24-10-2016, 04:08 PM
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INTRODUCTION
1.1 General
One of the greatest discovery of 20th century was oil and it has so many applications that
it cannot be separated from mankind. The oil exploration has started as early as 1900 and
the oil exploration initially was concentrated on on land. As the need for oil expands in an
explosive rate, need for find new discoveries was eminent. During the middle of 20th century,
oil discovery started in near shore and medium range of water depth.
The need for qualified offshore structural personnel are rapidly increasing as the oil industry
moves into deeper water in the search for additional supplies of oil and gas, new technology
is emerging at a rapid peace for the development of new concepts for offshore platforms.
This book gives brief introduction to offshore engineering with basic concepts of various
types of offshore structures and provide insight into various design issues and requirements,
fabrication and installation techniques.
Chapter 1 gives introduction in to types of offshore platforms based on water depth requirements,
geometry and installation concepts.
Chapter 2 gives some introduction to design methodology, and various design stages in a
offshore development project.
Chapter 3 gives basic loads applied on offshore structures and techniques of calculations of
such loading.
Chapter 4 gives introduction to material requirement for offshore structures including corrosion.
Chapter 5 gives introduction to materials used for offshore structures, and corrosion and
cathodic protection.
Chapter 6 describes inplace analysis methodology, load combinations and and various principles
involved in the design.
Chapter 7 describes methodology to carry out the dynamic analysis of an offshore platform
and its application to fatigue and seismic analyses.
Chapter 8 gives method of fatigue analysis such as deterministic and spectral methods including,
selection of S-N curves, SCF equations etc.
Chapter 9 give some introduction in to ship impact with offshore platforms and method to
carry out push over analysis.
1.2 Types of Offshore Structures
The offshore structures built in the ocean to explore oil and gas are located in depths from
very shallow water to the deep ocean. Depending on the water depth and environmental
conditions, the structural arrangement and need for new ideas required. Based on geometry
and behaviour, the offshore structures for oil and gas development has been divided into
following categories.
1. Fixed Platforms
• Steel template Structures
• Concrete Gravity Structures
2. Compliant tower
• Compliant Tower
• Guyed Tower
• Articulated Tower
• Tension Leg Platform
3. Floating Structures
• Floating Production System
• Floating Production, Storage and Offloading System
1.3 Fixed Platforms
The fixed type of platform shall exhibit a low natural period and deflection again environmental
loads.
1.3.1 Steel template Structures
The steel template type structure consists of a tall vertical section made of tubular steel
members supported by piles driven into the sea be with a deck placed on top, providing space
for crew quarters, a drilling rig, and production facilities. The fixed platform is economically
feasible for installation in water depths up to 500m.
These template type structures will be fixed to seabed by means of tubular piles either driven
through legs of the jacket (main piles) or through skirt sleeves attached to the bottom of the
jacket.
The principle behind the fixed platform design is to minimize the natural period of the
structure below 4 seconds to avoid resonant behaviour with the waves (period in the order
of 4 to 25 seconds. The structural and foundation configuration shall be selected to achieve
this concept.
1.3.2 Concrete Gravity Platforms
Concrete gravity platforms are mostly used in the areas where feasibility of pile installation is
remote. These platforms are very common in areas with strong seabed geological conditions
either with rock outcrop or sandy formation.
Some part of north sea oil fields and Australian coast, these kind of platforms are located.
The concrete gravity platform by its name derive its horizontal stability against environmental
forces by means of its weight. These structures are basically concrete shells assembled in
circular array with stem columns projecting to above water to support the deck and facilities.
Concrete gravity platforms have been constructed in water depths as much as 350m.
Compliant Structures
In addition to the developing technologies for exploration and production of oil and natural
gas, new concepts in deepwater systems and facilities have emerged to make ultra-deepwater
projects a reality. With wells being drilled in water depths of 3000m, the traditional fixed
offshore platform is being replaced by state-of-the-art deepwater production facilities. Compliant
Towers, Tension Leg Platforms, Spars, Subsea Systems, Floating Production Systems,
and Floating Production, Storage and Offloading Systems are now being used in water depths
exceeding 500m. All of these systems are proven technology, and in use in offshore production
worldwide.
