveling, binder, and surface course, whereas flexible

pavement refers to

the entire pavement structure from the subgrade up.)

The use of flexible pavements on airfields must be limited to

paved areas not subjected to detrimental effects of jet fuel spillage

and jet blast. In fact, their use is prohibited in areas where these

effects are severe.

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specified in certain

criticaloperational areas.MATERIALSSelect materials will normally be locally availableco

arse-grained soils, although fine-grained soils maybeused in certain cases. Lime rock, coral, shell,

ashes,cinders, caliche, disintegrated granite, and other suchmaterials should be considered

when they areeconomical.SubbaseSubbase materials may consist of naturallyoccurring coar

se-

grained soils or blended and processedsoils. Materials, such as lime rock, coral, shell, as

hes,cinders, caliche, and disintegrated granite, maybe

usedas subbases when they meet area specifications orproject specifications. Materials sta

bilized withcommercial admixes may be economical as subbases incertain instances. Portland

cement, cutback asphalt,emulsified asphalt, and tar are commonly used for

thispurpose.Base CourseA wide variety of gravels, sands, gravelly and sandysoils, and other

natural materials such as lime rock,corals, shells, and some caliches can be used alone

orblended to provide satisfactory base courses. In someinstances, natural materials will

require crushing orremoval of the oversize fraction to maintain gradationlimits. Other natural

materials may be controlled bymixing crushed and pit-

run materials to form asatisfactory base course material.Many natural deposits of sandy a

nd gravellymaterials also make satisfactory base materials. Graveldeposits vary widely in

the relative proportions ofcoarse and fine material and in the character of the rockfragments.

Satisfactory base materials often can

beproduced by blending materials from two or moredeposits. Abase course made from sandy

and gravel] ymaterial has a high-bearing value and can be used

tosupport heavy loads. However, uncrushed, cleanwashed gravel is not satisfactory for

a base coursebecause the fine material, which acts as the binder

andfills the void between coarser aggregate, has beenwashed away.Sand and clay in a natural

mixture maybe found inalluvial deposits varying in thickness from 1 to 20

content.With proper proportioning and construction methods,satisfactory results can be obt

ained with sand-clay soil.It is excellent in construction where a higher type ofsurface is to be

added later.Processed materials are prepared by crushing

andscreening rock, gravel, or slag. A properly gradedcrushed-

rock base produced from sound, durable rockparticles makes the highest quality of any base

material.Crushed rock may be produced from almost any type ofrock that is hard enough to

require drilling, blasting, andcrushing. Existing quarries, ledge rock, cobbles

andgravel, talus deposits, coarse mine tailings, and similarhard, durable rock fragments are

the usual sources ofprocessed materials. Materials that crumble on exposureto air or water should

not be used. Nor should processedmaterials be used when gravel or sand-clay is available,except

when studies show that the use of processedmaterials will save time and effort when they are

madenecessary by project requirements. Bases made

fromprocessed materials can be divided into three generaltypes-

stabilized, coarse graded, and macadam. Astabilized base is one in which all material ranging

fromcoarse to fine is intimately mixed either before or as thematerial is laid into place. A coarse-

graded base iscomposed of crushed rock, gravel, or slag. This

basemay be used to advantage when it is necessary toproduce crushed rock, gravel, or slag

on site or whencommercial aggregates are available. A macadam baseis one where a coarse,

crushed aggregate is placed in arelatively thin layer and rolled into place; then fineaggregate or

screenings are placed on the surface of thecoarse-

aggregate layer and rolled and broomed into thecoarse rock until it is thoroughly keyed in

place. Watermay be used in the compacting and keying process.When water is used, the base is a

water-bound macadam.The crushed rock used for macadam bases shouldconsist of clean,

angular, durable particles free of

clay,organic matter, and other objectional material orcoating. Any hard, durable crushed a

ggregate can beused, provided the coarse aggregate is primarily one sizeand the fine aggregate

will key into the coarse aggregate

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Definition of Airport Categories

1. Commercial Service Airports are publicly owned airports that have at

least 2,500 passenger boardings each calendar year and receive

scheduled passenger service.

