It's a technical seminar report on the topic ' Offshore Platform Designs'. The report says about the various kinds of Offshore Platforms and their characteristics and specifications.

1. A report on the topic
OFFSHORE PLATFORM DESIGN
A BRIEF OVERVIEW
Durgapur Institute of Advanced Technology
And Management
Presented By
Subham Dey
(CH/14/09)

2. ACKNOWLEDGEMENT
I would like to express my deepest appreciation to all of those who provided me the possibility
to complete this report. A special gratitude I give to the mentor of my NPTEL course ‘Offshore
structures under special loads including Fire resistance’, Dr. Shrinivasan Chandrasekaran of
Indian Institute of Technology (IIT), Madras.
Furthermore I would also like to acknowledge with much appreciation the crucial role of the
faculties of Chemical Engineering Department, who gave the permission and all the necessary
enthusiasm and equipments to complete this report.
Last but not the least, many thanks go to Dr. Pronay Kumar Sinha who have insisted his full
effort in guiding us to achieve the goal.

3. INTRODUCTION
Offshore structures are mainly used for exploration, drilling of oil from seabed as well as for processing also.
The first offshore structure was constructed 1n 1947 off the coast of Lousiana in 6 meter of water depth.
Now in present days there are over 7000 offshore structures all over the world.
The top side of the structure or commonly named as the ‘Superstructure’ depends on the equipments,
utilities and the facilities to be given to the structure.
Unique functionalities of Offshore structures
Offshore structures are mainly designed for critical combination of loads (Special issues), which makes them
functionally unique are
1. Although the service life is exceeded, they’re expected to serve.
2. As they are functionally very important, frequent intervention for repair and maintenance is not
advisable.
3. Under the given variation of environmental loads and strength degradation due to corrosion. It’s
important that offshore structures should be strengthen to enhance the service life.
NO SPECIAL CODE REGULATION ARE ADVISABLE TO CARRYOUT THE OFFSHORE
STRUCTURES.
Design of the Offshore structures should be cost effective , should be effective in dispersing or transmitting
the loads efficiency. Even the structures repair under damages may caused by environmental loads should
be cost effective and should provide a long time solution.
Factors Considered during design
a) Structural geometry which is proposed should be simple and stable.
b) Should easy to fabricate, install and decommissioning.
c) Should encounter Low Capital Investment (CAPEX).
d) Easy startup production (High Return of Investment).
e) To select a comfortable and successful geometry which is important in FEED ( Front End Engineering
Design).

4. There are various types of Offshore Structures, Fixed bed Structures and Compliant Structures. Under the
compliant structure category, there are several types of structures namely Tension Leg Platforms (TLP),
Guyed Tower, Articulated Tower, Spar Platforms and Floating Production Storage and Offloading (FPSO).
Attractive Features of Floating Structures
a) Minimum Environmental hazard. (Crude oil can be disconnected and moved to the shielded area
when severe storm).
b) Lower turret system with mooring lines are located below wave zone. (Not affected by storm).
c) The mooring system and the associated turret system are smaller in size. (Useful only when FPSOs
are needed to be moved).
ADVANTAGES
 Very Low cost due to mobility.
 Reduced lead time.
 Quick disconnecting ability.
 Requires very little infrastructure.
DISADVANTAGES
 They are very brittle in terms of modes of failure.
 As brittle mode of failure is initiated, they generally fail instantaneously. Because more or less they
are identified by rigid system.
 Because of fixity and high degree of rigidity, they are not suitable for cyclic loading ( Behaviour under
earthquake load is very worrying).
 The cost of the platform increases with increasing water depth.
 Very high initial cost.
 Very high delayed time of production ( takes about 3-5 years for commissioning).
 Most commonly used material is steel which results in corrosion or material degradation leads to loss
of strength as a result reduction of service life occurs.