1.4.1 Compliant Tower
Compliant Tower (CT) consists of a narrow, flexible tower and a piled foundation that
can support a conventional deck for drilling and production operations. Unlike the fixed
platform, the compliant tower withstands large lateral forces by sustaining significant lateral
deflections, and is usually used in water depths between 300m and 600m.
1.4.2 Guyed Tower
Guyed tower is an extension of complaint tower with guy wires tied to the seabed by means of
anchors or piles. This guy ropes minimises the lateral displacement of the platform topsides.
This further changes the dynamic characteristics of the system.
1.4.3 Tension Leg Platforms
A Tension-leg platform is a vertically moored floating structure normally used for the offshore
production of oil or gas, and is particularly suited for water depths around 1000m to 1200
metres (about 4000 ft). The platform is permanently moored by means of tethers or tendons
grouped at each of the structure’s corners. A group of tethers is called a tension leg. A
feature of the design of the tethers is that they have relatively high axial stiffness (low
elasticity), such that virtually all vertical motion of the platform is eliminated. This allows
the platform to have the production wellheads on deck (connected directly to the subsea
wells by rigid risers), instead of on the seafloor. This makes for a cheaper well completion
and gives better control over the production from the oil or gas reservoir.
Tension Leg Platform (TLP) consists of a floating structure held in place by vertical, tensioned tendons connected to the sea floor by pile-secured templates. Tensioned tendons
provide for the use of a TLP in a broad water depth range with limited vertical motion. The
larger TLP’s have been successfully deployed in water depths approaching 1250m.
Mini-Tension Leg Platform (Mini-TLP) is a floating mini-tension leg platform of relatively
low cost developed for production of smaller deepwater reserves which would be uneconomic
to produce using more conventional deepwater production systems. It can also be used as a
utility, satellite, or early production platform for larger deepwater discoveries. The world’s
first Mini-TLP was installed in the Gulf of Mexico in 1998.
SPAR Platform (SPAR) consists of a large diameter single vertical cylinder supporting a
deck. It has a typical fixed platform topside (surface deck with drilling and production
equipment), three types of risers (production, drilling, and export), and a hull which is
moored using a taut catenary system of six to twenty lines anchored into the seafloor. SPAR’s
are presently used in water depths up to 1000m, although existing technology can extend its use to water depths as great as 2500m.
1.4.4 Articulated Tower
Articulated tower is an extension of tension leg platform. The tension cables are replaced
by one single buoyant shell with sufficient buoyancy and required restoring moment against
lateral loads.
The main part of the configuration is the universal joint which connects the shell with the
foundation system. The foundation system usually consists of gravity based concrete block
or some times with driven piles.
The articulated tower concept is well suited for intermediate water depths ranging from 150m to 500m.
Floating Structures
1.5.1 Floating Production System
Floating Production System (FPS) consists of a semi-submersible unit which is equipped
with drilling and production equipment. It is anchored in place with wire rope and chain,
or can be dynamically positioned using rotating thrusters. Production from subsea wells is
transported to the surface deck through production risers designed to accommodate platform
motion. The FPS can be used in a range of water depths from 600m to 2500m feet.
1.5.2 Floating Production, Storage and offloading System
Floating Production, Storage and Offloading System (FPSO) consists of a large tanker type
vessel moored to the seafloor. An FPSO is designed to process and stow production from
nearby subsea wells and to periodically offload the stored oil to a smaller shuttle tanker.
The shuttle tanker then transports the oil to an onshore facility for further processing. An
FPSO may be suited for marginally economic fields located in remote deepwater areas where
a pipeline infrastructure does not exist. Currently, there are no FPSO’s approved for use in
the Gulf of Mexico. However, there are over 70 of these systems being used elsewhere in the world.
