ABSTRACT

This paper is concerned with the effects of wave drift damping upon the motions of a moored tanker and a moored offshore barge. The assignment of wave drift damping coefficients, whether predicted or measured, has a significant effect upon the excursions of the moored structure and hence upon the associated line tensions experienced by the mooring system. Procedures for simulating the random sea and determining the Quadratic Transfer Functions of second order wave effects are discussed in the context of including such effects in the time domain simulation of a moored vessel.

INTRODUCTION

The importance of low frequency damping in the motion studies of moored vessels has been accepted for the last 8-10 years. The inclusion of such effects however is quite new since only in the last few years have predictive methods been developed. Previously much of the pioneering work was undertaken by Wichers et al (1,2) who measured the damping and also provided explanations of how such a damping influence could arise.

Following a brief discussion of the choices regarding the prediction of wave drift damping the time domain equations of motion are discussed. Determination of the time dependent wave forces and evaluation of Quadratic Transfer Function (QTF) for the drift forces and fluid damping are then discussed. After outlining our method of simulating a random sea the solution and application of the time domain equations for a barge and tanker are discussed together with the statistical analysis of associated, generated time series. Following presentation of results our conclusions on this study are presented.

WAVE DKIFT DAHPDIG

In earlier papers (3,4) the theoretical development of 2D based and different full 3D based wave drift damping calculation procedures has been reported. Application of these techniques and comparison of predicted wave drift damping coefficients with experimentally measured values have also been reported. These different studies have led to the conclusion that the solution of the hydrodymanic problem using a simplified linearised 3D analysis, when coupled with a nearfield, pressure integration, second order force calculation procedure provides the "best" correlation between theoretical predictions and experimental measurements. This correlation is very good for tankers, quite acceptable in most cases for barges but is not always applicable in the case of certain semisubmersible forms. What makes the correlation good or bad in the case of semisubmersibles is not yet fully understood.

In two papers by Standing et al (5) and Brendling et al (6) it has been suggested that the solution of the forward oscillating vessel-wave interaction problem is not always possible or appropriate computer codes are not generally available for structures of arbitrary geometry. Consequently simpler methods of calculation should be considered. In particular to present a simplified procedure they have presented arguments which can be expressed mathematically as follows.

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