5. 1. Fixed Structures
Fixed structures are mainly supported on pile foundation, which is fixed at the bottom of the seabed.
Because of the fixity, it has a tendency to attract more forces. Fixed bed structures are expected to
withstand the huge forces by strengths and not by relative displacement of the structural members. It’s
generally stiff with the natural periods of 4-6 seconds and the natural frequency of 4-6 seconds.
The main ADVANTAGES of the fixity is
 It is relatively insensitive to lateral loads.
 It resist the lateral loads by bottom fixity and stiffness.
The main DISADVANTAGES are
 Difficult and complicated to install these platforms.
 Cost of the platforms increases with the increasing water depths.
 Structural action (Geometric Form) actually resist the lateral loads by virtue of their weights and fixity
i.e. support condition.
 Triggers brittle modes of failure.
THE MOST COMMONLY USED MATERIAL IS STEEL OR CONCRETE.
2. Compliant Structures
‘Compliancy’ is a term related to flexibility. This will go under large deformation due to loads and if done
on global. We call this as COMPLIANT.
Global displacement : The whole rigid body is moving with respect to the waves, so the structures and
the waves having relative displacement to each other, and this displacement reduces the forces on the
members.
“Offshore structures are flexible by geometry not by the reduced stiffness of the
members.”

6. a) Guyed Tower: Guyed towers are consist of the drilling derrick, fire bloom, residences, helipad
along with all the equipments like compressors, cooling towers, processing units, heat exchangers,
fire and blast resistance systems, storage units and many more utilities.
Guyed Towers mainly rest on the seabed using a Spud-Can arrangement, which acts like a inverted cone or
pendulum under the load action. This is actually a hinged joint enables the tower to undergo rotation about
the hinge. So under load, the platform gets rotated. This rotation enables the platforms to counteract the
wave loads.
As it is rotating about a hinged joint, it contains variable buoyancy and change in weight which challenges
the dynamic equation at any instant of time.
So, COMPLIANCY related to the
 Stability
 Large displacement
 Can’t resist loads only by its strength
 Resist load by relative displacement with respect to the wave action
The ADVANTAGES and DISADVANTAGES of Guyed Tower are as follows:
ADVANTAGES
 Lower cost compared to the fixed structures or bottom supported structures.
 Good stability.
 High reusability.
 Offer some decent level of decentering.
DISADVANTAGES
 Very high maintenance cost (Steel tower corrosion).
 Only applicable to small fields, (restoration depends upon buoyancy force).
 Difficult mooring system.
 Spud-can arrangement undergoes continuous rotation, which causes fatigue failure.

7. b) Articulated Tower: ‘Articulated’ term mainly related to the hinge joint. It is a type of platform
which is supported by a tower and rest on or connected to the seabed through a universal joint.
Ballast Chambers are introduced in Articulated tower mooring system to enable buoyancy induced
restoration or re- centering, storing the explored oil and reduce the weight of the mooring system to make
the structure light weight.
To ensure buoyancy and stability mainly two chambers are attached. They are buoyancy chamber and
ballast chamber.
Buoyancy chambers enable the buoyancy induced rotation, whereas the ballast chambers are used to store
explored oil and make the platform or the whole structure light weight.
The ADVANTAGES of Ballast Chambers are:
 Enables the shift of center of gravity to the bottom of the buoyancy point.
 It enables better stability under the action of lateral loads.
The Similarities and the differences between the Guyed Tower and Articulated Tower are:
DIFFERENCES
 In case of Articulated Towers, ballast chambers enable the shifting of C.G downwards which indicates
better stability than the Guyed Tower.
 Spud-can undergoes continuous rotation which causes fatigue failure. Universal Joints also gets
fatigue, but in this case recentering is quite gentle.
SIMILARITIES
 The moments at the joints are zero, which enables simple foundation system.
 Single point failure.
 Fatigue failure of joints.
STRUCTURAL RESPONSE IS QUITE HIGHER IN GUYED TOWER THAN THE ARTICULATED TOWER DUE TO MORE
RECEPROCATING ACTION.

