Marine Log - April 2008 - (Page 26)

OFFSHOREDRILLING motions in these degrees of freedom permit the use of dry trees. Dry trees offer direct vertical access to the wells from the deck, which allows the Spar to be configured for full drilling, workover, production processing, or any combination of these activities. TLP: GETTING A LEG UP ON HEAVE The main principal of the TLP is to ICE CLASS TANKERS assure that the vertical forces acting on the platform are in balance, i.e. fixed and variable platform loads plus tendon tension are equal to its displacement. The VCG should be close to the platform’s geometrical center. Positive displacement is obtained by locking the platform’s draft below the fixed and variable payload displacement draft. This will result in an upward force applied to the ten- dons, thereby keeping them in constant tension. As a consequence the vertical platform motions (heave) is almost eliminated, except for motions resulting from tendon elasticity and vertical motion as result of environmental introduced lateral platform motions. The tendons do allow a lateral motion of the platform as a result of wind, wave and current that is similar to an inverted pendulum— except that the displacement variation by pulling the hull down gives a restoring force to the lateral movement. The tendon tension is set within predefined values, or window of operation. If the variable load of the platform exceeds these values by adding risers or drilling loads etc., the tendon pretension is adjusted by re-ballasting of the platform. As an evolution of the Classic TLP design, the ETLP (Extended Tension Leg Platform) has been developed. In this design development the columns are moved in board. This gives a wider tendon base for greater pitch stiffness (stability), a smaller spacing of deck supports for more efficient structure and other advantages. SEMISUBMERSIBLE UNITS Semi units’ benefits, include large payload capacity, limited sensitivity to water depth, quayside integration and the ability to relocate after field abandonment. A typical semi has four columns connected at the bottom by pontoon with a nominally rectangular cross-section. A truss structure connects the column tops and supports topsides modules. This arrangement provides a high degree of flexibility in fabrication methodology. The semi is designated as a “column stabilized units” (USCG, ABS, DNV, etc.). The columns are “stability columns” and primarily provide floatation stability. Important design variables are columns dimension and spacing, pontoon size and the ratio of pontoon width to pontoon height, draft of the hull, etc. In order to satisfy the stability and motion requirements, ranges for the variables and critical parameters such as GM value, free board value, heave natural period, etc are set as the constraints. Columns are sized to provide adequate waterplane area to support all anticipated loading conditions, spaced to support topsides modules, and tuned for a natural period of at least 20 seconds. These columns are supported by two parallel pontoons or a ring pontoon. Pontoons are sized to provide adequate buoyancy to support all weights and vertical loads, and proportioned to maximize heave damping. ML 26 MARINE LOG APRIL 2008 www.marinelog.com http://www.tnghph.com.mx/ingles/index.htm http://www.tnghph.com.mx/ingles/index.htm 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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