In this paper, a typical floating production storage and off-loading system (FPSO) is chosen as a model of numerical simulation for investigating the effect of short crested waves on low frequency motions of moored tanker. In the calculations of drift forces, nonlinear interference of wave directional spreading is considered and its quadratic transfer functions are computed by three dimensional source technique. Equations of low frequency motions of moored tanker are derived for horizontal motions, surge, sway and yaw. For external forces, in addition to the waves, current and wind forces are also included. Finally, responses of moored tanker in long and short crested waves are compared. It is found that there are similar results in surge motions and enormous difference in sway and yaw motions.

1 Introduction

Slow drift oscillations of a moored structure in irregular waves are of importance in the design of mooring system. The large horizontal excursions can cause large tensions in mooring lines and exceed limitations in working operation. This phenomenon has been widely studied by model tests and numerical simulations. But these research work mainly focus on long crested waves. As for short crested waves, we cannot find many results in both of model tests and numerical simulations. This is because research is restricted by the model test facilities and sophisticated calculations. From the view point of safety of mooring system, it is necessary to investigate characteristics of slow drift oscillations of a moored structure in short crested waves. Pinkster[l] presented the concept and mathematical expressions of drift forces in short crested waves in 1985. He extended theory of drift forces in long crested waves to short crested waves and derived formulae of slowly varying wave drift forces. Kim et al.[2] developed a method to predict slowly varying drift forces in multidirectional seas.

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