Outer Continental Shelf National Center of Expertise (OCSNCOE)
Fixed OCS Facilities
Fixed OCS Facilities, commonly referred to as "Fixed Platforms" can be constructed from several different structural designs. In this article, we'll look at the various designs associated with the fixed structures that are typically seen on the U.S. Outer Continental Shelf (Gulf of Mexico and off the coast of central California).
Conventional fixed platforms used for the production of oil and/or natural gas are constructed atop steel jackets that are anchored with steel piles secured into the seabed. These platforms are strong and hold up well against environmental forces, but they are too costly for depths over 1,500 feet.
These platforms can be manned or unmanned. The owner/operator decides if the platform will be manned or not.
33 CFR 106 is applicable if the platform has 150 people onboard for 12 hours or more in each 24-hour period continuously for 30 days or more, or it produces greater than 100,000 barrels of oil per day, or produces greater than 200 million cubic feet of natural gas per day.
Compliant towers have a smaller seabed footprint than the conventional fixed platform jackets and are flexible, which allows them to move with environmental forces. Compliant towers can consist of two types, a guy wire type and a freestanding tower.
A guy wired tower utilizes guy lines attached to the seafloor to keep the structure level, while the freestanding tower has no mooring other than the piles attached to the structure at the seafloor.
Compliant towers are used at water depths from 1,500 feet to 3,000 feet.
These platforms may or may not be manned and may or may not meet the applicability of 33 CFR 106.
A bridged platform is a group of platforms (2 or more) attached to each other by a bridge (walkway that supports personnel transit and utilities between platforms). The platforms that are bridged can be conventional fixed platforms or compliant towers or a combination of the two, but they will normally be of one type (typically conventional fixed platforms).
The normal configuration is to have the accommodations on one of the platforms along with production equipment and the other platforms will only have production-related equipment.
Even though bridged, each platform is viewed as its own for the purposes for applying regulatory requirements and 33 CFR 106.
Junction platforms can be either a conventional fixed platform or a compliant tower. They are primarily used to combine multiple pipelines into one pipeline for transport oil crude oil or natural gas to refining facilities ashore.
In addition, they may also have equipment to boost the pressure going into the combined export pipeline.
Floating OCS Facilities
Like fixed platforms, Floating OCS Facilities (FOFs), also historically referred to as Floating Offshore Installations (FOIs), vary in the types of designs in service.
TLPs are vertically-moored, column-stabilized oil and gas production units. The columns are connected by pontoons at the bottom of each column and the topsides are placed on the column tops.
They are held in place by pipe-like “tendons” under significant tension to mooring pilings installed into the seabed. The tension is held on the unit by excess buoyancy in the hulls (also known as columns). Ballast (seawater) in the hulls can be added, moved or removed to adjust the tension as needed. The tension that is maintained on the facility limits the horizontal movements and prevents vertical (heave) movements. The number of tendons can vary based on the size of the facility, environment and design.
TLPs are generally limited to water depths of 6,000 feet and less.
A semi-submersible looks very similar to the TLP. They are column-stabilized with the columns connected by pontoons at the bottom and the topsides are placed on top of the columns tops like the TLPs.
The major difference is that semi-submersibles are free-floating and held in place with steel or synthetic moorings attached to seabed mooring piles, normally consisting of a coated cable or synthetic line with chain at both ends. The tension in the moorings from the catenary in the cables/synthetic line limits movement of the unit.
Semi-submersibles have been installed in water depths of 1,000 feet and greater.
The Spar has one long cylindrical hull (Classic Spar) or a cylindrical hull and open truss structure (Truss Spar) that extends several hundred feet below the sea. At the bottom of the hull, or truss, is a permanent ballast tank that is filled with a material (liquid slurry) that is denser than the surrounding water. This material and tank are used to lower the center of gravity for improved stability.
The hull is encircled with helical strakes that limit the formation of vortices from the flow of water around the cylindrical hull that would otherwise cause the spar to surge and sway.
Spars are attached to the seafloor utilizing moorings attached to seabed mooring piles, normally consisting of a coated cable or synthetic line with chain at both ends. This design and mooring system will allow some lateral and rotational movement and will keep the vertical movement nominal.
Spars have been installed in water depths ranging from over 1,500 feet to nearly 10,000 feet.