A Rational Dynamic Analysis Procedure for Turret Mooring Systems

ABSTRACT Until recently, most of the turret mooring systems have been designed using the so-called quasi-static approach. There is an increasing awareness that this approach may be deficient for certain applications, and that dynamic analyses are needed. To this end, this paper presents a rational dynamic analysis procedure for designing turret mooring systems. Specifically, practical methods for computing the low frequency motions and dynamic line tensions are presented, based on the latest analytical developments, model tests, and design experience. Model test data are presented for three turret mooring system in water depths of 70 meters, 75 meters, and 370 meters, respectively, to substantiate the significance of drag force on mooring lines on the low frequency motions, the modified Rayleigh distribution and the RINA method for interpolating the statistical low frequency responses, and line dynamic behavior. Finally, this procedure is illustrated using the turret mooring system of a 230K DWT tanker in 370-meter water depth as an example case. The results of the dynamic analyses are compared with model test data and with the results using a quasi-static analysis approach. INTRODUCTION Motion Response Characteristics The motions of a turret moored vessel can be described by three components: mean offset, low frequency motions and wave frequency motions. The mean offset is caused by the mean environmental force and is constant for the duration of interest. Low frequency motions refer to the resonant surge, sway, and yaw motions due to low frequency cyclic loads associated with wave groups or wind dynamics. These have periods varying from a low of 60 to 90 seconds to a high of a few hundred seconds. Wave frequency motions refer to the vessel motions excited directly by ocean waves. The periods vary from 3 to 25 seconds. The consensus is that these three types of motion can be considered as decoupled physical processes, and hence, they can be simulated individually and combined for the maximum offset. Computing the mean offset and the wave frequency motions is straightforward. The former involves only static catenary calculations. The latter can be accomplished using the highly developed frequency and time domain analysis techniques. The simulation of the low frequency motions remains a relatively complex task. The principal difficulties lie with obtaining quality data for wave drift force coefficients, estimates of low frequency damping, and low frequency wind spectra. While practical analytical tools exist, the prediction of low frequency motions is still in a research stage, Reference 1. Line Tension Response Characteristics A mooring line may respond to fairlead motions in three ways:change in the catenary configuration (catenary stiffness),stretch {elastic stiffness), andchange in the catenary configuration and stretch at the same time. If the fairlead moves slowly (low frequency motions) the line will move toward the new static-equilibrium position. The line will essentially be in static equilibrium at all times. The tension perturbation can be calculated using a static catenary equation.