Unit
- Fixed platforms: Steel jacket or concrete gravity base structures that rest on the seabed. Used in shallow to moderate water depths (up to 500 m). Jacket platforms have tubular steel frames; gravity platforms use massive concrete bases for stability.
- Compliant towers: Tall, slender steel towers designed to flex with wave forces rather than resist them rigidly. Suitable for water depths between 300 and 600 m. The flexibility reduces base overturning moment.
- Floating systems: Include tension leg platforms (TLP), spar platforms, and semi-submersibles. TLPs are tethered to the seabed with vertical tendons; spars have a large vertical cylindrical hull; semi-submersibles are buoyant with pontoons. Used in deep waters (500-3000 m).
- Subsea systems: Equipment placed on the seafloor, including wellheads, manifolds, and pipelines. Connected to surface facilities via risers and umbilicals. Enable production in ultra-deep water and satellite fields.
- Arctic structures: Designed for ice-prone regions. Features include conical hulls to break ice, reinforced hulls, and ice-resistant mooring systems. Must resist ice loading and low temperatures.
- FPSO vessels: Floating production, storage, and offloading vessels. Converted from tankers or purpose-built. Equipped with processing facilities and storage tanks. Used in remote areas without pipeline infrastructure.
- Compliant structures: Hybrid designs like guyed towers which use guylines to provide lateral support. Used in moderate depths to reduce steel weight while maintaining stiffness.
- Jacket structures: Most common fixed platform type. Consist of tubular steel members welded into a space frame. Piles driven through legs into seabed provide foundation. Used for drilling, production, and accommodation.
- Drillships: Self-propelled vessels equipped with a drilling derrick and dynamic positioning systems. Capable of operating in very deep waters for exploration and well construction. They maintain station using thrusters rather than anchors. Often used in remote or ultra-deep locations where mooring is impractical.
- Gravity base structures (GBS): Massive concrete or steel structures that rely on their own weight for seabed stability. Commonly used in shallow waters with soft soil conditions. The base is often constructed on land, towed offshore, and then sunk into place. Provide large storage capacity and resistance to ice or seismic loads.
- Tension leg platforms (TLP): Floating platforms held in place by vertical tendons under tension. Tendons connect the platform hull to foundation templates on the seabed. This design minimizes vertical motion, allowing dry wellheads on deck. Used in deepwater fields requiring stable production conditions.
System Architecture
Pattern: Fixed Platform (Steel Jacket)
Components:
- Deck: Topside modules for drilling/processing. Supported by deck legs.
- Jacket: Braced tubular frame. Transfers loads to foundation.
- Piles: Driven into seabed through jacket legs. Provide axial and lateral capacity.
- Conductors: Guide pipes for well drilling. Attached to jacket.
- Boat landing: Access for supply vessels. Attached to jacket.
- Helideck: Helicopter landing area on topsides.
Quick Revision
- Offshore structures are categorized by function and water depth: fixed, compliant, floating, and subsea.
- Fixed platforms include steel jackets and concrete gravity bases, suitable for up to 500 m depth.
- Floating systems (TLP, spar, semi-submersible) are used in deeper waters up to 3000 m.
- Arctic and FPSO vessels are specialized for ice conditions and remote production.
- Jacket structures use tubular steel frames with pile foundations for stability.