Investigation Into Scale Effects On Motions And Mooring Forces Of A Turret-Moored Tanker

Abstract

ABSTRACT A loaded 200 kDWT tanker moored by means of an internal bow-turret is exposed to survival conditions (colinearly directed irregular waves and wind). The water depth amounts to 82.5 m and the turret mooring system consists of 6 equally spaced chains. By means of time-domain computations of the coupled chain-mooring-tanker system the wave frequency (WF) and low frequency (LF) turret motions and chain/turret forces are simultaneously determined. The model tests were carried out on scale 82.5. The results of the computations are compared with the results of the model tests using the wave registration as calibrated in the basin. The comparison required the modelling of some additional damping which is expected to originate from small sway and yaw motions which were measured during the model test. Investigations on scale effects in the resistance coefficients of the hull in surge direction are presented. Finally the sensitivity to changes in the values of the drag and inertia coefficients of the chains on the mooring loads in the chains and turret construction is given. INTRODUCTION A moored tanker exposed to irregular head seas performs small amplitude WF pitch, heave and surge motions and relatively large amplitude low frequency surge motions around a mean surge displacement. The mean surge displacement is caused by the mean wave drift force and the steady wind force. The low frequency surge motions are due to the low frequency part of the second order wave drift forces. Since the total damping of the low frequency motions is small, and in irregular head seas the frequency of the slowly varying forces corresponds to the natural surge frequency of the system, resonance occurs. Large low frequency surge motions and consequently large mooring forces may be the result. For the design of the mooring system, the large amplitude motions and the corresponding forces are required and it is therefore of prime importance to have knowledge about the low frequency excitation and reaction forces. The excitation and reaction forces as acting on a chain-turret moored tanker are given below:velocity dependent wave drift forces (including wave drift damping)viscous damping forceswind damping forcesinertia forces due to the virtual mass of the tankerrestoring and damping forces caused by the mooring system. In order to determine the total restoring and damping forces of the mooring system the wave frequency tanker motions are required too. By means of an integrated computation (mooring-system-tanker) in the time domain, the instantaneous WF and LF restoring and damping forces as acting on the mooring lines and the turret construction will be used as input for the equation of low frequency motion of the tanker at every time interval to solve the new low frequency turret motions and velocities. The contribution of the hydrodynamic damping induced by the mooring chains on the low frequency part of the vessel motions may be considerable as was shown in Refs, [1] and [2](Available in full paper)