2. Nonprimary Commercial Service Airports are Commercial Service

Airports that have at least 2,500 and no more than 10,000 passenger

boardings each year.

3. Primary Airports are Commercial Service Airports that have more than

10,000 passenger boardings each year.

4. Cargo Service Airports are airports that, in addition to any other air

transportation services that may be available, are served by aircraft

providing air transportation of only cargo with a total annual landed weight

of more than 100 million pounds.

5. Reliever Airports are airports designated by the FAA to relieve congestion

at Commercial Service Airports and to provide improved general aviation

access to the overall community. These may be publicly or privately-owned.

commonly described as General Aviation Airports.

http://www.faa.gov/airports/planning_capacity/passenger_allcargo_stats/categories/

TYPE OFFSHORE

STRUCTURE

TYPE OFFSHORE

STRUCTURE

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TYPE OFFSHORE

STRUCTURE

TYPE OFFSHORE

STRUCTURE

TYPE OFFSHORE

STRUCTURE

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OFFSHORE

PLATFORM

DESIGN

Offshore platforms are used for

exploration of Oil and Gas from

under Seabed and processing.

The First Offshore platform was

installed in 1947 off the coast of

Louisiana in 6M depth of water.

Today there are over 7,000

Offshore platforms around the

world in water depths up to

1,850M

OVERVIEW

Platform size depends on facilities to be

installed on top side eg. Oil rig, living

quarters, Helipad etc.

Classification of water depths:

< 350 M- Shallow water

< 1500 M - Deep water

> 1500 M- Ultra deep water

US Mineral Management Service (MMS)

classifies water depths greater than 1,300

ft as deepwater, and greater than 5,000 ft

as ultra-deepwater.

Offshore platforms can broadly categorized

in two types.

Fixed structures that extend to the Seabed.

Steel Jacket

Concrete gravity Structure

Compliant Tower

Structures that float near the water

surface- Recent development

Tension Leg platforms

Semi Submersible

Spar

Ship shaped vessel (FPSO)

OVERVIEW

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• Space framed structure with

tubular members supported on

piled foundations.

• Used for moderate water depths

up to 400 M.

• Jackets provides protective

layer around the pipes.

• Typical offshore structure will

have a deck structure

containing a Main Deck, a

Cellar Deck, and a Helideck.

• The deck structure is supported

by deck legs connected to the

top of the piles. The piles

extend from above the Mean

Low Water through the seabed

and into the soil.

TYPE OF PLATFORMS (FIXED)

JACKETED PLATFORM

• Underwater, the piles are contained

inside the legs of a “jacket” structure

which serves as bracing for the piles

against lateral loads.

• The jacket also serves as a template

for the initial driving of the piles.

(The piles are driven through the

inside of the legs of the jacket

structure).

• Natural period (usually 2.5 second)

is kept below wave period (14 to 20

seconds) to avoid amplification of

wave loads.

• 95% of offshore platforms around

the world are Jacket supported.

• Narrow, flexible framed structures

supported by piled foundations.

• Has no oil storage capacity.

Production is through tensioned

rigid risers and export by flexible

or catenary steel pipe.

• Undergo large lateral deflections

(up to 10 ft) under wave loading.

Used for moderate water depths

up to 600 M.

• Natural period (usually 30

second) is kept above wave

period (14 to 20 seconds) to

avoid amplification of wave loads.

TYPE OF PLATFORMS (FIXED)

COMPLIANT TOWER

• Fixed-bottom structures made from

concrete

• Heavy and remain in place on the

seabed without the need for piles

• Used for moderate water depths

up to 300 M.

• Part construction is made in a dry

dock adjacent to the sea. The

structure is built from bottom up,

like onshore structure.

• At a certain point , dock is flooded

and the partially built structure

floats. It is towed to deeper

sheltered water where remaining

construction is completed.

• After towing to field, base is filled

with water to sink it on the seabed.

