What is a semi-submersible and how has this technology been used in offshore production?

A semi-submersible (or "semi") is another form of development system that allows oil and gas drilling and/or production from deepwater fields. The operating equipment is supported by large hollow structural components called pontoons and columns, which together comprise the hull of the platform. The pontoons are oriented horizontally and at the bottom of the structure while the columns are oriented vertically and span from the pontoons, through the waterline, and up to the deck that supports the equipment. The pontoons and columns provide enough buoyancy to support the deck and equipment and allow the structure to float. A semi is typically moored to the seafloor with wire rope, chains, and anchors. The flexibility of the system allows the platform to ride the waves while moving hundreds of feet sideways rather than resist the force of the waves. As opposed to a TLP, which is connected vertically to the seafloor with tendons that restrict vertical motion, a semi can also move vertically with the waves.

Submersible drilling rigs, i.e., rigs that could float to be moved from location to location but then flooded with water to set directly on the seafloor, were used in shallow marshes and other protected waters as early as 1949. In 1961 Shell converted an existing submersible rig such that it could remain floating while drilling and connected to the seafloor with steel cables, thus becoming the first semi-submersible rig. Figure 1 shows the original Blue Water Rig No. 1 as used by Shell in the Gulf of Mexico. The first purpose built drilling semi-submersible, Ocean Driller, was launched in 1963.Technology advanced rapidly, with modern day semi-submersible drilling rigs capable of drilling in 10,000 ft and greater water depths. Figure 2 shows a 5th generation vessel, Transocean's Deepwater Nautilus, built in 2000.

Although an ideal vessel for drilling in deepwater, semi-submersible technology wasn't applied to production facilities until the industry perfected the use of subsea production systems. While traditional offshore platforms combine both drilling and production activities, a subsea system effectively decouples drilling and production. It utilizes a production wellhead on the seafloor with a flexible riser carrying the fluids to the surface. Such a system can accommodate the large vertical and horizontal motions of a semi-submersible. Semi-submersibles have been used in deepwater production since the early 1990s, and the deepest system is the Atlantis semi in approximately 7000' of water in the Gulf of Mexico. Figure 3 illustrates the 15 deepest water semi-submersible production platforms worldwide, and Figure 4 shows the second deepest, Shell's NaKika, which was installed in 2003 in 6,300 ft of water.

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