1.6 Subsea System
Subsea System (SS) ranges from single subsea wells producing to a nearby platform, FPS,
or TLP to multiple wells producing through a manifold and pipeline system to a distant
production facility. These systems are presently used in water depths greater than 1500m.
1.7 Fixed Platform Concepts
For the last few decades, the fixed platform concept has been utilized extensively over 300m depth with various configurations.
Functional Classification
The offshore platforms for oil and gas exploration purpose can be classified based on functionality
and purpose of installation.
• Wellhead platform - primarily meant for drilling and supporting wellhead equipment.
It supports very few equipment such as wellhead control panel and piping. Occasionally
it also supports helicopter landing structure for emergency evacuation.
• Process Platform - primary meant for production facilities (oil or gas) and it may
support in addition to equipment for production, such as power generation, utilities
and living quarters.
• Riser Platform - This is another kind of structure specially built to support all the
incoming and outgoing risers on a planned complex. This will also be connected to the
main platform by bridge.
• Living Quarters Platform - Some times due to safety requirements, the living quarters
will be supported on a separate structure away from the wellhead and process
platforms. This types of platform will be located atleas 50m away from the neighboring
process platforms and will be connected by a bridge
Flare Support Platform- The flare boom structure to flare the excess gas from well
reservoirs may be supported on a separate structure either a tripod or four legged
jacket for safety reasons. This is to avoid excessive heat on wellhead and process
equipment on the neighboring platforms. Usually this will located away by a distance
to be calculated based on the heat output during flaring.
1.7.2 Geometrical Classification
The structural configuration of fixed template type structures vary extensively from location
to location depending on the requirement and environmental conditions such as water depth,
wave and current loads etc. Based on geometry, jackets can be classified in to following
categories.
• Tripod - basically to support minimum facility such as few wellhead and riser or to
support a bridge between two major platforms or to support a flare boom
• 4 Legged- typically for wellhead platforms
• 6 or 8 Legged - mainly for process complex
Foundation Concepts
The offshore platforms shall be fixed to the seabed by means of piles either driven through the
main legs of the jacket or through skirt sleeves attached to the jacket legs or the combinations
of both main and skirt piles. This kind of arrangement is shown in the following pictures.
DESIGN METHODOLOGY
2.1 General
The design of offshore structure is not an single step design process. The structural configuration,
arrangement, member sizes and its specification requirements can be arrived after
few design cycles. In order to achieve a optimum design suitable for the installation method
proposed and satisfy the final operating requirements, a design procedure suitable for the
project shall be developed.
In an offshore project, the design of structural elements cannot be initiated unless the basic
understanding of the needs are identified. The basic needs are
• What is the type of platform ? Oil or Gas, Process or Wellhead or Quarters etc
• What is floor area of topsides required ?
• Expected maximum weight of facilities ?
• What is basic water depth and environmental parameters such as wave and current ?
• Where is it located ?. Earthquake prone ?.
• What is type of installation ? Lift installed or Launch installed ?
• Any CAPEX constraints ?
The answer to the above questions will give some indication of type of jacket and topsides
required.
Design Stages
The various design stages in an offshore project is listed below.
• Front End Engineering Design (FEED) or Concept Selection
• Basic Design
• Detailed Design
2.2.1 FEED
The first step in initiating an offshore project is a FEED or concept selection. This stage of
project will involve following steps in all disciplines such as Process, Mechanical, Electrical
and Instrumentation in addition to Structural Engineering.
• Collection Process Data and identifying process needs and equipment
• Preliminary equipment sizing and area requirements
• Weight estimation based on past projects
• Identification of Structural configurations
• Preliminary estimation of structural weight
• Identification of installation methods
• Estimation of CAPEX within ±40%.
The above activities will define the project to a basic understanding and will provide enough
insight into carrying out further engineering activities.