• Advantage- Less maintenance

TYPE OF PLATFORMS (FIXED)

CONCRETE GRAVITY STRUCTURES

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• Tension Leg Platforms (TLPs) are

floating facilities that are tied down

to the seabed by vertical steel

tubes called tethers.

• This characteristic makes the

structure very rigid in the vertical

direction and very flexible in the

horizontal plane. The vertical

rigidity helps to tie in wells for

production, while, the horizontal

compliance makes the platform

insensitive to the primary effect of

waves.

• Have large columns and Pontoons

and a fairly deep draught.

TYPE OF PLATFORMS (FLOATER)

Tension Leg Platform (TLP)

• TLP has excess buoyancy which

keeps tethers in tension. Topside

facilities , no. of risers etc. have to

fixed at pre-design stage.

• Used for deep water up to 1200 M

• It has no integral storage.

• It is sensitive to topside

load/draught variations as tether

tensions are affected.

TYPE OF PLATFORMS (FLOATER)

SEMISUB PLATFORM

• Due to small water plane area ,

they are weight sensitive. Flood

warning systems are required to

be in-place.

• Topside facilities , no. of risers etc.

have to fixed at pre-design stage.

• Used for Ultra deep water.

• Semi-submersibles are held in

place by anchors connected to a

catenary mooring system.

• Column pontoon junctions and

bracing attract large loads.

• Due to possibility of fatigue

cracking of braces , periodic

inspection/ maintenance is

prerequisite

• Concept of a large diameter single

vertical cylinder supporting deck.

• These are a very new and

emerging concept: the first spar

platform, Neptune , was installed

off the USA coast in 1997 .

• Spar platforms have taut catenary

moorings and deep draught, hence

heave natural period is about 30

seconds.

• Used for Ultra deep water depth of

2300 M.

• The center of buoyancy is

considerably above center of

gravity , making Spar quite stable.

• Due to space restrictions in the

core, number of risers has to be

predetermined.

TYPE OF PLATFORMS (FLOATER)

SPAR

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• Ship-shape platforms are called

Floating Production, Storage and

Offloading (FPSO) facilities.

• FPSOs have integral oil storage

capability inside their hull. This

avoids a long and expensive

pipeline to shore.

• Can explore in remote and deep

water and also in marginal wells,

where building fixed platform and

piping is technically and

economically not feasible

• FPSOs are held in position over

the reservoir at a Single Point

Mooring (SPM). The vessel is

able to weathervane around the

mooring point so that it always

faces into the prevailing weather.

TYPE OF PLATFORMS (FLOATER)

SHIP SHAPED VESSEL (FPSO)

• Facilities are tailored to achieve

weight and space saving

• Incorporates process and utility

equipment

1. Drilling Rig

2. Injection Compressors

3. Gas Compressors

4. Gas Turbine Generators

5. Piping

6. HVAC

7. Instrumentation

• Accommodation for operating

personnel.

• Crane for equipment handling

• Helipad

PLATFORM PARTS

TOPSIDE

Used to tie platform in place

Material

1. Steel chain

2. Steel wire rope

a) Catenary shape due to

heavy weight.

b) Length of rope is more

3. Synthetic fiber rope

a) Taut shape due to

substantial less weight

than steel ropes.

b) Less rope length required

c) Corrosion free

PLATFORM PARTS

MOORINGS & ANCHORS

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• Pipes used for production,

drilling, and export of Oil and Gas

from Seabed.

• Riser system is a key component

for offshore drilling or floating

production projects.

• The cost and technical

challenges of the riser system

increase significantly with water

depth.

• Design of riser system depends

on filed layout, vessel interfaces,

fluid properties and

environmental condition.

PLATFORM PARTS

RISER

• Remains in tension due to self

weight

• Profiles are designed to reduce load

on topside. Types of risers

1. Rigid

2. Flexible - Allows vessel motion

due to wave loading and

compensates heave motion

• Simple Catenary risers:

Flexible pipe is freely

suspended between

surface vessel and the

seabed.

• Other catenary variants

possible

Various methods are deployed based

on availability of resources and size

of structure.

• Barge Crane

• Flat over - Top side is

installed on jackets. Ballasting

of barge

• Smaller jackets can be

installed by lifting them off

barge using a floating vessel

with cranes .

