ABSTRACT

Wave-drift damping has been calculated by the potential theory developed by Zhao and Faltmsen, [1] Model tests have been carried out to verify the numerical results Decay tests In regular head sea waves were analyzed to find the wave-drift damping and viscous drag damping An approach for establishing the viscous damping term is suggested. The agreement between calculated and

INTRODUCTION

The tension leg platform (TLP) is one of the most promising concepts for floating production of gas and of in deep waters It consists of a floating structure which is restrained from oscillating vertically by tethers (vertical anchor bases) that are tensioned by the platform buoyancy being larger than the platform weight In a hydrodynamic analysis of a TLP the main objectives are to calculate

  • the vertical dynamic loads on the platform with the purpose of estimating axial forces in thetethers

  • the horizontal motion of the platform with the purpose of predicting bending stresses In the risers and tethers, rotation capacity of the anchor and cross-load bearings and the set down of the platform and the resulting diminished air gap

  • the wave elevation close to the platform m order to estimate the air gap

The present work is focused on the second and third Items

It 18 equally important to find both the excitation forces and damping In order to establish a good prediction of the slow-drift motions of the TLP Most previous work concentrates on the excitation side while the damping has been treated with less effort It is the damping which will be emphasized here in Both mean and slowly varying wave-drift damping is an Important part of the total damping For the estimation of the standard deviation the mean value of the wave-drift damping is most important, while both mean and slowly varying wave-drift damping play a role In the extreme value distribution of slow-drift motions, see Zhao and Faltmsen, [3] The computation of slow-drift damping In regular waves would be based on a rational three-dimensional potentla1 theory developed by Zhao and Faltinsen, [I] In the free surface and body boundary conditions the interactions with slow-drift motions are taken care of This paper describes briefly he method applied with special attention to the calculation of mean drift forces In combined waves and current as well as the wave-drift damping In addition the wave field is calculated close to the platform The slow-drift motion has a significant influence on the wave elevation and air-gap between the waves and the platform deck The calculations give very large changes m the wave pattern between the platform legs depending on the wave frequency and slow-drift velocity

A TLP-concept has been tested hydro dynamically by using both computer programs and model tests The tests were done at MARINTEK The main dimensions are given in Figure 1 and Table 1

Figure 1 The main dimensions of the platform See Table 1(Available in full paper)

Figure 2 Computational model where the Inner domain IS bounded by the surfaces SB, SF1 and Sc The multipoles are marked as points on h e segments inside the TLP structure (Available in full paper)

Table 1 The main parameters In full and model scale (Available in full paper)

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