offshore platforms

07/30/2003 OFFSHORE PLATFORM DESIGN

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OFFSHORE PLATFORM DESIGNOFFSHORE PLATFORM DESIGN


Welcome aboard exciting world of Offshore platforms design. In Next 45 Welcome aboard exciting world of Offshore platforms design. In Next 45 minutes we will take you to educational trip of offshore platforms with minutes we will take you to educational trip of offshore platforms with breathtaking views and path breaking engineering accomplishments. breathtaking views and path breaking engineering accomplishments.


OVERVIEWOVERVIEW

Offshore platforms are used for Offshore platforms are used for exploration of Oil and Gas from exploration of Oil and Gas from under Seabed and processing.under Seabed and processing.

The First Offshore platform was The First Offshore platform was installed in 1947 off the coast of installed in 1947 off the coast of Louisiana in 6M depth of water. Louisiana in 6M depth of water.

Today there are over 7,000 Today there are over 7,000 Offshore platforms around the Offshore platforms around the world in water depths up to world in water depths up to 1,850M1,850M


OVERVIEWOVERVIEW

Platform size depends on facilities to be Platform size depends on facilities to be installed on top side eg. Oil rig, living installed on top side eg. Oil rig, living quarters, Helipad etc.quarters, Helipad etc.

Classification of water depths:Classification of water depths:

– < 350 M- Shallow water< 350 M- Shallow water

– < 1500 M - Deep water< 1500 M - Deep water

– > 1500 M- Ultra deep water > 1500 M- Ultra deep water

– US Mineral Management Service US Mineral Management Service (MMS) classifies water depths greater (MMS) classifies water depths greater than 1,300 ft as deepwater, and greater than 1,300 ft as deepwater, and greater than 5,000 ft as ultra-deepwater.than 5,000 ft as ultra-deepwater.


OVERVIEWOVERVIEW Offshore platforms can broadly categorized in two typesOffshore platforms can broadly categorized in two types

Fixed structures that extend to the Seabed.Fixed structures that extend to the Seabed. Steel JacketSteel Jacket Concrete gravity StructureConcrete gravity Structure Compliant TowerCompliant Tower


OVERVIEWOVERVIEW

Structures that float near the water surface- Recent developmentStructures that float near the water surface- Recent development Tension Leg platformsTension Leg platforms Semi SubmersibleSemi Submersible SparSpar Ship shaped vessel (FPSO)Ship shaped vessel (FPSO)

OFFSHORE PLATFORM DESIGN07/30/2003

TYPE OF PLATFORMS (FIXED)TYPE OF PLATFORMS (FIXED) JACKETED PLATFORMJACKETED PLATFORM

– Space framed structure with tubular Space framed structure with tubular members supported on piled foundations. members supported on piled foundations.

– Used for moderate water depths up to 400 Used for moderate water depths up to 400 M.M.

– Jackets provides protective layer around Jackets provides protective layer around the pipes.the pipes.

– Typical offshore structure will have a deck Typical offshore structure will have a deck structure containing a Main Deck, a Cellar structure containing a Main Deck, a Cellar Deck, and a Helideck. Deck, and a Helideck.

– The deck structure is supported by deck The deck structure is supported by deck legs connected to the top of the piles. The legs connected to the top of the piles. The piles extend from above the Mean Low piles extend from above the Mean Low Water through the seabed and into the soil.Water through the seabed and into the soil.


TYPE OF PLATFORMS (FIXED)TYPE OF PLATFORMS (FIXED) JACKETED PLATFORM (Cont.)JACKETED PLATFORM (Cont.)

– Underwater, the piles are contained Underwater, the piles are contained inside the legs of a “jacket” structure inside the legs of a “jacket” structure which serves as bracing for the piles which serves as bracing for the piles against lateral loads. against lateral loads.

– The jacket also serves as a template for The jacket also serves as a template for the initial driving of the piles. (The piles the initial driving of the piles. (The piles are driven through the inside of the legs are driven through the inside of the legs of the jacket structure).of the jacket structure).

– Natural period (usually 2.5 second) is Natural period (usually 2.5 second) is kept below wave period (14 to 20 kept below wave period (14 to 20 seconds) to avoid amplification of wave seconds) to avoid amplification of wave loads.loads.

– 95% of offshore platforms around the 95% of offshore platforms around the world are Jacket supported. world are Jacket supported.