8. c) Tension Leg Platform (TLP): To overcome the demerits of articulated and guyed towers,
Tension Leg Platforms are made. It’s a kind of structure that actually floats. When the structure
floats, the advantages are-
 Can be easily transported.
 Can be easily installed or commissioned.
 And most importantly, easily decommissioned.
TLP mainly follows a simple equation.
FB>>W
Where FB is the buoyancy force of the sea water and W is the weight of the platform. As buoyancy force is
very greater than the weight of the platform, to balance the equation a term T0 is added to the right hand
side, which is the pre-tension of the mooring system or the tethers by which the platform is attached to the
seabed.
FB=T0+W
The NOVELTIES in the TLP are:
 DO not have Spud-can or universal joint, so fatigue is avoided.
 Steel cases or towers on which the deck is placed are not suitable for deep water because of high
cost.
 Do not have Substructure, only have superstructure.
Generally the size of a TLP is 90m*90m*35m, which is 90m in length, 90m in width and 35m in height. As it’s
transparent to waves, it attracts lesser amount of forces.
Since weight of the platform is very less than the buoyancy force, it floats. Typical value of the pre-tension of
the tethers is about 15% to 20% of the platform.
Natural Periods of the TLPs are:
Degree of freedom Natural period of platform (Tn)
Surge 90-110 sec
Sway 90-110 sec
Heave 2-5 sec
Roll 2-5 sec
Pitch 2-5 sec
Yaw 60-90 sec
=

9. Where Wn is the natural frequency of the platform, k is the stiffness and m is the mass of the platform.
=
2
So, we can see here, for larger periods the frequency will be much lower (Surge, Sway, Yaw) and for lower
frequency, for the same mass m the stiffness is much lower.
So we can say TLPs are flexible in horizontal plane and rigid in vertical plane. So we call it a Hybrid structure.
MERITS
 High mobility
 High reusability
 Good stability
 Lower cost for increased water depth
 Deep water capability
 Easy installation
DEMERITS
 Very high initial cost
 Since tethers are suspected to high tension, causes fatigue failure
d) Spar Platform
Spar platforms consist of the superstructure with all the top side facilities like all the previous platforms.
Spar platforms are mainly use for ultra deep water (water depth> 1500m). Because in ultra deep water
tethering system does not work well as tension leg platforms, as because of very high buoyancy force,
for which tether pullout may occur. In Spar platforms a hollow cylinder also named as deep draft caisson
is used. Because of the extensive length of the cylinder compared to the thickness, vortex induced
vibration may take place. So to reduce or avoid it helical streaks are used to support it and strengthen it.
It consists of the multi storied deck (Production deck and Cellar deck). Mooring system goes through the
hollow cylinder to attach the deck with the seabed.
Structural action of the Spar platform is the buoyancy and the deep draft cylinder is similar to a large
buoy.
In surge degree of freedom, it’s flexible (Period 100-120 sec), in pitch degree relatively flexible (period 60
sec) and in heave degree natural period is about 25-40 sec.

10. The Differences between the Spar platforms and the Tension Leg Platforms are-
SPAR PLATFORM TENSION LEG PLATFORM
 Does not rely on mooring lines. It resist load by
the deep draft cylinder only.
 Spar does not have fatigue failure.
 Installation is quite difficult because topside need
to be assembled only after the cylinder is
upended.
 Derive restoring force from tendons or
mooring lines.
 Fatigue failure occurs in tendons.
 No such issues are there for TLPs.
ADVANTAGES
 Since more or less a free floating structure, the response in heave and pitch degree of freedom is
lesser.
 The drilling line goes through the cylinder which ensures the dry tree compliance (Protect the
risers from the wave action).
 Simple to fabricate
 Unconditional stability, because center of gravity always located lower than the center of
buoyancy which ensures good stability even when the mooring lines are disconnected.
DISADVANTAGES
 Difficult installation compared to the other platforms.
 Very little storage capacity.
 No drilling facilities are present there.
 Spar platforms are mainly used for storage and offloading, not for drilling.
 Because of the unusual length of spar hull, it’s prone to corrosion.
e) Floating Production Storage and Offloading (FPSO)
FPSOs are mainly ship shaped vessels, used to explore oil from the seabed where normal platforms
cannot be made or very difficult to made. A FPSO vessel is designed to receive hydrocarbons produced
by itself or from nearby platforms or subsea template, process them, and store oil until it can be
offloaded onto a tanker, or less frequently transported through a pipeline.
FPSOs are preferred in frontier offshore regions as they are easy to install, and do not require a local
pipeline infrastructure to explore oil.

