The operating depths of offshore platforms for the oil and gas industry have increased tremendously since the humble beginning of offshore drilling on a wooden pier off the coast of California in 1897 by H.L. Williams. With advancements in technology over the last 90 years, the push to go into deeper and deeper waters has opened access to more oil fields. As a supplier of portable accommodation modules and technical buildings for offshore platforms, Armoda’s units are utilized across the full range of platforms at every depth. This article will cover the depths in which offshore platforms operate, from fixed platforms located just offshore to drillships operating in the open ocean.

0-3000 Feet

Fixed Platform (FP) – Up to 1,700 ft. 

Fixed platforms are the oldest offshore platforms, dating back to the 1930s. They have legs made of steel or concrete that are fixed to the seabed and support a deck with the drilling rig, production, and crew accommodations. They only operate in shallow waters of up to 1,700 ft, as the additional cost to handle the deeper waters is prohibitive. 

Compliant Tower (CT) – 1,500 to 2,750 ft.

Emerging in the 1980s, Compliant tower platforms 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 2,750 feet deep.

3000-5000 Feet

Tension Leg Platform (TLP) – 1,000 to 5,000 ft. 

Tension leg platforms, or TLPs, and their variations, are used for drilling and production in up to 5,000 ft. of water. They were first used in the North Sea in 1984 and are now worldwide. They have a hull with four air-filled pillars and a square pontoon structure that makes them bouncy. They also have tendons that connect the hull to the seabed anchors and keep them tight. This tension keeps the platform steady against vertical and rotational forces but allows some horizontal movement. Traditional TLPs operate in waters of up to 3,900 ft. Innovations in TLPs over the years have led to new types of TLPs today, such as the Extended TLP and the Seastar TLP.

Extended TLP (E-TLP)—This design has narrower gaps between the air-filled pillars and uses rings for the pontoons. The pontoons have horizontal parts that attach the platform to the seabed with tendons. These changes make the platform lighter without affecting the deck's capacity to hold the same topside arrangements as regular TLPs. The E-TLP is allowed to operate in water depths up to 5,000 ft.

Seastar TLP—Unlike the traditional and extended TLPs, the Seastar uses a large singular column for its buoyancy with three extensions for the tendons to connect the platforms to the seabed. They are generally smaller and used in waters of up to 3,500 feet.

Over 5000 Feet

Spar Platform – Up to 10,000 ft.

The design of the spar platform has changed slightly since its introduction in 1996, but the overall principle of a platform being placed on a large vertical cylinder that allows the platform to float has stayed the same. Spar platforms are used for drilling and production and can work in water up to 10,000 feet deep. Two of the most common types of spar platforms in use today are the traditional spar and the truss spar.

Traditional Spar—A traditional spar platform operates on the principle of a platform placed on top of a large hollow cylinder that descends into the water. At the bottom of the cylinder is a ballasting section filled with a material heavier than water, which shifts the center of gravity below the platform's center of buoyancy to ensure stability. The spar is further stabilized by spiraling strakes around the vertical cylinder and is moored to the seabed with 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. The truss spar is also moored to the seabed like the traditional spar.

Floating Offshore Production Systems

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.

Floating Production Storage and Offloading (FPSO) – up to 8,500 ft.

FPSOs, which emerged in 1977, are usually big vessels equipped to handle and store oil and gas in deep water. They consist of a vessel or semi-submersible with a mooring system that keeps the FPSO in place while it connects to receive oil and gas from a platform or subsea well. Two main elements link the FPSO to subsea wells, flowlines and risers. Flowlines carry the oil from the wells on the seabed to the risers. The risers then transport the oil from the bottom of the ocean to the FPSO, where it is processed and stored. FPSO can work in waters up to 8,500 feet deep. 

Semi-Submersible Platform (SSP) – up to 10,000 ft.

Semi-submersibles 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 they can operate in waters up to 10,000 feet.

Drill Ships – up to 12,500 ft.

Drill ships are large vessels 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 lets the drilling equipment pass through the hull and down to the seabed. In shallower water locations, the drill ship will use multiple anchors to moor itself and keep it stable. The ship will utilize a dynamic positioning system (DPS) in deeper water. The DPS uses the ship's thrusters, propellers, and sensors to measure the wind and water to maintain the ship's position. These advanced systems allow the drill ship to operate in waters up to 12,000 feet deep.

The oil and gas industry has continued to innovate since the first fixed platforms were used off the coast of California. With advancements in technology, offshore platforms can now operate at depths of up to 12,500 ft. There is a facility for every depth, from fixed platforms to drillships. Armoda’s portable accommodation modules and technical buildings are utilized across various platforms, providing safe and comfortable living and working spaces for offshore personnel. If you need additional accommodations or working spaces for your facilities, contact Armoda today.