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Guide

Offshore installations: platform types and how a field is developed

The installation a field gets depends on the water depth, the size of the field, whether it holds oil or gas, and how far it is to existing pipelines and platforms. Here are the types you meet on the shelf, and the road from a discovery to a producing field.

Updated · about 5 min read · Written by OESA

What an installation does

An installation on a field does one or more of these jobs:

  • Drilling wells, either from the platform itself or from a mobile rig.
  • Processing: the well stream is a mix of oil, gas and water that has to be separated and treated before it can be sold.
  • Storage and offloading of oil to tankers, or export by pipeline.
  • Accommodation for the people working there.

Not every installation does everything. The Norwegian Offshore Directorate lists the functions of each installation in its FactPages. Troll B, for example, has processing and accommodation but no drilling; its wells are on the seabed around it. The word “platform” is often used for any installation with a deck above the water, including those that float.

Fixed installations: steel and concrete

Steel jacket

A lattice of steel tubes standing on the seabed, fixed with piles, with the topside on top. Jackets are the most common fixed type, especially in shallower water. The platforms on Johan Sverdrup stand on steel jackets in about 113 metres of water (Norwegian Offshore Directorate).

Concrete gravity base

A concrete structure that stands on the seabed under its own weight, without piles. In Norway the type is often called Condeep. The base has cells that on several of the platforms are used to store oil. Troll A is a Condeep with four shafts in 302 metres of water, in operation since 1996, and the Statfjord, Gullfaks and Oseberg A platforms are of the same kind (Norwegian Offshore Directorate).

Floating installations

In deeper water, fixed structures become too large and expensive. Instead, installations that float are used, held in place by mooring lines or tendons:

  • Semi-submersible (semi): the deck stands on columns above pontoons that sit below the waterline, so it moves little in the waves. Held in place by mooring lines. Most mobile drilling rigs on the Norwegian shelf are of this type, and Troll B is a concrete semi-submersible production platform.
  • Tension leg platform (TLP): a floating hull held down by taut tendons fixed to the seabed. It hardly moves up and down, so the wellheads can sit on the deck. Snorre A is a steel TLP and Heidrun a concrete one.
  • Spar: a tall, cylindrical floater with a deep draught, which makes it stable. Aasta Hansteen in the Norwegian Sea is a spar in about 1,300 metres of water, in operation since 2018 (Norwegian Offshore Directorate).
  • FPSO (floating production, storage and offloading): a ship-shaped hull that receives, processes and stores oil and offloads it to tankers, on the Norwegian shelf often shuttle tankers. Many are moored to a turret so the ship can turn with the weather. Johan Castberg in the Barents Sea started in 2025; Norne, Skarv, Alvheim and Goliat are other examples.

Storage vessels without processing are called FSUs (floating storage units).

Subsea systems

A subsea system has the wells and valves on the seabed, in templates. The well stream flows by pipeline to a platform, an FPSO or straight to shore, and is controlled through umbilicals.

  • Small discoveries are usually tied back to a field that is already producing. According to norskpetroleum.no, a tie-back to existing infrastructure is the most likely solution for most of the discoveries being considered for development.
  • Some larger fields have no platform at all. Snøhvit and Ormen Lange produce from the seabed and send the well stream by pipeline to the plants at Melkøya and Nyhamna.
  • Subsea wells are drilled and maintained from mobile rigs and vessels, see the guide to the rig market and offshore vessels.

What decides the choice

  • Water depth. Fixed structures in shallower water, floaters in deeper water.
  • Size and lifetime. A large field that will produce for decades can carry a platform of its own. A small discovery rarely can.
  • Oil or gas. Oil can be stored and loaded onto ships at the field. Gas needs a pipeline.
  • Distance to infrastructure and whether it has spare capacity, see the guide on how to assess a discovery.
  • The wells. Wellheads on the deck make drilling and well maintenance easier and cheaper; subsea wells need a rig or vessel.
  • Energy. For every new development the operator must show the energy demand and what it would cost to use power from shore instead of gas turbines (PDO/PIO guidelines, 2022). See the guide on the energy transition.

From discovery to producing field

The authorities divide the planning into three milestones (PDO/PIO guidelines, 2022; Norwegian abbreviations in brackets):

  1. Decision to concretise (BOK): the licensees have identified at least one technically and economically feasible concept and start the studies leading to a concept selection.
  2. Decision to continue (BOV): one concept has been selected and is studied further.
  3. Decision to implement (BOG): the investment decision, followed by submission of the plan for development and operation (PDO).

The PDO consists of a development part and an impact assessment covering the environment, fisheries and society, which is sent out for public consultation. The Ministry of Energy approves the plan; large or important projects go to the Storting (parliament) first. The authorities normally take three to six months to process it (norskpetroleum.no and the PDO/PIO guidelines, read 29 Sep 2026).

Then come construction (yards and fabrication), installation at the field, drilling and start-up. During operations new wells are drilled and new discoveries are tied in. As the field nears its end, the licensees must submit a decommissioning plan two to five years before the production licence expires, proposing what will happen to the installations.

At the end of 2025, 97 fields were producing and 19 projects were under development (norskpetroleum.no, read 29 Sep 2026).

See the installations yourself

Sources

The guides are written by OESA students to explain concepts and how things fit together. They are not investment advice. Figures that are not definitions carry a source and a date; if you find an error, let us know at styret@oesa.no.

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