2.2.2 Basic Design
At this stage of the project, the data collected during the FEED stage will be further verified
to make sure the authenticity and reliability of such data for further use. A detailed weight
estimates of all items involved in the project will be carried out. The process and mechanical
requirements will be further defined and identified. A Design Basis (DB) will be developed
for the proposed facility containing following information.
Process information containing type of well fluid (oil or gas) and its characteristics,
safety requirements and kind of process technology to be adopted.
• Mechanical requirement such as type of facility and basic equipment required for the
process, and material handling and safety
• Electrical requirement such power generation equipment, lighting and switch gears
etc
• Instrumentation requirement such as basic control system, feedback requirement etc.
• Piping information such as pressures, pipe sizes required etc.
• Meta-Ocean information such as water depth, wave, current, wind and tidal information
at the site.
• Structural requirement such as materials proposed or available for use in the country,
design method to be adopted, codes and specifications to be used etc.
• Installation information such as type of barge, lifting crane, loadout-method, piling
hammer etc.
At the basic design stage, the deck area required, number deck levels, etc will be defined.
This will lead to identification of number of legs required to support the deck. Normally the
spacing between deck legs for a typical platform can vary from 10m to 20m beyond which it
may become uneconomical to design a braced deck truss structure.
Basic weight estimates for various disciplines such as structural, mechanical, electrical, instrumentation
and piping will be carried out. Based on the weight of total deck, it may then
be decided to fabricate the deck in one piece or in various modules. This kind decision can
only be taken together with the viable installation options such ”Availability of Heavy Lift
vessels in the region” or use of float-over technique. In case such methods are not possible,
then the total topsides shall be divided in to various functional modules such as compression
module, process module module, utility module, quarters module, etc. These modules
are self contained units with structure, piping, equipment etc fabricated and transported to
the site. These modules are then installed on top of the ”module Support Frame”, which
transfers the loads to the jacket. Some times this module support frame may not needed, if
the modules are organised properly over the legs. This kind of basic ideas shall be made at
the basic design stage.
INTRODUCTION
1.1 General
One of the greatest discovery of 20th century was oil and it has so many applications that
it cannot be separated from mankind. The oil exploration has started as early as 1900 and
the oil exploration initially was concentrated on on land. As the need for oil expands in an
explosive rate, need for find new discoveries was eminent. During the middle of 20th century,
oil discovery started in near shore and medium range of water depth.
The need for qualified offshore structural personnel are rapidly increasing as the oil industry
moves into deeper water in the search for additional supplies of oil and gas, new technology
is emerging at a rapid peace for the development of new concepts for offshore platforms.
This book gives brief introduction to offshore engineering with basic concepts of various
types of offshore structures and provide insight into various design issues and requirements,
fabrication and installation techniques.
Chapter 1 gives introduction in to types of offshore platforms based on water depth requirements,
geometry and installation concepts.
Chapter 2 gives some introduction to design methodology, and various design stages in a
offshore development project.
Chapter 3 gives basic loads applied on offshore structures and techniques of calculations of
such loading.
Chapter 4 gives introduction to material requirement for offshore structures including corrosion.
Chapter 5 gives introduction to materials used for offshore structures, and corrosion and
cathodic protection.
Chapter 6 describes inplace analysis methodology, load combinations and and various principles
involved in the design.
Chapter 7 describes methodology to carry out the dynamic analysis of an offshore platform
and its application to fatigue and seismic analyses.
Chapter 8 gives method of fatigue analysis such as deterministic and spectral methods including,
selection of S-N curves, SCF equations etc.
Chapter 9 give some introduction in to ship impact with offshore platforms and method to
carry out push over analysis.
1.2 Types of Offshore Structures
The offshore structures built in the ocean to explore oil and gas are located in depths from
very shallow water to the deep ocean. Depending on the water depth and environmental
conditions, the structural arrangement and need for new ideas required. Based on geometry
and behaviour, the offshore structures for oil and gas development has been divided into
following categories.
