INTRODUCTION TO OFFSHORE TECHNOLOGY

• Fixed Platforms – Jacket and Gravity

• Compliant Platforms – Articulated tower, Guyed tower and Tension leg platform

• Semi-submersibles, Jack-up Platform, FPSO and Spar

• Mooring, Anchors

• Submarine pipelines

• Corrosion

Purpose of the offshore structures

These installations are built for varied applications:

• Drilling.

• Preparing water or gas for injection into the reservoir.

• Processing oil and gas.

• Cleaning the produced water for disposal into the sea.

• Accommodation facilities.

Jacket platform• Design: Production;

• Steel framed tubular structure attached to sea bed with piles which are

driven into the seafloor (legs act as a guiding device or “jacket” for the

piles); constructed in sections and transported to the site;

• design lifetime - 10-25 years.

• Suitable for water depths 150-250m

• Soil conditions – Clay - penetrable –because of piles

A fixed platform may be described as consisting of two main components

the substructure - Jacket and Foundations the superstructure (Topsides) – Decks

Jacket platformSuperstructure• These are supported on a deck, which is fixed (mounted) on the jacket

structure.• These consist of a series of modules which house drilling equipment,

production equipment including gas turbine, generating sets, pumps, compressors, a gas flare stack, revolving cranes, survival craft, helicopter pad and living quarters with hotel and catering facilities.

• It can weigh up to 40,000 tonnes.• The topsides or the superstructure can be of modular decks or Integrated deck

(several storeys)• MSF- Module support frame supports the modules • Modular constructions used when the deck weight or geometry exceeds

available crane capacity• If total deck weight is within crane lifting capacity then integrated module is

used. (Results in steel saving)• Hybrid concepts also exists

- Jacket Platform

• The jacket surrounds the piles and holds the pile extension in position all the way

from mudline to the deck substructure. It supports and protects the well

conductors, pumps, sumps, risers and hence the name “jacket”

• The jacket legs serve as guides for the driving of piles and therefore it is called a

“template” – template structure

Elements / Parts of a Jacket Platform

Helideck

A raised level on a platform to facilitate helicopter landings. Normally, solar panels

are mounted just below the helideck framing to facilitate auxiliary-power for the

platform’s equipment.

Vent / flare boom

A long truss that supports a vent or a flare line.

Topsides (or Deck structure)

The upper part of the platform (generally above the reach of the highest wave

height) that houses most of the equipment related to Process, Mechanical, Electrical,

Piping and Instrumentation. Also houses generally some form of housing (Dog

house, LQ- Living Quarters or Workshop, Battery room, etc).

Jacket structure

The part of the platform that supports the topsides and is generally submerged below

the water line. A jacket may simply be called a supporting frame of the platform. Its

design is mainly governed by the wave loads.

Crane pedestal

A large structural tube that supports an offshore crane for lifting purposes. Sometimes,

crane pedestals also function as diesel storage tanks since their diameter is large

enough to house fuel.

Piles, Skirt piles

Major structural members are driven through hollow leg tubes to reach and embed

below sea-bed for depth that is adequate to support the platform in its worst ‘inservice’

conditions.

The pile embodiment is generally governed by the capacity of soil to withstand platform

loads and forces of environment (wave & wind) that act upon the platform. Pile also

connects the jacket structure to the topsides. Skirt piles are required when the soil is

very weak and the existing number of piles formed by the geometry of the platform is not

adequate. Skirt piles are run close to the main pile/leg as a cluster of 2, 3, 4, etc

Transition piece

A structural element generally in the form of a cone that links the topsides and jacket

structure. It is generally a cone since the topsides leg sizes are smaller in diameter

compared to that of jacket legs.

Conductors

Long hollow straight or curved tubes that embed into the seabed and through which

drilling for oil, gas or both is performed. To support such long length of tube, conductor

framings are provided to act as lateral support guides to conductors. Hence such plan

framings are called conductor framings. Diameter 50 cm to 75 cm.

Risers

• Long slender tubes that carry the crude or partially processed oil/gas to another location for

further processing either to a land based facility or to another platform by way of underwater

pipelines. Risers are generally clamped to the jacket structure.

• They transfer oil and gas from the seabed pipeline to the deck piping.

Boat landing, Barge bumper and Riser guards

• As the name suggests, this component of the platform is for berthing of supply vessels.

• To facilitate a smooth berthing, barge bumpers (that are equipped with shock cells) are

mounted on each side of boat landing to facilitate a reduced vessel impact on the structure.

• A riser guard is another protective structure that protects the oil/gas carrying risers. They are

designed for accidental vessel impacts.

Launch truss

• Sometimes, the jacket structures are very large and cannot be lifted even with large cranes,

hence permanent structures like launch trusses are provided on one side of the jacket to

facilitate the loading out on to the barge.

• If the jacket is designed for buoyancy, the jacket is launched in the sea after reaching its

destined position for a natural append and levelling.

• When the jacket is launched, it floats due to buoyancy. The jacket legs are then

sequentially flooded to make it upright and stand over the sea-bed before the piles are

driven through the legs to fix it to the sea-bed.

