Offshore platforms are an integral part of the offshore oil and gas industry. There are many different types of platforms today, and differentiating the different types can be difficult.
Armoda has built technical buildings for and supplied portable accommodations and support modules to offshore platforms around the world. This article breaks down five of the most common types of offshore platforms utilized by the offshore oil and gas industry.
Offshore platforms consist of a large deck supported by legs reaching into the seabed floor or by a hull providing buoyancy to the structure. The equipment and facilities placed on the deck are referred to as topsides. Topsides can include the drilling rig, the production facilities, and the accommodations for the crew.
Fixed platforms, first introduced in the 1930s, are supported by legs embedded into the seabed. The legs are constructed of either steel or concrete, and they support a deck where the drilling rig, production, and crew accommodations are located. They are designed with an operational life of at least 25 years. Their construction also limits them shallow waters, up to 1,700 ft, as the additional cost to handle the deeper waters is prohibitive.
Compliant tower platforms, which emerged in the 1980s, are similar to fixed platforms in that they are supported by legs embedded into the ocean floor. Unlike fixed platforms, the support structure (tower) that supports the platform is narrower and allows for some flexibility. This flexibility enables the platform to handle the stronger forces found in deeper water. Compliant towers can be utilized in waters up to 2,750ft deep.
Tension leg platforms were first put into use in the North Sea in 1984 and can now be found worldwide. They are designed around a hull comprised of four air-filled pillars connected by a square-shaped pontoon structure that provides the buoyancy to support the deck. Tendons are anchored to the seabed, then connected to the hull and pulled tightly by the hull’s buoyancy. This tension enables the platform to remain stationary against vertical and rotational forces but does permit for some horizontal movement. TLPs are used for drilling and production and can operate in depths of up to 10,000ft. New designs of TLPs have been introduced since 1984, including the Extended TLP and the Seastar TLP.
Extended TLP (E-TLP) – This design brings the air-filled pillars closer together and incorporates a ring shape for the pontoons. Horizontal extensions are added to the pontoons enabling the tendons to connect the platform to the seabed. These changes allow for a reduction in the platform’s overall weight while still providing the ability to handle the same topsides arrangements on the deck compared to traditional TLPs.
Seastar TLP – Unlike the traditional and extended TLPs, the Seastar uses a large singular column for its buoyancy with three extensions out of it for the tendons to connect the platforms to the seabed. They are generally smaller in size than and are used in waters up to 3,500 ft.
The first spar platform was installed in 1996. Since that time, the platform’s design has seen some changes; but the concept of a platform being placed on a large vertical cylinder allowing the platform to float has stayed the same. They are used for drilling and production and can work in deep water up to 10,000ft. Two of the most common types of spar platforms in use today are the traditional spar and the truss spar.
Traditional Spar – The platform sits on top of a large hollow cylinder that descends into the water. At the end of the cylinder is a ballasting section filled with a material that weighs more than water. This enables the center of gravity to be moved below the platform’s center of buoyancy, ensuring that the facility does not topple over. To help keep the spar stable, spiraling strakes surround the outside of the vertical cylinder. The spar is then moored to the seabed by a series of cables and lines.
Truss Spar – Constructed similarly to the traditional spar, the cylinder is shortened, and a truss structure is used to extend the ballasting section to the required depth. The truss structure consists of four vertical legs with -braces and heave plates. This design change allows the truss spar to be produced at a lower cost as less steel is needed for its construction. Like the traditional spar, the truss spar is also moored to the seabed.
The platforms mentioned so far have been designed to be permanently moored to the seabed. Floating Offshore Production Systems (FPS) are designed to move from location to location and provide a more cost-effective solution for deeper waters. Three of the most common types of FPS in use today are semi-submersibles, floating production storage and offloading vessels, and drillships.
Semi-Submersible Platform (SSP)
Semi-submersibles, introduced in 1961, place a deck on top of cylindrical columns that connect to horizontal pontoons submerged beneath the water’s surface. The SSP can be self-propelled or towed to location where they partially flood the pontoons and columns to lower the platform and create greater stability in the water. Depending on the design, multiple anchors or a system of positioning thrusters are used to keep the platform in the desired location. Semi-submersible platforms are used for mobile offshore drilling units (MODUs), production, and heavy lift cranes and can operate in waters up to 10,000ft.
Floating Production Storage and Offloading (FPSO)
FPSOs, first built in 1977, are typically large vessels that have been outfitted to process and store oil and gas in deep water. They consist of a vessel or semi-submersible with a mooring system designed to keep the FPSO stationary while it connects to receive oil and gas from a platform or subsea well. There are two main components used to connect the FPSO to subsea wells, flowlines, and risers. Flowlines transport the oil from the seabed wells to the risers. The aptly named risers then transport the oil up from the ocean floor to the FPSO, where it is processed and stored. FPSO can operate in waters up to 8,500 ft deep.
Drillships, first deployed in 1955, are large vessels that have been outfitted to drill for oil in deep water. They are designed with a derrick that sits over a moon pool in the hull. The moon pool enables the drilling equipment to pass through the hull and down to the seabed. In shallower water locations, the drillship will use multiple anchors to moor itself and keep stable. In deeper water, the ship will utilize a dynamic positioning system (DPS). The DPS uses the ship’s thrusters and propellers, and sensors measuring the wind and water to maintain the ship’s position. These advanced systems allow the drillship to operate in waters up to 12,000 ft deep.
These are the types of offshore platforms used by the offshore oil and gas industry. It is important to note that these are general descriptions. Each platform is uniquely designed to meet the specific requirements of the location where it is deployed. There's also a lot to think about when it comes to getting an offshore project started and also in servicing offshore buildings and modules that deployed on these platforms.
For project planning, it’s essential to understand the types of platforms utilized in the industry and the various codes (ABS, USCG, DNV, etc.) and regulations that differ depending on the platform, its function, and location. Each type has specific requirements that can impact the equipment and facility specifications utilized for its topsides structure.
Armoda can assist you with project planning and help develop an understanding of these facility requirements. Contact us today with any questions you may have.