ABSTRACT

This paper is concerned with the evaluation of the extreme environmental The paper describes some recent advances in the prediction of wave loading, based on a series of projects completed in the recent joint SERC/Industry Programme on Floating Production Systems. It is organised as follows. Numerical analysis and experimental modelling of low frequency responses is first addressed, including both second order wave drift forcing and hydrodynamic damping This is followed by a discussion of TLP springing, and its implications for tether fatigue. Next consideration is given to the effects of directional sea spreading in storms, and finally some brief concluding remarks are offered on areas of continuing uncertainty.

INTRODUCTION

Effective design of floating production systems is placing increasing demands on the prediction of hydrodynamic loads. Substantial advances in understanding the responses of certain moored systems in waves are being made by the development and application of numerical methods based on potential flow theory. Several major uncertainties still remain; but recent theoretical research coupled with programmes of model tests confirms the feasibility of pursuing such an approach, at least as a first stage in the achievement of robust mathematical models using the tools of computational hydrodynamics.

The key to some of the recent advances has been the break from a purely linear description of hydrodynamics to an expansion which incorporates quadratic non linearities. By this means it has been possible to model low frequency drift behaviour of moored systems; and a plausible explanation has been provided for the high frequency "springing" motions in the vertical modes (heave, pitch and roll) of a tension leg platform. In both cases the phenomenon is one of a lightly damped resonant response, excited by forces which are quadratic in the wave amplitude. In the former case the frequencies of these forces are at the difference between the frequencies of components In a random seaway; whereas in the latter case they are at double and sum frequencies. To make reliable theoretical predictions of the resulting resonant motions, it is necessary both to compute reliably the second order forces themselves, and to predict accurately the associated damping Another desirable objective is to be able to quantify the implications of wave directional spreading effects on these phenomena. While intuition may suggest that responses in spread seas should be less than in unidirectional seas, it is easy to disprove this as a general proposition, even when non-linear effects are ignored.

This paper does not aim to provide a literature review of the major research contributions that have been made in this general area, by laboratories in many countries. Rather it attempts to shed further light by summarising some of the relevant results of a small series of closely linked projects, initiated as part of the Hanaged Programme on Floating Production Systems (1987–89). Full information concerning the background to the projects, the methodologies adopted, and the detailed results, are given in the individual reports of the projects completed by investigators in a number of universities (and to which reference is made below).

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