Offshore platform-design

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PRESENTATION ON
OFFSHORE PLATFORM DESIGN

07/30/2003 OFFSHORE PLATFORM DESIGN

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.


OVERVIEW
s Offshore platforms are used for
exploration of Oil and Gas from
under Seabed and processing.
s The First Offshore platform was
installed in 1947 off the coast of
Louisiana in 6M depth of water.
s Today there are over 7,000
Offshore platforms around the
world in water depths up to
1,850M


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


OVERVIEW
Offshore platforms can broadly categorized in two types

s Fixed structures that extend to the Seabed.
s Steel Jacket
s Concrete gravity Structure
s Compliant Tower

OVERVIEW

Structures that float near the water surface- Recent development
s Tension Leg platforms
s Semi Submersible
s Spar
s Ship shaped vessel (FPSO)

TYPE OF PLATFORMS (FIXED)
s 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.

s 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.

s 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.


s 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)
s 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.

s 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.


s 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.


s SEMISUB PLATFORM
– Column pontoon junctions and bracing
attract large loads.
– Due to possibility of fatigue cracking of
braces , periodic inspection/
maintenance is prerequisite



s 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.
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.

s 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
s TOPSIDE:
– Facilities are tailored to achieve
weight and space saving
– Incorporates process and utility
equipment
s Drilling Rig
s Injection Compressors
s Gas Compressors
s Gas Turbine Generators
s Piping
s HVAC
s Instrumentation
– Accommodation for operating
personnel.
– Crane for equipment handling
– Helipad

PLATFORM PARTS
s MOORINGS & ANCHORS:
– Used to tie platform in place
– Material
s Steel chain

s Steel wire rope

– Catenary shape due to heavy
weight.
– Length of rope is more
s Synthetic fiber rope
– Taut shape due to substantial
less weight than steel ropes.
– Less rope length required
– Corrosion free


PLATFORM PARTS
s 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
s RISER:
– Remains in tension due to self
weight
– Profiles are designed to reduce
load on topside. Types of risers
s Rigid
s 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
s BARGE LOADOUT:
– Various methods are deployed based
on availability of resources and size of
structure.
s Barge Crane
s Flat over - Top side is installed on
jackets. Ballasting of barge
s 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
s 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.
s 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.
s The steelwork in the splash zone is
usually protected by a sacrificial wall
thickness of 12 mm to the members.


PLATFORM
FOUNDATION
s 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
s 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 s 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 yaw
Structural response - Involving structural deformations


STRUCTURAL DESIGN
s Loads:
s Offshore structure shall be designed for
following types of loads:
– Permanent (dead) loads.
– Operating (live) loads.
– Environmental loads
s Wind load
s Wave load
s Earthquake load
– Construction - installation loads.
– Accidental loads.
s The design of offshore structures is
dominated by environmental loads,
especially wave load

STRUCTURAL DESIGN

s Permanent Loads:
Weight of the structure in air,
including the weight of ballast.
– 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.


STRUCTURAL DESIGN
s Operating (Live) Loads:
– Operating loads include the weight of all non-
permanent equipment or material, as well as forces
generated during operation of equipment.
s The weight of drilling, production facilities,
living quarters, furniture, life support systems,
heliport, consumable supplies, liquids, etc.
s Forces generated during operations, e.g. drilling,
vessel mooring, helicopter landing, crane
operations.
s Following Live load values are recommended in
BS6235:
Crew quarters and passage ways: 3.2 KN/m2
Working areas: 8,5 KN/m2


STRUCTURAL DESIGN
s Wind Loads:
s Wind load act on portion of platform above
the water level as well as on any equipment,
housing, derrick, etc.
s 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.
s 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
Wave load:
s The wave loading of an offshore structure is usually the most important of all environmental
loadings.
s The forces on the structure are caused by the motion of the water due to the waves
s 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
s 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:
sLarge 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.
s 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
sAs 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.
s 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
Earthquake load:
sOffshore structures are designed for
two levels of earthquake intensity.
s Strength level :Earthquake,
defined as having a "reasonable
likelihood of not being exceeded
during the platform's life" (mean
recurrence interval ~ 200 - 500
years), the structure is designed
to respond elastically.
s 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.


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
Accidental Load :
s According to the DNV rules , accidental
loads are loads, which may occur as a result
of accident or exceptional circumstances.
sExamples of accidental loads are, collision
with vessels, fire or explosion, dropped
objects, and unintended flooding of
buoyancy tanks.
s Special measures are normally taken to
reduce the risk from accidental loads.


STRUCTURAL DESIGN
Load Combinations :
s The load combinations depend upon the design method used, i.e. whether limit
state or allowable stress design is employed.
sThe load combinations recommended for use with allowable stress procedures are:
s Normal operations
Dead loads plus operating environmental loads plus maximum live loads.
Dead loads plus operating environmental loads plus minimum live loads.
s Extreme operations
Dead loads plus extreme environmental loads plus maximum live loads.
Dead loads plus extreme environmental loads plus minimum live loads
sEnvironmental loads,should be combined in a manner consistent with their joint
probability of occurrence.
sEarthquake loads, are to be imposed as a separate environmental load, i.e., not to
be combined with waves, wind, etc.

STRUCTURAL ANALYSIS
s ANALYSIS MODEL:
s The analytical models used in offshore
engineering are similar to other types of on
shore steel structures
s The same model is used throughout the
analysis except supports locations.
s Stick models are used extensively for
tubular structures (jackets, bridges, flare
booms) and lattice trusses (modules,
decks).
s Each member is normally rigidly fixed at
its ends to other elements in the model.
s 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.

s STRUCTURAL ANALYSIS:
– 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.


s Acceptance Criteria:
s The verification of an element consists of comparing
its characteristic resistance(s) to a design force or
stress. It includes:
s a strength check, where the characteristic resistance is
related to the yield strength of the element,
s a stability check for elements in compression related
to the buckling limit of the element.
s An element is checked at typical sections (at least both
ends and mid span) against resistance and buckling.
s 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.
s Elements should also be verified against fatigue,
corrosion, temperature or durability wherever relevant.


STRUCTURAL DESIGN
s Design Conditions:
Operation
Survival
Transit.

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

s 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.


CODES
s Offshore Standards (OS):
Provides technical requirements and acceptance
criteria for general application by the offshore
industry eg.DNV-OS-C101
s Recommended Practices(RP):
Provides proven technology and sound engineering
practice as well as guidance for the higher level
publications eg. API-RP-WSD
s BS 6235: Code of practice for fixed
offshore structures.
– British Standards Institution 1982.
– Mainly for the British offshore sector.


REFERENCES
s W.J. Graff: Introduction to offshore
structures.
– Gulf Publishing Company, Houston 1981.
– Good general introduction to offshore
structures.
s B.C. Gerwick: Construction of offshore
structures.
– John Wiley & Sons, New York 1986.
– Up to date presentation of offshore design and
construction.
s Patel M H: Dynamics of offshore
structures
– Butterworth & Co., London.

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