Offshore Platform Types in Oil and Gas Explained

Around a quarter of the world’s oil and gas comes from offshore fields, from shallow coastal waters to ultra deep water far from land. An offshore platform must support drilling, processing, living quarters and export equipment while resisting waves, wind, currents and sometimes ice for 20 to 40 years.

What a production platform does

A production platform’s topsides typically include:

• Wellheads and manifolds, or connections to subsea wells through flexible risers.
• Separation of oil, gas and water in pressure vessels.
• Gas compression, dehydration and sometimes reinjection.
• Water treatment and injection for pressure support.
• Power generation, utilities, safety systems and accommodation.
• Export by pipeline or, for floaters, to shuttle tankers.

The substructure or hull carries the topsides and transfers loads to the seabed or to moorings.

Download diagram (WebP)

Bottom founded platforms

Steel jacket platforms

A jacket is a welded tubular steel space frame fixed to the seabed by piles driven through its legs. It is the most common offshore structure, used from a few metres to about 400 m water depth. Jackets are fabricated onshore, transported on barges and lifted or launched into place. Topsides are installed in one lift or several modules. Tubular members follow principles similar to land based steel sections, designed for wave fatigue and corrosion.

Gravity based structures (GBS)

Massive concrete bases sit on the seabed and are held in place by their own weight, needing no piles. Their hollow cells can store crude oil. GBS platforms suit harsh environments such as the North Sea and iceberg prone waters off Canada, in depths up to about 300 m.

Compliant towers

Slender towers that flex with wave forces instead of resisting them rigidly, used in about 300 to 900 m of water. Few have been built, as floaters have become more economical at these depths.

Jack-up platforms

A floating hull with three or four legs that are lowered to the seabed, then the hull is jacked up above the waves. Jack-ups are mostly mobile drilling rigs in up to about 120 to 150 m of water, but some serve as production or accommodation units.

Floating platforms

Tension leg platforms (TLP)

A buoyant hull is held down by vertical steel tendons anchored to the seabed and kept in high tension by excess buoyancy. Heave (vertical motion) is almost eliminated, allowing dry trees, wellheads on deck, which simplifies well access. Used in about 300 to 1500 m.

Spar platforms

A very deep draft vertical cylinder, often 150 to 200 m long, ballasted at the bottom and held by catenary or taut mooring lines. The deep draft gives small motions, also allowing dry trees in water depths up to about 3000 m.

Semi-submersibles

A deck supported by columns on submerged pontoons. The small waterplane area reduces wave motion. Semi-submersibles serve as drilling rigs and production platforms, usually with subsea wells, in depths from about 100 m to over 3000 m.

FPSO

A ship shaped vessel, new built or converted from a tanker, with process plant on deck and crude storage in its hull. It is moored by a turret that lets it weathervane with the wind and waves, or spread moored in calm regions. Shuttle tankers offload the oil. FPSOs dominate deep water developments offshore Brazil and West Africa and are widely used elsewhere. Gas fields can use FLNG vessels that also liquefy gas, using the LNG process.

Offshore platform types compared

Buoyancy in floating platforms

A floater’s hull displaces water equal to the total weight it supports (Archimedes’ principle):

$$\rho_w\,g\,V_\text{displaced} = W$$

Decommissioning

At the end of field life, platforms must be removed or made safe. Topsides are usually lifted off in modules or in one piece by heavy lift vessels, jackets are cut and removed, and wells are permanently plugged with cement barriers. Regulations in many regions require full removal of structures in shallow water, while some large concrete bases may stay in place under agreed conditions. Some regions allow parts of jackets to remain as artificial reefs.

Choosing a concept

Selection weighs water depth, reservoir size and life, distance to shore and existing infrastructure, environmental loads, need for dry trees, oil storage and export route, schedule, and cost. Small fields near existing hubs are often developed with subsea wells tied back to a host platform. The wells themselves are drilled by mobile rotary drilling rigs and maintained with pipeline pigging and inspection programmes across the life of the field.