1. Fixed Platforms
• Steel template Structures
• Concrete Gravity Structures
2. Compliant tower
• Compliant Tower
• Guyed Tower
• Articulated Tower
• Tension Leg Platform
3. Floating Structures
• Floating Production System
• Floating Production, Storage and Offloading System
1.3 Fixed Platforms
The fixed type of platform shall exhibit a low natural period and deflection again environmental
loads.
1.3.1 Steel template Structures
The steel template type structure consists of a tall vertical section made of tubular steel
members supported by piles driven into the sea be with a deck placed on top, providing space
for crew quarters, a drilling rig, and production facilities. The fixed platform is economically
feasible for installation in water depths up to 500m.
These template type structures will be fixed to seabed by means of tubular piles either driven
through legs of the jacket (main piles) or through skirt sleeves attached to the bottom of the
jacket.
The principle behind the fixed platform design is to minimize the natural period of the
structure below 4 seconds to avoid resonant behaviour with the waves (period in the order
of 4 to 25 seconds. The structural and foundation configuration shall be selected to achieve
this concept.
1.3.2 Concrete Gravity Platforms
Concrete gravity platforms are mostly used in the areas where feasibility of pile installation is
remote. These platforms are very common in areas with strong seabed geological conditions
either with rock outcrop or sandy formation.
Some part of north sea oil fields and Australian coast, these kind of platforms are located.
The concrete gravity platform by its name derive its horizontal stability against environmental
forces by means of its weight. These structures are basically concrete shells assembled in
circular array with stem columns projecting to above water to support the deck and facilities.
Concrete gravity platforms have been constructed in water depths as much as 350m.
Compliant Structures
In addition to the developing technologies for exploration and production of oil and natural
gas, new concepts in deepwater systems and facilities have emerged to make ultra-deepwater
projects a reality. With wells being drilled in water depths of 3000m, the traditional fixed
offshore platform is being replaced by state-of-the-art deepwater production facilities. Compliant
Towers, Tension Leg Platforms, Spars, Subsea Systems, Floating Production Systems,
and Floating Production, Storage and Offloading Systems are now being used in water depths
exceeding 500m. All of these systems are proven technology, and in use in offshore production
worldwide.
1.4.1 Compliant Tower
Compliant Tower (CT) consists of a narrow, flexible tower and a piled foundation that
can support a conventional deck for drilling and production operations. Unlike the fixed
platform, the compliant tower withstands large lateral forces by sustaining significant lateral
deflections, and is usually used in water depths between 300m and 600m.
1.4.2 Guyed Tower
Guyed tower is an extension of complaint tower with guy wires tied to the seabed by means of
anchors or piles. This guy ropes minimises the lateral displacement of the platform topsides.
This further changes the dynamic characteristics of the system.
1.4.3 Tension Leg Platforms
A Tension-leg platform is a vertically moored floating structure normally used for the offshore
production of oil or gas, and is particularly suited for water depths around 1000m to 1200
metres (about 4000 ft). The platform is permanently moored by means of tethers or tendons
grouped at each of the structure’s corners. A group of tethers is called a tension leg. A
feature of the design of the tethers is that they have relatively high axial stiffness (low
elasticity), such that virtually all vertical motion of the platform is eliminated. This allows
the platform to have the production wellheads on deck (connected directly to the subsea
wells by rigid risers), instead of on the seafloor. This makes for a cheaper well completion
and gives better control over the production from the oil or gas reservoir.
Tension Leg Platform (TLP) consists of a floating structure held in place by vertical, tensioned tendons connected to the sea floor by pile-secured templates. Tensioned tendons
provide for the use of a TLP in a broad water depth range with limited vertical motion. The
larger TLP’s have been successfully deployed in water depths approaching 1250m.
