The prediction of slow drift motions for the design of a mooring system is usually made using the Newman approximation [1], based on the assumption of a very low resonance frequency of the system and small contribution of the second order wave fields. This hypothesis is commonly satisfied for most parts of the mooring systems in deep water. However, this is not the case for LNG terminals moored in shallow water.
Unlike the Newman approximation, the FullQTF formulation to compute the low frequency wave loads is more accurate but requires much longer time of computation, which presents limitations in practice when a large quantity of simulations is needed.
Further to the work presented in [2] on the quadratic transfer function (QTF) of low-frequency loading, a new approximation has been developed in [3]. The F1 approximation gives comparable results to the FullQTF and presents the interesting aspect that the loads time series can be reconstructed by means of simple summations, presenting the same efficiency in computation time as Newman approximation.
In this paper, main parameters of mooring systems are analyzed to evaluate the impact of the choice of each method, Newman, F1 or FullQTF. Indeed, this choice is a compromise between calculation time and accuracy of results. The conclusions raised are underlined in the study of an LNG terminal.