• The launch truss helps in skidding the jacket from the barge (large flat surface vessel)

to the sea.

Mudmat

• As the name suggests, mudmat is the bottommost framing of the platform that helps

resist the natural forces towards stability of the platform before the piles are

driven through the legs.

• The mudmat is like a large raft made up generally of timber that helps resist platform’s

sinking deeper if the soil is too soft near the top layer of sea-bed and also provides

adequate resistance to overturning.

Advantages

(1) support large deck loads

(2) may be constructed in sections &transported

(3) large field, long term production(supports a large number of wells)

(4) piles result in good stability,

(5) little effect from seafloor scour

Disadvantages

(1) costs increase exponentially with depth,

(2) high initial & maintenance costs,

(3) not reusable,

(4) steel structural members subject to corrosion

(5)Corrosion protection measures to be adopted

(6)Corrosion is severe near the SWL (waves)

Corrosion protection –

Anodes(Cathodic Protection)

• Submerged portions of the steel jackets are usually left uncoated,

Cathodic protection is provided to protect these areas

• It is a process where an electric current is created at the jacket

surface so the current flows from an external source into the metal

preventing the formation and loss of ions

• Impressed current system - When a current is supplied from an

external source it is called impressed current system

• Or a sacrificial anode are attached at selected points on the jacket

structure

• Sacrificial anodes are made up of magnesium, aluminum or zinc and

these are anodic with respect to protected steel structure

Foundation - Piles (piling)

• Piles penetrate great distances to ocean bottom to resist loads

Penetration – 150m

• Piles extend through the jacket legs

Total length – 250m

Typical dia 1.2m and length 200m

Thickness 16mm to 32mm

(Largest thickness - Near mudline)

• Axial load = skin friction and end bearing

Pile capacity Q= fs As + q Ae

• As - embedded surface area

• Ae - end bearing area

• fs - skin friction

• q - end bearing

Construction and Installation

• Most of the fabrication is carried out at the construction yard onshore

(close to coasts)

• On site installation ( in sea – environment)

• Launching and Upending of the jacket, driving pilings, placing

• Deck structure and welding all of these into a single unit

• Components are prefabricated into the largest units that can be

economically and quickly transported from the fabrication yard to the site

(sea)

• Construction can last 4 to 12 months depending upon complexity

Fabrication

• Assembled by constructing its narrow dimension frames lying flat on the

ground

• These are rotated by cranes into vertical position

• Cross bracings, guides and other members are added

• Finished jacket lies on the ground on its long sides

• The two legs in the middle are usually parallel (see fig, launch runners)

and the jacket is constructed with these legs lying on the launch beams

used to skid the jacket off the shore on to a barge

Installation

• After assembling the jacket and deck on the shore, the components are pulled

or lifted on to barges and transported to the site(sea – environment)

• At the site winch and cable assembly pull the jacket off its barge into the

water

• Flooding of the lower portion of the legs brings into an upright position of

the jacket (upending)

• A large derrick barge lifts the uprighted jacket and place it in the designated

spot

• Piles are driven through the jacket legs and through the skirt pile guide tubes

(if any)

• The deck sections are then mounted in place on top of the piles and welded

• Final phase: Prefabricated modules containing living quarters, pump

assemblies and other equipment are brought out by barge and lifted into

place on the deck substructure

Transportation and Installation procedure for a typical template structure

• Load out

• Towing

• Launching

• Floating

• Upending

• Vertical Position

• Piling

• Deck mating

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Installation

• Installation of a jacket by launching

• Installation of a Self floating-jacket

• Installation of a Horizontally connected sectionalized jacket

• Installation of a Vertically connected sectionalized jacket

Installation of a jacket by launching 1)Ballasting launch end of barge, 2)Moving jacket along skid beams, 3)Jacket pivots on rocker arms, 4)Floating in water, 5)Upending with derrick barge, 6)In place

Installation of a Self floating jacket1) Arriving at site, flooding begins 2) Controlled flooding lower legs 3) Controlled flooding of upper legs 4) Upending 5)Positioning 6) In place

Installation of a horizontally connected sectionalized jacket1) Launching lower section from a barge 2) Launching upper section from a barge 3) Aligning section in water 4) Upending by controlled flooding 5) In place

Installation of a vertically connected sectionalized jacket1) Launch base from a barge 2) Prepare to lower base 3) Lowering base 4) Launch and lower midsection 5) Launch and lower top section

Major Classes of Loads

• Deck and equipment loads - Self weight - Dry (self) and Wet (self plus

contents) of deck equipment and facilities, storage (mud, cement, pipe),

operations, buoyancy and self weight of structural elements

• Environmental loads - Met ocean loads – Meteorological and Oceanographic

loads, Include wind, wave, current and tide effects, Ice loads, Seismic loads,

Seabed settlements and movements

• Construction loads - Assembly loads, Transportation loads Installation loads

• Accidental loads - Occur due to human error, operational or equipment failures

• Vessel impact loads (supply boats, crew boats, tankers etc) Dropped objects

• Fires and explosions, Other events like environmental events exceeding than

considered in design