Mini-Tension Leg Platform (Mini-TLP) is a floating mini-tension leg platform of relatively
low cost developed for production of smaller deepwater reserves which would be uneconomic
to produce using more conventional deepwater production systems. It can also be used as a
utility, satellite, or early production platform for larger deepwater discoveries. The world’s
first Mini-TLP was installed in the Gulf of Mexico in 1998.
SPAR Platform (SPAR) consists of a large diameter single vertical cylinder supporting a
deck. It has a typical fixed platform topside (surface deck with drilling and production
equipment), three types of risers (production, drilling, and export), and a hull which is
moored using a taut catenary system of six to twenty lines anchored into the seafloor. SPAR’s
are presently used in water depths up to 1000m, although existing technology can extend its use to water depths as great as 2500m.
1.4.4 Articulated Tower
Articulated tower is an extension of tension leg platform. The tension cables are replaced
by one single buoyant shell with sufficient buoyancy and required restoring moment against
lateral loads.
The main part of the configuration is the universal joint which connects the shell with the
foundation system. The foundation system usually consists of gravity based concrete block
or some times with driven piles.
The articulated tower concept is well suited for intermediate water depths ranging from 150m to 500m.
Floating Structures
1.5.1 Floating Production System
Floating Production System (FPS) consists of a semi-submersible unit which is equipped
with drilling and production equipment. It is anchored in place with wire rope and chain,
or can be dynamically positioned using rotating thrusters. Production from subsea wells is
transported to the surface deck through production risers designed to accommodate platform
motion. The FPS can be used in a range of water depths from 600m to 2500m feet.
1.5.2 Floating Production, Storage and offloading System
Floating Production, Storage and Offloading System (FPSO) consists of a large tanker type
vessel moored to the seafloor. An FPSO is designed to process and stow production from
nearby subsea wells and to periodically offload the stored oil to a smaller shuttle tanker.
The shuttle tanker then transports the oil to an onshore facility for further processing. An
FPSO may be suited for marginally economic fields located in remote deepwater areas where
a pipeline infrastructure does not exist. Currently, there are no FPSO’s approved for use in
the Gulf of Mexico. However, there are over 70 of these systems being used elsewhere in the world.
1.6 Subsea System
Subsea System (SS) ranges from single subsea wells producing to a nearby platform, FPS,
or TLP to multiple wells producing through a manifold and pipeline system to a distant
production facility. These systems are presently used in water depths greater than 1500m.
1.7 Fixed Platform Concepts
For the last few decades, the fixed platform concept has been utilized extensively over 300m depth with various configurations.
Functional Classification
The offshore platforms for oil and gas exploration purpose can be classified based on functionality
and purpose of installation.
• Wellhead platform - primarily meant for drilling and supporting wellhead equipment.
It supports very few equipment such as wellhead control panel and piping. Occasionally
it also supports helicopter landing structure for emergency evacuation.
• Process Platform - primary meant for production facilities (oil or gas) and it may
support in addition to equipment for production, such as power generation, utilities
and living quarters.
• Riser Platform - This is another kind of structure specially built to support all the
incoming and outgoing risers on a planned complex. This will also be connected to the
main platform by bridge.
• Living Quarters Platform - Some times due to safety requirements, the living quarters
will be supported on a separate structure away from the wellhead and process
platforms. This types of platform will be located atleas 50m away from the neighboring
process platforms and will be connected by a bridge
Flare Support Platform- The flare boom structure to flare the excess gas from well
reservoirs may be supported on a separate structure either a tripod or four legged
jacket for safety reasons. This is to avoid excessive heat on wellhead and process
equipment on the neighboring platforms. Usually this will located away by a distance
to be calculated based on the heat output during flaring.
1.7.2 Geometrical Classification
The structural configuration of fixed template type structures vary extensively from location
to location depending on the requirement and environmental conditions such as water depth,
wave and current loads etc. Based on geometry, jackets can be classified in to following
categories.