TYPE OF PLATFORMS (FIXED)TYPE OF PLATFORMS (FIXED) COMPLIANT TOWERCOMPLIANT TOWER

– Narrow, flexible framed structures Narrow, flexible framed structures supported by piled foundations. supported by piled foundations.

– Has no oil storage capacity. Production is Has no oil storage capacity. Production is through tensioned rigid risers and export through tensioned rigid risers and export by flexible or catenary steel pipe. by flexible or catenary steel pipe.

– Undergo large lateral deflections (up to 10 Undergo large lateral deflections (up to 10 ft) under wave loading. Used for ft) under wave loading. Used for moderate water depths up to 600 M.moderate water depths up to 600 M.

– Natural period (usually 30 second) is kept Natural period (usually 30 second) is kept above wave period (14 to 20 seconds) to above wave period (14 to 20 seconds) to avoid amplification of wave loads.avoid amplification of wave loads.


TYPE OF PLATFORMS (FIXED)TYPE OF PLATFORMS (FIXED) CONCRETE GRAVITY CONCRETE GRAVITY

STRUCTURES:STRUCTURES:– Fixed-bottom structures made from concrete Fixed-bottom structures made from concrete – Heavy and remain in place on the seabed without Heavy and remain in place on the seabed without

the need for pilesthe need for piles– Used for moderate water depths up to 300 M.Used for moderate water depths up to 300 M.– Part construction is made in a dry dock adjacent Part construction is made in a dry dock adjacent

to the sea. The structure is built from bottom up, to the sea. The structure is built from bottom up, like onshore structure.like onshore structure.

– At a certain point , dock is flooded and the At a certain point , dock is flooded and the partially built structure floats. It is towed to partially built structure floats. It is towed to deeper sheltered water where remaining deeper sheltered water where remaining construction is completed.construction is completed.

– After towing to field, base is filled with water to After towing to field, base is filled with water to sink it on the seabed. sink it on the seabed.

– Advantage- Less maintenanceAdvantage- Less maintenance


TYPE OF PLATFORMS (FLOATER)TYPE OF PLATFORMS (FLOATER) Tension Leg Platform (TLP)Tension Leg Platform (TLP)

– Tension Leg Platforms (TLPs) are Tension Leg Platforms (TLPs) are floating facilities that are tied down to floating facilities that are tied down to the seabed by vertical steel tubesthe seabed by vertical steel tubes called called tethers.tethers.

– This characteristic makes the structure This characteristic makes the structure very rigid in the vertical direction and very rigid in the vertical direction and very flexible in the horizontal plane. The very flexible in the horizontal plane. The vertical rigidity helps to tie in wells for vertical rigidity helps to tie in wells for production, while, the horizontal production, while, the horizontal compliance makes the platform compliance makes the platform insensitive to the primary effect of insensitive to the primary effect of waves.waves.

– Have large columns and Pontoons and a Have large columns and Pontoons and a fairly deep draught. fairly deep draught.


TYPE OF PLATFORMS (FLOATER)TYPE OF PLATFORMS (FLOATER) Tension Leg Platform (TLP)Tension Leg Platform (TLP)

– TLP has excess buoyancy which keeps TLP has excess buoyancy which keeps tethers in tension. Topside facilities , tethers in tension. Topside facilities , no. of risers etc. have to fixed at pre-no. of risers etc. have to fixed at pre-design stage. design stage.

– Used for deep water up to 1200 MUsed for deep water up to 1200 M

– It has no integral storage.It has no integral storage.

– It is sensitive to topside load/draught It is sensitive to topside load/draught variations as tether tensions are variations as tether tensions are affected.affected.


TYPE OF PLATFORMS (FLOATER)TYPE OF PLATFORMS (FLOATER) SEMISUB PLATFORMSEMISUB PLATFORM

– Due to small water plane area , they are Due to small water plane area , they are weight sensitive. Flood warning systems are weight sensitive. Flood warning systems are required to be in-place.required to be in-place.

– Topside facilities , no. of risers etc. have to Topside facilities , no. of risers etc. have to fixed at pre-design stage. fixed at pre-design stage.

– Used for Ultra deep water.Used for Ultra deep water.

– Semi-submersibles are held in place by Semi-submersibles are held in place by anchors connected to a catenary mooring anchors connected to a catenary mooring system.system.


