OFFSHORE PLATFORMS

BY

ABHISHEK KUMAR

Indian Maritime University

Visakhapatnam Campus

WHAT IS AN OFFSHORE PLATFORM?

A large structure at sea used to house crew and mechinery for exploration and/ or production of natural resouces such as fossil fuels from under the ocean bed.

Normally located in the continental shelf, but could be in deeper waters.

Could be fixed to sea bed or floating.

Could be dumb or mobile.

WHERE ARE OFFSHORE PLATFORMS?

Around 6,500 offshore oil and gas installations distributed in some 53 countries

US Gulf of Mexico: 4,000; Asia: 950; Middle East: 700; Europe, North Sea and North East Atlantic: 490; West Africa coast: 380; and South America: 340

TYPES OF OFFSHORE PLATFORMS

Fixed Platforms Compliant towers Jack-up Platforms Semi-submersible Platforms Drillships Tension-leg platforms SPAR Platforms Normally unmanned installations Floating production systems

FIXED OFFSHORE PLATFORMS

Superstructure: 'topsides' supported on a deck mounted on jacket structure. Has modules which house drilling equipment, production equipment, power generating sets, pumps, compressors, a gas flare stack, revolving cranes, survival craft, helicopter pad and living quarters with hotel and catering facilities. Weight - up to 40,000 tonnes. Substructure: (a) steel tubular jacket or (b)pre-stressed concrete structure.

FIXED OFFSHORE PLATFORMS(CONTD.)

Long term use production platform

Shallow depth installations (from 5m to 500 feet)

Load on platform depends on jacket length and can be very severe

A TYPICAL STEEL JACKET PLATFORM

A steel jacket structure A concrete gravity platform

A typical concrete platform.

FIXED CONCRETE PLATFORM

(TROLL A PLATFORM, NS, NORWAY)

FIXED CONCRETE PLATFORM

(TROLL A PLATFORM, NS, NORWAY)

OFFSHORE STRUCTURES – COMPLIANT TOWERS

Narrow, flexible towers on a piled foundation supporting a conventional deck for drilling and production operations.

With the use of flex elements such as flex legs or axial tubes, resonance is reduced and wave forces are de-amplified.

Used for conventional oil production from much greater depth upto 900m.

MAIN ATTRACTION: Due to its flexibility, the compliant tower system is strong enough to withstand wind and sea, even hurricane conditions.

COMPLIANT TOWERS - RECENT EXAMPLES

COMPLIANT TOWERS PETRONIUS, GULF OF

MEXICO, USA

OFFSHORE STRUCTURES – JACK-UP RIGS

Self-contained drilling rig on a floating barge, fitted with long support legs that can be raised or lowered independently of each other.

The rig is towed onto location by a supply vessel with its legs elevated and the barge section floating on the water.

Upon arrival at the drilling location, three legs are jacked down to the seabed, raising the entire barge slowly above water, to a predetermined height.

Used for exploration and production upto a depth of 100m, even upto 130m or 400 feet

A TYPICAL JACK-UP PLATFORM

SELF ELEVATING PLATFORM JB-109/JB-110,

SINGAPORE

FLOATING PLATFORMS

SEMI-SUBMERSIBLE PLATFORMS

Two or more submerged Hulls

Four or six pillars or legs

Platform about 100 feet above water

Remains afloat by weight and buoyancy balance

Height above water can be varied by buoyancy adjustment in tanks

SEMI-SUBMERSIBLE PLATFORMS (CONTD.)

Can be moved from place to place

Remains in position by multi-point anchoring to sea bed and

Dynamic positioning system

Can be operated inshallow as well as deep waters upto 5000 feet depth

A TYPICAL SEMI-SUBMERSIBLE PLATFORM

SEMI-SUBMERSIBLE PLATFORM THUNDER

HORSE, USA

DRILL SHIPS

It is a ship equipped with a drilling rig and station-keeping equipment such as anchor chains and/ or a DP system

They carry larger payloads than semi-submersible rigs and due to their mobility and ability to carry large amounts of drilling equipment, drill-ships are well suited to operate in remote and deep oceans

A TYPICAL DRILL SHIP

DRILL SHIP, CHIKYU, JAPAN

TENSION LEG PLATFORM

Semi-submersible construction with submerged hulls, four or six pillars and a platform

Vertical tethers hold the TLP to the sea floor and these are in tension always

Tension in tethers =

buoyancy - weight

TENSION LEG PLATFORM(CONTD.)

Vertical motion of structure restricted

Can be used in very deep waters upto 2000m depth

TENSION-LEG PLATFORM

FUNCTIONAL SCHEMATIC OF A TYPICAL TLP

TENSION-LEG PLATFORM: MARS, USA

MINI TLP

SPAR PLATFORMS

Single cylindrical strucures moored to sea bed holding a production platform above water Moored to the seabed conventional mooring lines. Three configurations: Conventional SPAR - one-piece cylindrical hull; Truss SPAR - where the midsection is composed of truss elements and Cell SPAR - which is built from multiple vertical cylinders. SPAR is more economical to build for small and medium sized rigs than the TLP, and has more inherent stability than a TLP.

SCHEMATIC OF A SPAR PLATFORM

DEPTH SCHEMATIC OF TYPICAL SPAR PLATFORMS

GENESIS SPAR PLATFORM, NEW ORLEANS, USA

OFFSHORE PLATFORMS – UNMANNED INSTALLATION (UI)

Offshore facility designed to be operated remotely without the constant presence of personnel. Small size with a helipad on top. These are easy to build and maintain, while avoiding the high operating costs of a full production platform. Used only in shallower water, where constructing many small UIs is a relatively easy and cheap option as compared to the cost of using subsea wells.

A TYPICAL UNMANNED INSTALLATION

SLEIPNER UNMANNED INSTALLATION, NORWAY

FLOATING PRODUCTION SYSTEMS

Large ships equipped with processing facilities and moored to a location for a long period.

FPSs: FPSO (floating production, storage, and offloading system), FSO (floating storage and offloading system), and FSU (floating storage unit).

FPSO is a "unit" or a "system“ of floating tanks designed to take all of the oil or gas produced from itself and/ or nearby platforms, process and store it.

From FPSO oil or gas is offloaded onto waiting tankers, or sent through a pipeline.

FSO is a similar system, but without facility for production and processing of the oil or gas.

FLOATING PRODUCTION SYSTEMS(CONTD.)

FPSOs are effective in remote or deepwater locations where seabed pipelines are not cost effective. FPSOs are economical in smaller oil fields which can be exhausted in a few years and do not justify the expense of installing a fixed oil platform. Once the field is depleted, the FPSO can be moved to a new location Large tankers have been converted to work as FPSO or FSO

SOME TYPICAL FLOATING PRODUCTION SYSTEMS

A TYPICAL SHUTTLE TANKER

SCHEMATIC OF A TYPICAL FPSO

FUNCTIONAL SCHEMATIC OF A TYPICAL FPSO

FPSO, MODEC V II

THANK YOU