ABSTRACT:

Based on the review of present force coefficients estimation methods, a new method in frequency domain, which is revised cross-spectrum estimation method, Is presented in this paper. Compared to the oases of vertical cylinders, there have not been that much data available for Inclined cylinders. In this paper, some experiments on the wave-current force on Inclined cylinder and the wave current force coefficients estimated by the revised cross-spectrum estimation method are presented. The results show that the wave and current directions have some consistent effect on them. Based on the results, empirical formulas are obtained which are used to modify the wave-current force coefficients on the Inclined cylinders Into unified coefficients. By comparison, It is found that the unified coefficients fit very well with the others results of the cases on the vertical cylinders.

INTRODUCTION

In offshore engineering, most structures are consisted of circular cylinders. The diameters of the cylinders are usually much smaller than the wave length. For the wave force on small diameter fixed cylinders, an empirical formula by Morison et al (1950) is proposed to be used. According to the Morton formula, the wave force can be divided Into two parts, the drag force and Inertia force. The drag force coefficient and Inertia force coefficient are needed to be determined by experimental results. For the same experimental data, the results may differ when different coefficient estimation methods and different wave theory are used. For regular waves, the peak value method, Fourier and least-square methods are commonly used. For the Irregular wave force on cylinder, Wiegel, Beebe and Moon (1957) analyzed each wave to obtain the force coefficients Co and CM. Although the high order wave theory can be used, the results are very scattered and are not convenient to use In engineering practice.

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