TYPE OF PLATFORMS (FLOATER)TYPE OF PLATFORMS (FLOATER)

SEMISUB PLATFORMSEMISUB PLATFORM– Column pontoon junctions and bracing Column pontoon junctions and bracing

attract large loads.attract large loads.

– Due to possibility of fatigue cracking of Due to possibility of fatigue cracking of braces , periodic inspection/ braces , periodic inspection/ maintenance is prerequisitemaintenance is prerequisite



SPAR:SPAR:– Concept of a large diameter single vertical Concept of a large diameter single vertical

cylinder supporting deck.cylinder supporting deck.– These are a very new and emerging concept: the These are a very new and emerging concept: the

first spar platform, first spar platform, NeptuneNeptune, was installed off the , was installed off the USA coast in 1997USA coast in 1997.

– Spar platforms have taut catenary moorings and Spar platforms have taut catenary moorings and deep draught, hence heave natural period is about deep draught, hence heave natural period is about 30 seconds.30 seconds.

– Used for Ultra deep water depth of 2300 M.Used for Ultra deep water depth of 2300 M.– The center of buoyancy is considerably above The center of buoyancy is considerably above

center of gravity , making Spar quite stable.center of gravity , making Spar quite stable.– Due to space restrictions in the core, number of Due to space restrictions in the core, number of

risers has to be predetermined.risers has to be predetermined.



SHIP SHAPED VESSEL (FPSO)SHIP SHAPED VESSEL (FPSO)– Ship-shape platforms are called Floating Ship-shape platforms are called Floating

Production, Storage and Offloading (FPSO) Production, Storage and Offloading (FPSO) facilities. facilities.

– FPSOs have integral oil storage capability inside FPSOs have integral oil storage capability inside their hull. This avoids a long and expensive their hull. This avoids a long and expensive pipeline to shore.pipeline to shore.

– Can explore in remote and deep water and also in Can explore in remote and deep water and also in marginal wells, where building fixed platform marginal wells, where building fixed platform and piping is technically and economically not and piping is technically and economically not feasiblefeasible

– FPSOs are held in position over the reservoir at a FPSOs are held in position over the reservoir at a Single Point Mooring (SPM). The vessel is able Single Point Mooring (SPM). The vessel is able to weathervane around the mooring point so that to weathervane around the mooring point so that it always faces into the prevailing weather.it always faces into the prevailing weather.


PLATFORM PARTSPLATFORM PARTS TOPSIDE:TOPSIDE:

– Facilities are tailored to achieve weight Facilities are tailored to achieve weight and space savingand space saving

– Incorporates process and utility Incorporates process and utility equipment equipment

Drilling RigDrilling Rig Injection CompressorsInjection Compressors Gas CompressorsGas Compressors Gas Turbine GeneratorsGas Turbine Generators PipingPiping HVACHVAC InstrumentationInstrumentation

– Accommodation for operating personnel.Accommodation for operating personnel.

– Crane for equipment handlingCrane for equipment handling

– HelipadHelipad


PLATFORM PARTSPLATFORM PARTS

MOORINGS & ANCHORS:MOORINGS & ANCHORS:– Used to tie platform in place Used to tie platform in place

– MaterialMaterial Steel chainSteel chain Steel wire rope Steel wire rope

– Catenary shape due to heavy Catenary shape due to heavy weight. weight.

– Length of rope is moreLength of rope is more Synthetic fiber ropeSynthetic fiber rope

– Taut shape due to substantial Taut shape due to substantial less weight than steel ropes.less weight than steel ropes.

– Less rope length requiredLess rope length required

– Corrosion freeCorrosion free



RISER:RISER:– Pipes used for production, drilling, Pipes used for production, drilling,

and export of Oil and Gas from and export of Oil and Gas from Seabed.Seabed.

– Riser system is a key component Riser system is a key component for offshore drilling or floating for offshore drilling or floating production projects.production projects.

– The cost and technical challenges The cost and technical challenges of the riser system increase of the riser system increase significantly with water depth.significantly with water depth.

– Design of riser system depends on Design of riser system depends on filed layout, vessel interfaces, filed layout, vessel interfaces, fluid properties and environmental fluid properties and environmental condition.condition.



