New Developments in Prediction of Mooring System Damping

Abstract

SUMMARY Previous OTC papers (Refs. 1, 2, 3) have shown that drag forces on the mooring lines can represent the dominating contribution to the surge damping of moored vessels. They have also shown that in order to calculate this damping effect correctly one normally has to take into account the effect of the wave-frequency motions of the vessel, superposed on the low-frequency, resonant motions, whose damping we wish to predict. The present paper suggests a simple and practical procedure for this purpose. It applies the concept of "apparent drag coefficient" in combination with an iterative use of the catenary equations to obtain the low-frequency damping. The range of applicability of this procedure covers most practical mooring configurations. Only in cases of very deep water, heavy lines, and suspended clump weights it may be necessary to use a complete FEM procedure to get the effect of wave-frequency motions and line dynamics accurately described. The present paper also publishes for the first time results from investigations of the possible heave damping effect of the mooring system. It turns out to be normally negligible for moored ships, moderate but significant for semisubmersibles, and can be dominating the heave damping of spar buoys. 1. INTRODUCTION An important feature of most moored offshore structures is the pronounced oscillatory motions at their low, resonant frequencies. Together with the motions at wave frequencies they determine peak offsets, peak mooring line loads, riser performance, and other important engineering aspects of the structure. These low frequency oscillations take place outside the frequency range of the waves. They are excited by second order wave forces, by non-linear viscous reaction forces to wave frequency responses, and to some extent by wind gusting. Since we are dealing with low-damped systems at resonance, the LF (= low frequency) motion amplitudes are very dependent on system damping. Main contributions to for instance the surge damping of a moored vessel are due to wave drift, drag and skin friction on the main structure, and drag forces on the mooring lines. Until just a few years ago it was customary to assume that the mooring system influenced the motions of a moored structure only by its horizontal stiffness, determining the surge and sway resonant periods of the structure. Any effect of the mooring system upon the damping of resonant motions was neglected, assuming that the drag area of the mooring lines was too small to be significant, compared to the drag damping of the structure itself. Then in 1986 a first publication was issued, showing that the mooring system can be the main source of surge damping of a moored structure (Ref. 1). Further OTC publications in 1988 and 1989 (Refs.2 and 3) presented different attempts at calculating the surge damping effect of the mooring system, in addition to presenting more experimental information on the subject. The objective of the present paper is to give a survey of some recent developments of numerical methods to predict mooring line damping.