Large 400’ x 100’ deck barges

capable of carrying up to 12,000 tons

are available

PLATFORM INSTALLATION

BARGE LOADOUT

• The usual form of corrosion

protection of the underwater part

of the jacket as well as the upper

part of the piles in soil is by

cathodic protection using

sacrificial anodes.

• A sacrificial anode consists of a

zinc/aluminium bar cast about a

steel tube and welded on to the

structures. Typically approximately

5% of the jacket weight is applied

as anodes.

• The steelwork in the splash zone

is usually protected by a sacrificial

wall thickness of 12 mm to the

members.

CORROSION PROTECTION

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• The loads generated by

environmental conditions plus by

onboard equipment must be

resisted by the piles at the seabed

and below.

• The soil investigation is vital to the

design of any offshore structure.

Geotech report is developed by

doing soil borings at the desired

location, and performing in-situ

and laboratory tests.

• Pile penetrations depends on

platform size and loads, and soil

characteristics, but normally range

from 30 meters to about 100

meters.

PLATFORM FOUNDATION

FOUNDATION

• Stability is resistance to capsizing

• Center of Buoyancy is located at

center of mass of the displaced

water.

• Under no external forces, the

center of gravity and center of

buoyancy are in same vertical

plane.

• Upward force of water equals to

the weight of floating vessel and

this weight is equal to weight of

displaced water

• Under wind load vessel heels, and

thus CoB moves to provide

righting (stabilizing) moment.

• Vertical line through new center of

buoyancy will intersect CoG at

point M called as Metacenter

NAVAL ARCHITECTURE

HYDROSTATICS AND STABILITY

• Intact stability requires righting moment

adequate to withstand wind moments.

• Damage stability requires vessel

withstands flooding of designated volume

with wind moments.

• CoG of partially filled vessel changes,

due to heeling. This results in reduction

in stability. This phenomena is called

Free surface correction (FSC).

• HYDRODYNAMIC RESPONSE:

• Rigid body response

• There are six rigid body motions:

1. Translational - Surge, sway and

heave

2. Rotational - Roll, pitch and yaw

• Structural response - Involving structural

deformations

Loads:

Offshore structure shall be designed

for following types of loads:

1. Permanent (dead) loads.

2. Operating (live) loads.

3. Environmental loads

a) Wind load

b) Wave load

c) Earthquake load

4. Construction - installation

loads.

5. Accidental loads.

The design of offshore structures is

dominated by environmental loads,

especially wave load

STRUCTURAL DESIGN

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Permanent Loads:

Weight of the structure in air,

including the weight of ballast.

1. Weights of equipment,

and associated

structures permanently

mounted on the

platform.

2. Hydrostatic forces on

the members below the

waterline. These forces

include buoyancy and

hydrostatic pressures.

STRUCTURAL DESIGN

Operating (Live) Loads:

Operating loads include the weight of

all non-permanent equipment or

material, as well as forces generated

during operation of equipment.

1. The weight of drilling,

production facilities, living

quarters, furniture, life support

systems, heliport, consumable

supplies, liquids, etc.

2. Forces generated during

operations, e.g. drilling, vessel

mooring, helicopter landing,

crane operations.

3. Following Live load values are

recommended in BS6235:

4. Crew quarters and passage

ways: 3.2 KN/m 2

5. Working areas: 8,5 KN/m 2

STRUCTURAL DESIGN

Wind Loads:

• Wind load act on portion of platform

above the water level as well as on any

equipment, housing, derrick, etc.

• For combination with wave loads, codes

recommend the most unfavorable of the

following two loadings:

• 1 minute sustained wind speeds

combined with extreme waves.

• 3 second gusts .

• When, the ratio of height to the least

horizontal dimension of structure is

greater than 5, then API-RP2A requires

the dynamic effects of the wind to be

taken into account and the flow induced

cyclic wind loads due to vortex shedding

must be investigated.

STRUCTURAL DESIGN

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Wave load :

• The wave loading of an offshore structure is usually the most important

of all environmental loadings.

