The Tension Leg Semisubmersible (TLS): The Hybrid TLP-Semisubmersible Floater With the Spar Response

The Tension Leg Semisubmersible (TLS) floater is the second generation of the Free-Hanging Solid Ballast Semisubmersible (FHSB Semi) design. The FHSB Semi is the state-of-the-art design that maintains the advantages of the conventional Semisubmersibles and provides the motions of Spars. The FHSB Semi design is a conventional semisubmersible with an added feature of a Free-Hanging Solid Ballast tank (SBT) located deep below the hull keel level and connected through four groups of chains (on its four corners) to the semisubmersible hull. During the pre-service conditions, the chains are used to retract the SBT and maintain it at a close proximity to the hull keel to facilitate the transportation and installation operations. Once the semisubmersible is moored in-place, the chain system is operated to lower the SBT to its desired elevation below the hull keel and maintain it permanently connected to the semisubmersible hull during the in-service condition. The SBT provides substantial in-water weight and added mass that significantly reduce the platform response and maintains the chains in tension. In this paper, detailed description of the Tension Leg Semisubmersible (TLS) design as the second generation of the FHSB Semi is presented. The paper also includes discussions on the proposed project execution plan for the TLS covering the fabrication, transportation and installation as well as the added risks and risk mitigations associated with the new semisubmersible design and execution. A case study is introduced where a TLS is used as a dry tree host for a field located at 6,000 ft of water in the central Gulf of Mexico (GoM) harsh environment.