ABSTRACT

This paper presents analyses of experimental data and an empirical model for horizontal forces on cylinders due to laboratory-generated asymmetric waves. Given the time history of the incident wave elevation, the model can predict the horizontalforce. Four single cylinders of different diameters are tested in three transient asymmetric waves and two high seas.CM and Cd are determined for each cylinder and the question of the effects of hydrostatic force fluctuations and convectiveacceleration has been clarified.

INTRODUCTION

Column-supported structures, e.g., tension leg platforms (TLPs) are very sensitive to high seas. The ringings of TLP tendons aredue to steep asymmetric waves (Natvig, 1993). The ringinghydrodynamic problems have not yet been completely solved, because the waves that can cause the ringings in association withspringings are very special. In addition to this, one must literallymeasure the ringings experimentally or simulate ringing loads dueto the waves and predict the ringing tensions. Our research has begun with the study of the kinematics of laboratory-generated extreme waves including the transient wavegroup (TWG) (Kim et al., 1992). The TWG has a steep asymmetriccrest, and the horizontal particle velocities in the crest aremuch higher than those of the Stokes 5th-order-like regular wave(SRW) whose height is equivalent to the TWG's. It was alsofound that the particle velocities in the crest of a typical high sea(HS) of significant wave height (SWH) 0.10 m are much lowerthan those of the SRW. It was clearly observed that the high particlevelocity comes from the crest of the TWG's concave frontand convex rear, while the lower velocity results from the crest ofthe SRW's convex front and convex rear. Impact was measured on a vertical truncated cylinder fixed inthe TWG of nominal slope 0.28.

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