• The forces on the structure are caused by the motion of the water due to

the waves

• Determination of wave forces requires the solution of ,

a) Sea state using an idealization of the wave surface profile and the

wave kinematics by wave theory.

b) Computation of the wave forces on individual members and on the

total structure, from the fluid motion.

Design wave concept is used, where a regular wave of given height and period

is defined and the forces due to this wave are calculated using a high-order

wave theory.

Usually the maximum wave with a return period of 100 years, is chosen. No

dynamic behavior of the structure is considered. This static analysis is

appropriate when the dominant wave periods are well above the period of the

structure. This is the case of extreme storm waves acting on shallow water

structures.

STRUCTURAL DESIGN

Wave Load: (Contd.)

Wave theories

Wave theories describe the

kinematics of waves of water. They

serve to calculate the particle

velocities and accelerations and the

dynamic pressure as functions of

the surface elevation of the waves.

The waves are assumed to be long-

crested, i.e. they can be described

by a two-dimensional flow field, and

are characterized by the

parameters: wave height (H), period

(T) and water depth (d).

STRUCTURAL DESIGN

Wave theories: (Contd.)

Wave forces on structural members

• Structures exposed to waves experience forces much higher than wind

loadings. The forces result from the dynamic pressure and the water

particle motions. Two different cases can be distinguished:

• Large volume bodies, termed hydrodynamic compact structures, influence

the wave field by diffraction and reflection. The forces on these bodies

have to be determined by calculations based on diffraction theory.

• Slender, hydro-dynamically transparent structures have no significant

influence on the wave field. The forces can be calculated in a straight-

forward manner with Morison's equation. The steel jackets of offshore

structures can usually be regarded as hydro-dynamically transparent

• As a rule, Morison's equation may be applied when D/L < 0.2, where D is

the member diameter and L is the wave length.

• Morison's equation expresses the wave force as the sum of,

•An inertia force proportional to the particle acceleration

•A non-linear drag force proportional to the square of the particle

velocity.

STRUCTURAL DESIGN

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Earthquake load:

• Offshore structures are designed for two

levels of earthquake intensity.

• Strength level :Earthquake, defined

as having a & quot; reasonable

likelihood of not being exceeded

during the platform's life & quot;

(mean recurrence interval ~ 200 -

500 years), the structure is

designed to respond elastically.

• Ductility level : Earthquake, defined

as close to the & quot; maximum

credible earthquake & quot; at the

site, the structure is designed for

inelastic response and to have

adequate reserve strength to avoid

collapse.

STRUCTURAL DESIGN

Ice and Snow Loads:

Ice is a primary problem for marine structures in the arctic and sub-arctic zones.

Ice formation and expansion can generate large pressures that give rise to

horizontal as well as vertical forces. In addition, large blocks of ice driven by

current, winds and waves with speeds up to 0,5 to 1,0 m/s, may hit the structure

and produce impact loads. Temperature Load: Temperature gradients produce

thermal stresses. To cater such stresses, extreme values of sea and air

temperatures which are likely to occur during the life of the structure shall be

estimated. In addition to the environmental sources , accidental release of

cryogenic material can result in temperature increase, which must be taken into

account as accidental loads. The temperature of the oil and gas produced must

also be considered. Marine Growth: Marine growth is accumulated on

submerged members. Its main effect is to increase the wave forces on the

members by increasing exposed areas and drag coefficient due to higher

surface roughness. It is accounted for in design through appropriate increases

in the diameters and masses of the submerged members.

STRUCTURAL DESIGN

Installation Load :

These are temporary loads and arise during fabrication and installation of the

platform or its components. During fabrication, erection lifts of various

structural components generate lifting forces, while in the installation phase

forces are generated during platform load out, transportation to the site,

launching and upending, as well as during lifts related to installation. All

members and connections of a lifted component must be designed for the

forces resulting from static equilibrium of the lifted weight and the sling

tensions. Load out forces are generated when the jacket is loaded from the

fabrication yard onto the barge. Depends on friction co-efficient

STRUCTURAL DESIGN

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Accidental Load :

According to the DNV rules , accidental loads are loads, which may occur as a

result of accident or exceptional circumstances.