RISER:RISER:– Remains in tension due to self Remains in tension due to self

weightweight– Profiles are designed to reduce load Profiles are designed to reduce load

on topside. Types of riserson topside. Types of risers RigidRigid Flexible - Allows vessel motion Flexible - Allows vessel motion

due to wave loading and due to wave loading and compensates heave motioncompensates heave motion

– Simple Catenary risers: Simple Catenary risers: Flexible pipe is freely Flexible pipe is freely suspended between surface suspended between surface vessel and the seabed.vessel and the seabed.

– Other catenary variants Other catenary variants possiblepossible


PLATFORM PLATFORM INSTALLATIONINSTALLATION

BARGE LOADOUT:BARGE LOADOUT:– Various methods are deployed based Various methods are deployed based

on availability of resources and size of on availability of resources and size of structure.structure.

Barge CraneBarge Crane

Flat over - Top side is installed on Flat over - Top side is installed on jackets. Ballasting of bargejackets. Ballasting of barge

Smaller jackets can be installed Smaller jackets can be installed by lifting them off barge using a by lifting them off barge using a floating vessel with cranesfloating vessel with cranes..

– Large 400’ x 100’ deck barges capable Large 400’ x 100’ deck barges capable of carrying up to 12,000 tons are of carrying up to 12,000 tons are availableavailable


CORROSION PROTECTIONCORROSION PROTECTION 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.


PLATFORM PLATFORM FOUNDATIONFOUNDATION

FOUNDATION:FOUNDATION:– The loads generated by environmental The loads generated by environmental

conditions plus by onboard equipment conditions plus by onboard equipment must be resisted by the piles at the must be resisted by the piles at the seabed and below.seabed and below.

– The soil investigation is vital to the The soil investigation is vital to the design of any offshore structure. design of any offshore structure. Geotech report is developed by doing Geotech report is developed by doing soil borings at the desired location, soil borings at the desired location, and performing in-situ and laboratory and performing in-situ and laboratory tests.tests.

– Pile penetrations depends on platform Pile penetrations depends on platform size and loads, and soil characteristics, size and loads, and soil characteristics, but normally range from 30 meters to but normally range from 30 meters to about 100 meters. about 100 meters.


NAVAL ARCHITECTURENAVAL ARCHITECTURE HYDROSTATICS AND STABILITY:HYDROSTATICS AND STABILITY:

– Stability is resistance to capsizingStability is resistance to capsizing

– Center of Buoyancy is located at center of Center of Buoyancy is located at center of mass of the displaced water.mass of the displaced water.

– Under no external forces, the center of Under no external forces, the center of gravity and center of buoyancy are in gravity and center of buoyancy are in same vertical plane.same vertical plane.

– Upward force of water equals to the Upward force of water equals to the weight of floating vessel and this weight weight of floating vessel and this weight is equal to weight of displaced wateris equal to weight of displaced water

– Under wind load vessel heels, and thus Under wind load vessel heels, and thus CoB moves to provide righting CoB moves to provide righting (stabilizing) moment.(stabilizing) moment.

– Vertical line through new center of Vertical line through new center of buoyancy will intersect CoG at point M buoyancy will intersect CoG at point M called as Metacentercalled as Metacenter


NAVAL ARCHITECTURENAVAL ARCHITECTURE HYDROSTATICS AND HYDROSTATICS AND STABILITY:STABILITY:– Intact stabilityIntact stability requires righting requires righting

moment adequate to withstand moment adequate to withstand wind moments. wind moments.

– Damage stabilityDamage stability requires vessel requires vessel withstands flooding of withstands flooding of designated volume with wind designated volume with wind moments. moments.

– CoG of partially filled vessel CoG of partially filled vessel changes, due to heeling. This changes, due to heeling. This results in reduction in stability. results in reduction in stability. This phenomena is called Free This phenomena is called Free surface correction (FSC).surface correction (FSC).

HYDRODYNAMIC RESPONSE:Rigid body response

There are six rigid body motions:•Translational - Surge, sway and heave•Rotational - Roll, pitch and yaw

Structural response - Involving structural deformations


STRUCTURAL DESIGNSTRUCTURAL DESIGN Loads:Loads: Offshore structure shall be designed for Offshore structure shall be designed for

following types of loads: following types of loads: – Permanent (dead) loads. Permanent (dead) loads.

– Operating (live) loads. Operating (live) loads.

