What are the different types of offshore deepwater platforms?

Deepwater is considered any water depth deeper than 800 to 1,000 feet (244 to 305 meters), which is the approximate depth at which it becomes economically infeasible to utilize a fixed-based platform to support drilling and/or production equipment. All deepwater platforms have one common feature - they are based on a floating structure whose buoyancy supports the weight of the equipment. What differentiates one type of platform from another are the configuration of the floating component of the platform and the way that floating component is attached to the seafloor. The following are the main systems in use today:

• Semi-submersible (or "semi" or "semi-sub"): A semi is a floating structure consisting of a number of large hollow structural components called pontoons and columns, which together comprise the hull of the platform. The pontoons are oriented horizontally at the bottom of the structure and remain submerged at all times while the columns are oriented vertically and span from the pontoons, through the waterline, and up to a deck that supports the equipment. The pontoons and columns provide enough buoyancy to support the deck and equipment and allow the structure to float. The semi is moored (or connected) to the seafloor with a flexible system of wire rope, chains, and anchors (see Figure 1). This flexibility allows the platform to ride the waves much like a ship. It can move vertically (heave), and rotate (pitch, roll, and yaw) unrestrained. It can also move horizontally (surge and sway) until all the slack is taken up from the mooring lines, at which point the mooring system will restrain the motion and eventually pull the platform back to its original position. Depending on water depth and how taut a mooring sytem is used, a semi could move hundreds of feet sideways.
  
  
• Tension Leg Platform (TLP): A TLP is a floating structure with a hull that is similar to a semi-submersible, but with relatively small diameter hollow tubular pipes called tendons that run vertically and connect it to the sea floor. As opposed to a more conventional moorng system, these tendons are rigid vertically and prevent the platform from heavng, pitching, or rolling with the waves. However, given their long length, the tendon mooring system is flexible laterally and allows the platform to move sideways. Although not as flexible as a semi, a TLP is able to ride with hurricanes or other large waves while maintaining a relatively level deck. Figure 2 shows a schematic of the TLP concept, and Figure 3 shows a computer model that illustrates the hull (floating portion) and deck that supports the process and/or drilling equipment.
  
  
• Spar: A spar is a version of the semi with a hull that consists of a single large-diameter, deep draft, vertical cylinder supporting a deck as shown in Figure 4. The hull is connected to the seafloor using a taut catenary mooring system of 6 to 20 lines. The hull configuration minimizes heave motions under large sea conditions. The structure will move laterally under the restraint of mooring system and will rotate (tilt), but its vertical motion will be relatively small. As spar technology developed, the single very large diameter cylinder design was altered to replace deeper sections with a truss framework as shown in Figure 5.
  
  
• Floating Production Storage and Offloading System (FPSO): FPSOs are large, ship-shaped structures that are used to support offshore oil developments in locations where there is no infrastructure to move oil production to markets. Originally, FPSOs were converted oil tankers, which provided large decks with lots of area for production equipment and large cargo holds that could temporarily store produced oil. Converting an existing tanker was a relatively quick and inexpensive way to develop remote fields. However, specially designed and built FPSOs are often used for very large or unique developments. FPSOs are permanently moored in the field and connected to subsea wells, typically through a turret that is installed at the bow and which allows the vessel to weather-vane (rotate) in response to the wind and waves. Produced oil is treated on deck and then transferred to cargo tanks in the FPSO ship's hull and held until it is offloaded to other tankers that transport it to market. Produced gas is treated and either utilized to fuel the FPSOs onboard systems, reinjected into the reservoir, or transported via pipeline to shore. See Figure 6.

The majority of deepwater platforms are located in the Gulf of Mexico; however, there are other deepwater areas around the world including offshore Brazil, the west coast of Africa, and the North Sea.

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