ABSTRACT:

The second order wave exciting force on a floating ship considering tank liquid dynamic effect is examined. The second order drift forces are quadratic products of the first order quantities. The ship motions in wave considering tank liquid dynamic effect are calculated by linear potential theory. Then the second order drift forces are computed by near field method based on the pressure integration and validity is confirmed by far field method. For a numerical calculation example, the motions and drift forces of 250K LNG FPSO in wave condition are demonstrated.

INTRODUCTION

To estimate the ship motion in waves containing large liquid tanks are very complex problem. In most seakeeping computations, the liquid inside the tanks has been generally treated as a rigid mass. However, for LNG carriers and FPSO vessels during loading and unloading cargo operations the ship motion is very important topic. Coupled global ship motion and tank liquid dynamic effect have been studied numerically and experimentally by Chen, X. B. et al.(2007), Gaillarde, G. et al.(2004), Rognebakke, O. F.(2003) and Kim J. W. et al (2005). FPSO vessels are operated at a fixed station point by some kind of mooring systems. Therefore in addition to estimate the ship motion ability the evaluation of the drift force acting FPSO is required from the operational view point. Newman (2005) conducted the ship drift force calculation considering tank liquid dynamic effect based on the commercial program WAMIT. The result showed that the tank liquid dynamic effect on the drift force is significant at sloshing resonance wave period. This suggest that during loading and unloading cargo operations between FPSO and shuttle tanker which are considering the tank liquid dynamic effect on the ship motion and drift forces are essential problem.

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