Abstract
In design and operation of floating offshore structures, one has to avoid fatigue failures caused by action of ocean waves. The aim of this paper is to investigate the applicability of WaveWatch-III wave model to fatigue assessment of offshore floating structures. The applicability was investigated for Bluewaters’ FPSO (Floating Production, Storage and Offloading) which had been turret moored at Sable field for half a decade. The waves were predicted as sea-state time series consisting of one wind sea and one swell. The predicted waves were compared with wave data obtained from ERA-interim and buoy measurements. Furthermore, the fatigue calculations were also carried out for main deck and side shell locations. It has been concluded that predicted fatigue damages of main deck using WaveWatch-III are in a very good agreement regardless of differences in predicted wind waves and swells caused by differences in wave system partitioning. When compared to buoy measurements, the model underestimates fatigue damages of side shell by approximately 30 %. The reason for that has been found in wider directional spreading of actual waves. The WaveWatch-III wave model has been found suitable for the fatigue assessment. However, more attention should be paid on relative wave directionality, wave system partitioning and uncertainty analysis in further development.
Highlights
A floating offshore system consists of a floater, risers, subsea installation and either mooring lines or a dynamic positioning system which is an attractive alternative for deep water applications
In order to validate WaveWatch-III wave model, the fatigue calculations were carried out with the wave data obtained from ERA-interim and from buoy measurement
The wave data predicted by WaveWatch-III are compared Fig. 3 Examples of response amplitude operators of midship vertical with the wave data obtained from ERA-interim and the buoy bending moment
Summary
A floating offshore system consists of a floater, risers, subsea installation and either mooring lines or a dynamic positioning system which is an attractive alternative for deep water applications. Further crack propagation may even lead to a structural disjointing failure In both the design and operation stages of floaters, one has to demonstrate that fatigue failures cannot occur during the design lifetime of e.g. 25 years multiplied by the fatigue safety factor varying between 1 and 10 depending on criticality and reliability of considered structural members or joints. The wave simulation ability of WaveWatch-III has already been validated by many studies, but no one has ever considered the applications of this model to fatigue assessment of floaters (Tolman 2002; Mentaschi et al 2015). The suitability of WaveWatch-III wave model for fatigue assessment of floaters is investigated by comparing predicted and measured waves and fatigue damages.
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