• Tripod - basically to support minimum facility such as few wellhead and riser or to
support a bridge between two major platforms or to support a flare boom
• 4 Legged- typically for wellhead platforms
• 6 or 8 Legged - mainly for process complex
Foundation Concepts
The offshore platforms shall be fixed to the seabed by means of piles either driven through the
main legs of the jacket or through skirt sleeves attached to the jacket legs or the combinations
of both main and skirt piles. This kind of arrangement is shown in the following pictures.
DESIGN METHODOLOGY
2.1 General
The design of offshore structure is not an single step design process. The structural configuration,
arrangement, member sizes and its specification requirements can be arrived after
few design cycles. In order to achieve a optimum design suitable for the installation method
proposed and satisfy the final operating requirements, a design procedure suitable for the
project shall be developed.
In an offshore project, the design of structural elements cannot be initiated unless the basic
understanding of the needs are identified. The basic needs are
• What is the type of platform ? Oil or Gas, Process or Wellhead or Quarters etc
• What is floor area of topsides required ?
• Expected maximum weight of facilities ?
• What is basic water depth and environmental parameters such as wave and current ?
• Where is it located ?. Earthquake prone ?.
• What is type of installation ? Lift installed or Launch installed ?
• Any CAPEX constraints ?
The answer to the above questions will give some indication of type of jacket and topsides
required.
Design Stages
The various design stages in an offshore project is listed below.
• Front End Engineering Design (FEED) or Concept Selection
• Basic Design
• Detailed Design
2.2.1 FEED
The first step in initiating an offshore project is a FEED or concept selection. This stage of
project will involve following steps in all disciplines such as Process, Mechanical, Electrical
and Instrumentation in addition to Structural Engineering.
• Collection Process Data and identifying process needs and equipment
• Preliminary equipment sizing and area requirements
• Weight estimation based on past projects
• Identification of Structural configurations
• Preliminary estimation of structural weight
• Identification of installation methods
• Estimation of CAPEX within ±40%.
The above activities will define the project to a basic understanding and will provide enough
insight into carrying out further engineering activities.
2.2.2 Basic Design
At this stage of the project, the data collected during the FEED stage will be further verified
to make sure the authenticity and reliability of such data for further use. A detailed weight
estimates of all items involved in the project will be carried out. The process and mechanical
requirements will be further defined and identified. A Design Basis (DB) will be developed
for the proposed facility containing following information.
Process information containing type of well fluid (oil or gas) and its characteristics,
safety requirements and kind of process technology to be adopted.
• Mechanical requirement such as type of facility and basic equipment required for the
process, and material handling and safety
• Electrical requirement such power generation equipment, lighting and switch gears
etc
• Instrumentation requirement such as basic control system, feedback requirement etc.
• Piping information such as pressures, pipe sizes required etc.
• Meta-Ocean information such as water depth, wave, current, wind and tidal information
at the site.
• Structural requirement such as materials proposed or available for use in the country,
design method to be adopted, codes and specifications to be used etc.
• Installation information such as type of barge, lifting crane, loadout-method, piling
hammer etc.
At the basic design stage, the deck area required, number deck levels, etc will be defined.
This will lead to identification of number of legs required to support the deck. Normally the
spacing between deck legs for a typical platform can vary from 10m to 20m beyond which it
may become uneconomical to design a braced deck truss structure.
Basic weight estimates for various disciplines such as structural, mechanical, electrical, instrumentation
and piping will be carried out. Based on the weight of total deck, it may then
be decided to fabricate the deck in one piece or in various modules. This kind decision can
only be taken together with the viable installation options such ”Availability of Heavy Lift
vessels in the region” or use of float-over technique. In case such methods are not possible,
then the total topsides shall be divided in to various functional modules such as compression
module, process module module, utility module, quarters module, etc. These modules
are self contained units with structure, piping, equipment etc fabricated and transported to
the site. These modules are then installed on top of the ”module Support Frame”, which
transfers the loads to the jacket. Some times this module support frame may not needed, if
the modules are organised properly over the legs. This kind of basic ideas shall be made at
the basic design stage.