– Environmental loads Environmental loads

Wind loadWind load

Wave loadWave load

Earthquake loadEarthquake load

– Construction - installation loads. Construction - installation loads.

– Accidental loads.Accidental loads.

The design of offshore structures is The design of offshore structures is dominated by environmental loads, dominated by environmental loads, especially wave loadespecially wave load


STRUCTURAL DESIGNSTRUCTURAL DESIGN

Permanent Loads:Permanent Loads:Weight of the structure in air, Weight of the structure in air, including the weight of ballast.including the weight of ballast.– Weights of equipment, and Weights of equipment, and

associated structures permanently associated structures permanently mounted on the platform.mounted on the platform.

– Hydrostatic forces on the members Hydrostatic forces on the members below the waterline. These forces below the waterline. These forces include buoyancy and hydrostatic include buoyancy and hydrostatic pressures.pressures.


STRUCTURAL DESIGNSTRUCTURAL DESIGN Operating (Live) Loads:Operating (Live) Loads:

– Operating loads include the weight of all non-Operating loads include the weight of all non-permanent equipment or material, as well as forces permanent equipment or material, as well as forces generated during operation of equipment. generated during operation of equipment.

The weight of drilling, production facilities, The weight of drilling, production facilities, living quarters, furniture, life support systems, living quarters, furniture, life support systems, heliport, consumable supplies, liquids, etc.heliport, consumable supplies, liquids, etc.

Forces generated during operations, e.g. drilling, Forces generated during operations, e.g. drilling, vessel mooring, helicopter landing, crane vessel mooring, helicopter landing, crane operations.operations.

Following Live load values are recommended in Following Live load values are recommended in BS6235:BS6235:

Crew quarters and passage ways: 3.2 KN/mCrew quarters and passage ways: 3.2 KN/m22

Working areas: 8,5 KN/mWorking areas: 8,5 KN/m22


STRUCTURAL DESIGNSTRUCTURAL DESIGN Wind Loads:Wind Loads:

Wind load act on portion of platform above Wind load act on portion of platform above the water level as well as on any equipment, the water level as well as on any equipment, housing, derrick, etc.housing, derrick, etc.

For combination with wave loads, codes For combination with wave loads, codes recommend the most unfavorable of the recommend the most unfavorable of the following two loadings:following two loadings:

– 1 minute sustained wind speeds 1 minute sustained wind speeds combined with extreme waves.combined with extreme waves.

– 3 second gusts3 second gusts.. When, the ratio of height to the least When, the ratio of height to the least

horizontal dimension of structure is greater horizontal dimension of structure is greater than 5, then API-RP2A requires the dynamic than 5, then API-RP2A requires the dynamic effects of the wind to be taken into account effects of the wind to be taken into account and the flow induced cyclic wind loads due to and the flow induced cyclic wind loads due to vortex shedding must be investigated.vortex shedding must be investigated.



Wave loadWave load:: The wave loading of an offshore structure is usually the most important of all environmental The wave loading of an offshore structure is usually the most important of all environmental loadings.loadings.

The forces on the structure are caused by the motion of the water due to the waves 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 ,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 a) Sea state using an idealization of the wave surface profile and the wave kinematics by wave theory. theory.

b) Computation of the wave forces on individual members and on the total structure, from the b) Computation of the wave forces on individual members and on the total structure, from the fluid motion.fluid motion.

Design wave concept is used, where a regular wave of given height and period is defined and 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 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 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 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 well above the period of the structure. This is the case of extreme storm waves acting on shallow water structures. shallow water structures.



Wave Load: (Contd.)

•Wave theoriesWave theories

Wave theories describe the kinematics of waves of water. They serve to calculate the particle 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 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-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) dimensional flow field, and are characterized by the parameters: wave height (H), period (T) and water depth (d).and water depth (d).


STRUCTURAL DESIGNSTRUCTURAL DESIGNWave theories: Wave theories: (Contd.)(Contd.)

•Wave forces on structural membersWave forces on structural members Structures exposed to waves experience forces much higher than wind loadings. The forces 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 result from the dynamic pressure and the water particle motions. Two different cases can be distinguished:distinguished:

Large volume bodies, termed hydrodynamic compact structures, influence the wave field by 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 diffraction and reflection. The forces on these bodies have to be determined by calculations based on diffraction theory. based on diffraction theory.