Examples of accidental loads are, collision with vessels, fire or explosion,

dropped objects, and unintended flooding of buoyancy tanks.

Special measures are normally taken to reduce the risk from accidental loads.

STRUCTURAL DESIGN

Load Combinations :

The load combinations depend upon the design method used, i.e. whether limit

state or allowable stress design is employed.

The load combinations recommended for use with allowable stress procedures

are:

Normal operations

Dead loads plus operating environmental loads plus maximum live loads .

Dead loads plus operating environmental loads plus minimum live loads .

Extreme operations

Dead loads plus extreme environmental loads plus maximum live loads.

Dead loads plus extreme environmental loads plus minimum live loads

Environmental loads,should be combined in a manner consistent with their joint

probability of occurrence.

Earthquake loads, are to be imposed as a separate environmental load, i.e.,

not to be combined with waves, wind, etc.

STRUCTURAL DESIGN

The analytical models used in offshore

engineering are similar to other types of on

shore steel structures

The same model is used throughout the

analysis except supports locations.

Stick models are used extensively for

tubular structures (jackets, bridges, flare

booms) and lattice trusses (modules,

decks).

Each member is normally rigidly fixed at its

ends to other elements in the model.

In addition to its geometrical and material

properties, each member is characterized

by hydrodynamic coefficients, e.g. relating

to drag, inertia, and marine growth, to allow

wave forces to be automatically generated.

STRUCTURAL ANALYSIS

ANALYSIS MODEL

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Integrated decks and hulls of floating platforms

involving large bulkheads are described by plate

elements.

Deck shall be able to resist crane’s maximum

overturning moments coupled with corresponding

maximum thrust loads for at least 8 positions of the

crane boom around a full 360° path.

The structural analysis will be a static linear analysis

of the structure above the seabed combined with a

static non-linear analysis of the soil with the piles.

Transportation and installation of the structure may

require additional analyses

Detailed fatigue analysis should be performed to

assess cumulative fatigue damage

The offshore platform designs normally use pipe or

wide flange beams for all primary structural

members.

STRUCTURAL ANALYSIS

ANALYSIS MODEL

• The verification of an element consists of

comparing its characteristic resistance(s) to a

design force or stress. It includes:

• a strength check, where the characteristic

resistance is related to the yield strength of the

element,

• a stability check for elements in compression

related to the buckling limit of the element.

• An element is checked at typical sections (at least

both ends and mid span) against resistance and

buckling.

• Tubular joints are checked against punching.

These checks may indicate the need for local

reinforcement of the chord using larger thickness

or internal ring-stiffeners.

• Elements should also be verified against fatigue,

corrosion, temperature or durability wherever

relevant.

Acceptance Criteria

Design Conditions:

Operation

Survival

Transit.

The design criteria for strength should

relate to both intact and damaged

conditions.

Damaged conditions to be considered may

be like 1 bracing or connection made

ineffective, primary girder in deck made

ineffective, heeled condition due to loss of

buoyancy etc.

STRUCTURAL DESIGN

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Offshore Standards (OS):

Provides technical requirements and

acceptance criteria for general

application by the offshore industry

eg.DNV-OS-C101

Recommended Practices(RP): Provides

proven technology and sound

engineering practice as well as guidance

for the higher level publications eg. API-

RP-WSD

BS 6235: Code of practice for fixed

offshore structures.

British Standards Institution 1982.

Mainly for the British offshore sector.

CODES

W.J. Graff: Introduction to offshore

structures.

• Gulf Publishing Company, Houston

1981.

• Good general introduction to

offshore structures.

B.C. Gerwick: Construction of offshore

structures.

• John Wiley & Sons, New York 1986.

• Up to date presentation of offshore

design and construction.

Patel M H: Dynamics of offshore

structures

Butterworth & Co., London.

http://www.slideshare.net/surya3303/offshore-structures-presentation

REFERENCES

Q & A

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THANK

YOU