Effects of Directionality of the Waves in the Mooring Systems Design

In the analysis of the mooring system, one of the most important steps concerns the computation of the low-frequency wave loads that will excite the system at its resonance periods. Therefore, over the last decades much work has been devoted to this topic, especially for unidirectional waves although some publications can be found which deal with the computation of second-order loads in multi-directional waves. The common practice in the design of mooring systems is to ignore the interaction between waves coming from different directions even if we know that the unidirectional sea is an idealized situation that does not exist in practice. In Rezende & Chen (OMAE 2010), new method for computation of the low-frequency loading has been presented which is more efficient than the method based on the computations of the complete QTF in multi-directional waves, even though this method is still much more time consuming than the usual approximations in unidirectional seas. In this paper, the effects of the directionality of the waves are evaluated for typical mooring systems in deep water and in shallow water. The contribution of the directional interaction terms in the loads will be assessed in order to conclude on its importance to the mooring design.