11. FPSOs can be a conversion of an oil tanker or can be a vessel built specially for the application. A
vessel only to store oil (without processing) is referred to as a floating storage and offloading (FSO)
vessel.
ADVANTAGES
 These type of platforms are particularly effective in remote or deep water locations, where
seabed pipelines are not cost effective.
 Eliminate the need to lay long distance pipelines from the processing facility to to an offshore
terminal.
 This can provide an economically better solution for smaller oil fields, which can be exhausted
in a few years and do not justify the expense of installing a pipeline.
FPSOs can be of different types. They are-
1. FSO- Floating Storage and Offloading
2. FPSO- Floating Production Storage and Offloading
3. FDPSO- Floating, Drilling and Production, Storage and Offloading
4. FSRU- Floating Storage Regasification Unit
Some attractive features of floating structures are
 Minimum environmental hazard.
 Crude oil can be disconnected and moved to the shielded area when severe storm.
 Lower turret system and mooring are located below wave zone, which is not affected by the
storm.
 The mooring system and the associated turret system are smaller in size (useful only when
FPSOs need to be moved).
The DISADVANTAGES of this type of platforms are
 Useful only in small fields
 Have very low deck load capacity
 Since risers are exposed to wave action, they can be damaged
 Very little storage capacity
 Need actually offload the explored crude oil to the shuttle tankers for further processing

12. Some problems associated with the tendons in TLPs are
Design Objectives
1) Fatigue damage, caused by the change in pretension should be checked.
2) Initial pretension (T0) should always be positive.
3) The tendons should not slack. If happens, tendons will fail by buckling.
4) Maximum axial tension should not be more than allowable value. Otherwise it will result in
tether/tendon pullout.
The material which is commonly used as tendons is steel. Steel hollow tubes are used as tendons. It is
usually air filled at atmospheric pressure to reduce their weight in water
Recently Carbon fiber composite of specific gravity 1.59 is used as tendon material, which is
slightly more than water. It’s used because it’s light weight and have good fatigue resistant
and also have high capacity to withstand static loads.
Necessity of new generation platforms
There were several lacunas the existing platforms had. They are-
I. Very large hull displacement.
II. Very quick restoration, which can damage connecting risers.
III. Fatigue failure.
IV. Snapping effect.
V. Corrosion due to extended large deep draft caissons.
VI. Hull displacement in rotational degree of freedom is very challenging.
To avoid these problems, a new concept of making or developing the platforms arrived. Which is BASE
ISOLATION system.
The concept here is to isolate the sub and super structure by a medium or layer which then do not transform
the undesirable responses to submergence. When force causing on the substructure elements, they will not
transfer or influence the deck due to wave action. For superstructure, similar thing happens. Wind forces
acting on the superstructure which can cause high movement on the deck will not now influence the
substructure elements.
The DEMERITS of this kind of system are-
System should remain stiff in heave degree of freedom, which saves the platform from fatigue failure and
ensures the gentle recentering under the action of added mass (variable submergence).
Research works are being done on several topics and issues. Some of them are-

13.  How to keep the deck response local
 How to ensure the stiffness in heave degree of freedom
Researchers are struggling very hard to find a new way to resolve these issues. And as for base isolated
systems, various new generation platforms are developed which are now being used widely. Some of them
are
1. Offshore Triceratops
2. Buoyant Leg Structures (BLS)
3. Regasification Platforms
4. Buoyant Leg Storage Regasification Platform (BLSRP)
So, after all these discussions we can come to a conclusion that Offshore Structure are-
 Form dominated design
 Not function dominated usually
 Preferred that they remain compliant, but hybrid design is needed
 By Deck isolation deck responses are minimized.

14. REFERENCES
Here are all the references from where all the datum are taken to
complete this report.
 www.wikipedia.org
 www.elsvier.com
 www.offshorestructures.uk
 www.onlinecourses.nptel.ac.in
 Notes of Prof. Dr. Shrinivasan Chandrasekaran of the online course
‘Offshore Structures under special loads including Fire Resistance’.
 Images are hand drawn.