Fixed and Floating Facilities

Fixed structures

• Steel jacket platforms — a welded tubular space frame piled to the seabed, supporting a topsides deck. The standard for shallow to moderate water depth and the most numerous type worldwide.
• Concrete gravity base structures — massive concrete structures held in place by their own weight, often with storage in the base. Used in harsh environments such as the North Sea.
• Compliant towers — slender structures designed to flex with wave loading rather than resist it rigidly, for deeper water than a conventional jacket.
• Minimal facilities and unmanned installations — small normally unattended platforms controlled remotely from a host, increasingly common for marginal fields.

Floating facilities

Beyond the depth at which a fixed structure is economic, facilities float.

• FPSO (floating production, storage and offloading) — a ship-shaped vessel with processing on deck and crude storage in the hull, offloading to shuttle tankers. It needs no export pipeline, can be relocated and reused, and is therefore the dominant solution for deepwater and for remote fields. Turret mooring allows weathervaning; internal or external turret designs suit different conditions.
• Semi-submersible production platforms — columns on submerged pontoons, giving good motion characteristics. No storage, so they require an export pipeline.
• Tension leg platforms — a buoyant hull held down by vertical tendons in tension, giving very low vertical motion, which allows dry trees and direct well access from the platform.
• Spars — a deep, slender cylindrical hull with excellent motion behaviour, also allowing dry trees.

Dry trees versus wet trees

This is one of the defining architecture decisions. Dry trees sit on the facility, giving direct, cheap well access for intervention — a major advantage over field life. They require a platform with very low vertical motion, which means a TLP, spar or fixed structure.

Wet trees sit on the seabed, connected by flowlines and risers. They allow an FPSO or semi-submersible to be used, permit wells spread over a wide area, and cost far more to intervene in. Intervention cost therefore feeds back into completion design, reliability requirements and the case for intelligent completions.

Topsides

The process plant offshore performs the same functions as onshore, under three constraints that shape everything: weight, because every tonne must be supported and installed; space, because the deck is finite and congested; and the fact that everyone lives on top of the plant.

That last point drives offshore layout philosophy: hazardous process areas are separated from the accommodation and control room by distance, by blast walls and by the arrangement of decks, with the temporary refuge, escape routes and evacuation systems positioned so that they remain usable during a major accident. Layout is a safety measure, not a space-planning exercise.