Presentation on offshore platform design

Presentation on offshore platform design

07/30/2003 OFFSHORE

WELCOME

PRESENTATION ON OFFSHORE PLATFORM DESIGN

07/30/2003 OFFSHORE

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

07/30/2003 OFFSHORE

OVERVIEW

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

07/30/2003 OFFSHORE

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.

07/30/2003 OFFSHORE

OVERVIEW Offshore platforms can broadly categorized in two types

Fixed structures that extend to the Seabed.

Steel JacketConcrete gravity StructureCompliant Tower

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OVERVIEW

Structures that float near the water surface- Recent development

Tension Leg platforms Semi Submersible

Spar Ship shaped vessel (FPSO)

TYPE OF PLATFORMS (FIXED)

JACKETED PLATFORM

– 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 (Cont.)

– 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

TYPE OF PLATFORMS (FIXED)

COMPLIANT TOWER

– 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)

CONCRETE GRAVITY STRUCTURES:

– 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 (FLOATER)

Tension Leg Platform (TLP)

– 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 design stage

pre-– 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.

TYPE OF PLATFORMS (FLOATER)

SEMISUB PLATFORM

– Column pontoon junctions and bracing

attract large loads

– Due to possibility of fatigue cracking of

braces , periodic inspection/

maintenance is prerequisite

TYPE OF PLATFORMS (FLOATER)

SPAR:

– 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

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)

SHIP SHAPED VESSEL (FPSO)

– 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

PLATFORM PARTS

TOPSIDE:

– Facilities are tailored to achieve

weight and space saving

– Incorporates process and utility

equipment Drilling RigInjection CompressorsGas Compressors

Gas Turbine GeneratorsPiping

HVACInstrumentation

– Accommodation for operating

personnel.

– Crane for equipment handling – Helipad

PLATFORM PARTS

MOORINGS & ANCHORS:

– Used to tie platform in place – Material

Steel chain Steel wire rope – Catenary shape due to heavy weight

– Length of rope is moreSynthetic fiber rope

– Taut shape due to substantial less weight than steel ropes.– Less rope length required– Corrosion free

PLATFORM PARTS

RISER:

– 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 risersRigid

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

PLATFORM INSTALLATION BARGE LOADOUT:

– Various methods are deployed based

on availability of resources and size of structure

Barge CraneFlat 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

CORROSION 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 FOUNDATION FOUNDATION:

– 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

NAVAL ARCHITECTURE

HYDROSTATICS AND STABILITY:

– 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:

•Translational - Surge, sway and heave

•Rotational - Roll, pitch and yawStructural response - Involving structural deformations

– Permanent (dead) loads

– Operating (live) loads

– Environmental loads

Wind loadWave loadEarthquake load– Construction - installation loads – Accidental loads.

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

– Weights of equipment, and associated

structures permanently mounted on the platform

– Hydrostatic forces on the members

below the waterline These forces include buoyancy and hydrostatic pressures

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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

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

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

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

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

dimensional flow field, and are characterized by the parameters: wave height (H), period (T) and water depth (d).

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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

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

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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

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

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

objects, and unintended flooding of buoyancy tanks

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