Marine Log - April 2008 - (Page 25)

approaching storm. In addition to the ship mono hull, three floating structure designs —TLP, Semi, and Truss Spar—have been successfully developed over the past several decades, says FloaTEC, a 50:50 joint venture company created by J. Ray McDermott and Keppel FELS to deliver deepwater floating production systems (FPS). Each of these floater designs has been evolving along with subsea and riser technology to meet new field development challenges mainly related to increased water depth and reservoir operating pressures and temperatures. FloaTEC says that the functional requirements for floating structures can be generalized as: • Drilling facilities Number, type and location of drilling rigs • Production facilities Weight, area and center of gravity • Drilling / production risers Number and arrangement • Well systems Number of wells, completion and workover methods, minimum well spacing and well bay location • Hull compartmentalization Damage stability considerations • Air gap requirements The relative hull responses of the three designs vary considerably. In all cases the objective is to minimize the response to the environment. FloaTEC says that efficient design of floating structures is predicated on functionality and performance—it should be capable of supporting all the necessary equipment for production and related tasks while meeting all performance criteria. The structure should provide sufficient space and robustness to fulfill its intended purpose; also, the floating structure should be built at a minimum of cost, which is governed mainly by the hull steel weight. SPAR TECHNOLOGY The classic Spars evolved into the Truss Spar by replacing the lower section of the caisson hull with a truss. Spars are often considered along with TLPs for dry tree solutions because they offer small vertical motion. However, Spars differ from both Semis and TLPs in the mechanism of motion control. One of the distinctions of the Spar, notes FloaTEC, is that its center of gravity is always lower than the center of buoyancy which guarantees a positive GM. This makes the Spar unconditionally stable. The Spar derives no stability from its www.marinelog.com mooring system, so does not list or capsize even when completely disconnected from its mooring. The deep draft is a favorable attribute for minimal heave motions, its deep draft and large inertia filter wave frequency motions in all but the larger storms. The natural period in heave and pitch are above the range of wave energy periods. The long response periods for Spars mitigate the mooring and riser dynamic responses common to ship shaped FPSOs and Semis. The deep draft, along with protected centerwell, significantly reduce the current and wave loading on the riser system These loads normally control the tension and fatigue requirements of the production risers on TLP or Semis. One of the principal advantages of the Spar lies in its reduced heave and pitch motions. Low APRIL 2008 MARINE LOG 25 http://www.bollingershipyards.com http://www.bollingershipyards.com http://www.marinelog.com

Table of Contents Feed for the Digital Edition of Marine Log - April 2008

Marine Log - April 2008 Contents Editorial Second Thoughts Update Inside Washington Offshore Vessels Evolve to Meet Subsea Demands Picking the Right Floaters for Deepwater Tugs: The Good Times Roll, But for How Long? Interactive TV at Sea Jo-Kell's Suzan Kelly Tugs & Barges Exhibitors Preview Beyond Imagination Tech News Newsmakers Contracts Events Website Directory Infodirect ML Marketplace Last Page

Marine Log - April 2008

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