In the study of behaviour of moored offshore structures were drift forces play an important role. Research on slowly verying wave drift forces is usually carried out for a single body numerically or with model tests, which are based on long crested waves. But in practical seas, the waves are short crested in nature and as incase of carrier vessels (e.g. LNG) are usually connected in parallel or in tandem to off-load from floating production, stage & offolding unit (FPSO). Due to the presence of neighbouring structures the motion as well as the second order drift forces are different from those for a single floating body. Hence, in this paper some results of numerical calculations of mean drift forces for a side-by-side and tandem arrangement for an FPSO and LNG are presented. Since the slowly varying drift forces are very important external forces for any moored floating bodies these forces are calculated in short crested directional irregular waves using the bi-directional transfer functions in cross waves. The calculated drift forces are verified by published results. Comparisons are also made of the results for mean drift forces obtained from short crested waves with those of the results in long crested waves. The results indicate that directional interference for both cases are dominant and hydrodynamic interaction is significant for LNG in parallel arrangement.
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