Effect of wave spreading including multi-directional wave interaction on the responses of a spar platform

Abstract

This paper deals with the effect of directionality of waves on the responses of a spar-type floating offshore platform. Here, a spar-type offshore platform restrained by four catenary mooring cables is modelled as per the work done by Chen et al. (Ocean Eng 28:863–887, 2001). The numerical model and numerical simulation have been validated by comparing the results (natural periods, RAOs and statistics of response) available in the literature (Steen et al. in Offshore Technology Conference 2004; Wangz et al. in J Mar Sci Appl 8:168–174, 2010). The multi-directional sea spectrum is represented by the product of a cosine power spreading function and a uni-directional JONSWAP wave spectrum. The Quadratic Transfer Functions have been generated for the calculation of drift loads, taking into account the multi-directional wave interaction. The responses of the platform have been obtained under long-crested (uni-directional) and short-crested (multi-directional) sea states and the results thus obtained are compared. Further, the variations in responses due to change in power of spreading function and total spreading angle are also investigated. It is found that due to the multi-directional wave interaction, the second order drift loads assume higher values in all degrees of freedom. Subsequently, in short-crested seas, the structure exhibits higher responses in all degrees of freedom when multi-directional wave interaction is considered. This highlights the need for incorporation of the multi-directional wave interaction while modelling short-crested seas for a realistic assessment of response behaviour of the system.