Slender, hydro-dynamically transparent structures have no significant influence on the wave 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 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 transparentsteel 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 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.diameter and L is the wave length.

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

– An inertia force proportional to the particle acceleration An inertia force proportional to the particle acceleration

– A non-linear drag force proportional to the square of the particle velocity.A non-linear drag force proportional to the square of the particle velocity.


STRUCTURAL DESIGNSTRUCTURAL DESIGNEarthquake load: Earthquake load: Offshore structures are designed for Offshore structures are designed for two levels of earthquake intensity.two levels of earthquake intensity.

Strength level :Earthquake, Strength level :Earthquake, defined as having a "reasonable defined as having a "reasonable likelihood of not being exceeded likelihood of not being exceeded during the platform's life" (mean during the platform's life" (mean recurrence interval ~ 200 - 500 recurrence interval ~ 200 - 500 years), the structure is designed years), the structure is designed to respond elastically. to respond elastically.

Ductility level : Earthquake, Ductility level : Earthquake, defined as close to the defined as close to the "maximum credible earthquake" "maximum credible earthquake" at the site, the structure is at the site, the structure is designed for inelastic response designed for inelastic response and to have adequate reserve and to have adequate reserve strength to avoid collapse.strength to avoid collapse.


STRUCTURAL DESIGNSTRUCTURAL DESIGNIce and Snow Loads:Ice and Snow Loads:

Ice is a primary problem for marine structures in the arctic and sub-arctic zones. Ice 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 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 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.speeds up to 0,5 to 1,0 m/s, may hit the structure and produce impact loads.

Temperature Load:Temperature Load:

Temperature gradients produce thermal stresses. To cater such stresses, extreme values of 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 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 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 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.loads. The temperature of the oil and gas produced must also be considered.

Marine Growth: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 DESIGNSTRUCTURAL DESIGNInstallation Load :Installation Load :

These are temporary loads and arise during These are temporary loads and arise during fabrication and installation of the platform or fabrication and installation of the platform or its components. During fabrication, erection its components. During fabrication, erection lifts of various structural components lifts of various structural components generate lifting forces, while in the generate lifting forces, while in the installation phase forces are generated during installation phase forces are generated during platform load out, transportation to the site, platform load out, transportation to the site, launching and upending, as well as during launching and upending, as well as during lifts related to installation.lifts related to installation.

All members and connections of a lifted All members and connections of a lifted component must be designed for the forces component must be designed for the forces resulting from static equilibrium of the lifted resulting from static equilibrium of the lifted weight and the sling tensions.weight and the sling tensions.

Load out forces are generated when the jacket Load out forces are generated when the jacket is loaded from the fabrication yard onto the is loaded from the fabrication yard onto the barge. Depends on friction co-efficientbarge. Depends on friction co-efficient


STRUCTURAL DESIGNSTRUCTURAL DESIGNAccidental LoadAccidental Load : :

According to the DNV rules , accidental According to the DNV rules , accidental loads are loads, which may occur as a result loads are loads, which may occur as a result of accident or exceptional circumstances. of accident or exceptional circumstances.

Examples of accidental loads are, collision Examples of accidental loads are, collision with vessels, fire or explosion, dropped with vessels, fire or explosion, dropped objects, and unintended flooding of objects, and unintended flooding of buoyancy tanks. buoyancy tanks.

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


STRUCTURAL DESIGNSTRUCTURAL DESIGNLoad CombinationsLoad 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 ANALYSISSTRUCTURAL ANALYSIS ANALYSIS MODEL:ANALYSIS MODEL: The analytical models used in offshore The analytical models used in offshore

engineering are similar to other types of on engineering are similar to other types of on shore steel structuresshore steel structures

The same model is used throughout the The same model is used throughout the analysis except supports locations.analysis except supports locations.

Stick models are used extensively for Stick models are used extensively for tubular structures (jackets, bridges, flare tubular structures (jackets, bridges, flare booms) and lattice trusses (modules, decks).booms) and lattice trusses (modules, decks).

Each member is normally rigidly fixed at its Each member is normally rigidly fixed at its ends to other elements in the model.ends to other elements in the model.

In addition to its geometrical and material In addition to its geometrical and material properties, each member is characterized by properties, each member is characterized by hydrodynamic coefficients, e.g. relating to hydrodynamic coefficients, e.g. relating to drag, inertia, and marine growth, to allow drag, inertia, and marine growth, to allow wave forces to be automatically generated.wave forces to be automatically generated.


