Response of Tension Leg Platform to Wave Drift Forces

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ABSTRACT The wave drift forces on tension leg platforms (TLP) are contributed by second-order potential and viscous wave load effects, the fluctuations in wave surface elevation and the influence of platform displaced position on the wave excitation. These forces are expressed in terms of in-phase and out-of-phase drift forces. In this study, computationally efficient time domain and frequency domain based schemes are developed to evaluate the TLP response to drift forces. These schemes retain the statistical relationship that exists among these drift forces and the first-order wave forces which is important for the combined response analysis. A parameter study is conducted to delineate the relative significance of different drift forces for several representative sea-states. I. INTRODUCTION In addition to the wave forces at the typical wave frequencies in a sea-state, a compliant structure experiences slowly varying wave drift forces. These forces originate from various mechanisms involving the nonlinearity of the viscous drag term, nonlinearity in the potential forces, variations in free water surface elevation near TLP columns and the nonlinear feedback of the structural response to the wave loads. The tension leg platform has low natural frequencies in the compliant modes, i.e., motion in horizontal plane. The wave drift force at the difference frequency contributes significantly to its horizontal displacement. In view of the complexity ofloading and TLP response characteristics, an enhanced response prediction capacity with respect to the environmental loads promises to provide useful input to the reliable design of TLPs in deep water. This paper focuses on the computation of waveinduced drift forces and associatedplatform response utilizing computationally efficient schemes developed in this study. Many researchers (e.g., de Boom et al, 1985) have described the drift forces on a TLP as a result of the second-order terms in the wave diffraction force. This force is often called potential drift force. Others (Finnigan et al., 1984 and Botelho et al .,1984, Salvensen et al., 1982) have focussed on the fact that the drag force on the TLP components can cause viscous drift force resulting from the nonlinearity in Morisonls drag force formulation. Normally the wave forces on a TLP are calculated at their undisplaced position. McIver (1976) and Rainey (1977) have emphasized that the wave effects on TLPs should be evaluated at their displaced position. In regular waves, by neglecting this effect one may underestimate response by a steady offset. In random wave conditions, this offset results in another form of drift force (Li, 1988, Li and Kareem, 1992c), which herein is called displacement feedback drift force. In this study, numerical scheme were developed, in both time and frequency domains, to describe the TLP response induced by different drift forces on TLP horizontal motion. It is noted that the total drift force is not a simple summation of the individual drift forces.