Evaluating the Validity of Quasi-Static Analysis for Prediction of Vessel Mooring Line Forces

Quasi-Static analysis of moored vessels is vastly used for engineering designs, as a substitute to the numerical simulation of dynamic mooring analysis. Yet, the level of validity of the results of quasi-static analysis is a matter of discussion. In the present study, the validation of the assumptions behind the quasi-static analysis of mooring vessels is examined with application of a dynamic model for a number of vessel sizes and mooring configurations. The results show that in the moderate wave condition (wave height less than 1.2m), the main assumption of not applicability of wave forces in prediction of line forces is acceptable the differences being less than 5 percent. However, distribution of forces between spring and breasting lines is not coincided with the anticipation of response of the system. The parametric study shows that the critical condition for line tensions is not necessarily corresponding to the exact coincidence of directions of wind, current and wave forces. Also, the forces exerted on fenders in some cases are maxima in direction, different from the perpendicular to the vessel. This means that the mooring line arrangement considerably dictates the distribution of the tension in mooring lines and reaction in fenders. Hence, the accuracy of the quasi-static analysis highly depends on the mooring lines configuration.