STRUCTURAL ANALYSIS:STRUCTURAL ANALYSIS:– Integrated decks and hulls of floating platforms Integrated decks and hulls of floating platforms

involving large bulkheads are described by plate involving large bulkheads are described by plate elements.elements.

– Deck shall be able to resist crane’s maximum Deck shall be able to resist crane’s maximum overturning moments coupled with corresponding overturning moments coupled with corresponding maximum thrust loads for at least 8 positions of the maximum thrust loads for at least 8 positions of the crane boom around a full 360° path.crane boom around a full 360° path.

– The structural analysis will be a static linear analysis The structural analysis will be a static linear analysis of the structure above the seabed combined with a of the structure above the seabed combined with a static non-linear analysis of the soil with the piles. static non-linear analysis of the soil with the piles.

– Transportation and installation of the structure may Transportation and installation of the structure may require additional analysesrequire additional analyses

– Detailed fatigue analysis should be performed to Detailed fatigue analysis should be performed to assess cumulative fatigue damageassess cumulative fatigue damage

– The offshore platform designs normally use pipe or The offshore platform designs normally use pipe or wide flange beams for all primary structural wide flange beams for all primary structural members.members.


Acceptance CriteriaAcceptance Criteria:: The verification of an element consists of comparing The verification of an element consists of comparing

its characteristic resistance(s) to a design force or its characteristic resistance(s) to a design force or stress. It includes:stress. It includes:

a strength check, where the characteristic resistance is a strength check, where the characteristic resistance is related to the yield strength of the element, related to the yield strength of the element,

a stability check for elements in compression related a stability check for elements in compression related to the buckling limit of the element.to the buckling limit of the element.

An element is checked at typical sections (at least both An element is checked at typical sections (at least both ends and mid span) against resistance and buckling. ends and mid span) against resistance and buckling.

Tubular joints are checked against punching.These Tubular joints are checked against punching.These checks may indicate the need for local reinforcement checks may indicate the need for local reinforcement of the chord using larger thickness or internal ring-of the chord using larger thickness or internal ring-stiffeners.stiffeners.

Elements should also be verified against fatigue, Elements should also be verified against fatigue, corrosion, temperature or durability wherever relevant.corrosion, temperature or durability wherever relevant.



Design Conditions:Design Conditions: OperationOperation

Survival Survival

Transit. Transit.

The design criteria for strength should relate to both intact and The design criteria for strength should relate to both intact and damaged conditions. damaged conditions.

Damaged conditions to be considered may be like 1 bracing or Damaged conditions to be considered may be like 1 bracing or connection made ineffective, primary girder in deck made connection made ineffective, primary girder in deck made ineffective, heeled condition due to loss of buoyancy etc.ineffective, heeled condition due to loss of buoyancy etc.


CODES CODES

Offshore Standards (OS):Offshore Standards (OS): Provides technical requirements and acceptance Provides technical requirements and acceptance criteria for general application by the offshore criteria for general application by the offshore industry eg.DNV-OS-C101industry eg.DNV-OS-C101

Recommended Practices(RP):Recommended Practices(RP): Provides proven technology and sound engineering Provides proven technology and sound engineering practice as well as guidance for the higher level practice as well as guidance for the higher level publications eg. API-RP-WSDpublications eg. API-RP-WSD

BS 6235: Code of practice for fixed BS 6235: Code of practice for fixed offshore structures.offshore structures. – British Standards Institution 1982.

– Mainly for the British offshore sector.


REFERENCESREFERENCES

W.J. Graff: Introduction to offshore W.J. Graff: Introduction to offshore structures. structures.

– Gulf Publishing Company, Houston 1981.Gulf Publishing Company, Houston 1981.

– Good general introduction to offshore structures.Good general introduction to offshore structures.

B.C. Gerwick: Construction of offshore B.C. Gerwick: Construction of offshore structures. structures.

– John Wiley & Sons, New York 1986.John Wiley & Sons, New York 1986.

– Up to date presentation of offshore design and Up to date presentation of offshore design and construction.construction.

Patel M H: Dynamics of offshore Patel M H: Dynamics of offshore structuresstructures

– Butterworth & Co., London.Butterworth & Co., London.

offshore platforms

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