Preliminary Design of Tendons for Deepwater TLP's

ABSTRACT This paper describes a procedure for preliminary design of tubular steel tendons for steel hulled tension leg platforms (TLPs) in deep water. First order wave forces and long period drift (quasi-static motions) are analyzed with the hull and tendons uncoupled, while motions at the fundamental heave and pitch periods of the TLP are analyzed using a simplified coupled model. Also considered are effects of tendon vortex shedding and an approximation to springing response. INTRODUCTION TLP tendons serve to restrain vertical and lateral motions of TLP hull. The procedure described in this paper was developed to perform preliminary cost estimates of steel TLP vessels with unpressurized tubular steel tendons in deep water. "Deep water" as used here means water depths in which unpressurized tubular steel tendons are no longer neutrally buoyant and in which platform heave and pitch resonance requires special consideration in tendon design. With this definition, "deep water" means depths greater than 3000 to 4000 feet. Figures 1, 2, and 3 show a design flowchart associated with this design procedure. The first section of the paper describes the analysis procedures. The second section briefly describes an example preliminary design for 7000 feet of water. The final section presents some qualitative conclusions reached from studies performed in various water depths using the procedure. This procedure considers three effects which become increasingly important to design of TLP tendons as water depth increases. First, tendon lateral and axial stiffnesses do not remain constant as the tendon is offset from a vertical position, but decrease. This occurs because unpressurized tubular steel tendons must resist increasing hydrostatic pressure as water depth increases, and so require decreasing diameter-to-thickness ratios. If the tendon is heavier than neutrally buoyant, it will "sag" when its top is offset and exert less restoring force than would a straight tendon. Current acting on a tendon also acts to reduce the restoring force. Figure 4 shows some typical deflected tendon shapes. Second, as tendons become longer, their axial stiffness decreases unless cross sectional area increases proportionately. Also, hull displacement must increase to support the longer, heavier tendons. Both of these effects tend to move the fundamental heave and pitch periods of the TLP into the range where significant wave energy exists. The result may be large tendon stresses and possibly inadequate tendon fatigue life. Third, tendon vortex shedding may occur, and its effects need to be considered. As stated above, this procedure uncouples analysis of the hull and tendons when evaluating first order waves and drift. The hull is modeled with tendon and riser reactions and a portion of their mass, while tendons are modeled using hullmean responses and response amplitude operators (RAOs). Use of the uncoupled analysis assumes that tendon and risermotions do not affect hull motions; this assumption becomes increasingly invalid as tendon and riser mass increase in deeper water. Use of the uncoupled analysis is justified for preliminary work by the widespreadavailability of computer programs for such an analysis.