Yanfei Deng

Influence of Wave Group Characteristics on the Motion of a Semisubmersible in Freak Waves

In the last few decades, the hydrodynamic performance of offshore structures has been widely studied to ensure their safety as well as to achieve an economical design. However, an increasing number of reported accidents due to rough ocean waves call for in-depth investigations on the loads and motions of offshore structures, particularly the effect of freak waves.The aim of this paper is to determine the sea conditions that may cause the maximum motion responses of offshore structures, which have a significant effect on the loads of mooring systems because of their tight relationship. As a preliminary step, the response amplitude operators (RAOs) of a semisubmersible platform of 500 meters operating depth are obtained with the frequency-domain analysis method. Subsequently, a series of predetermined extreme wave sequences with different wave group characteristics, such as the maximum crest amplitude and the time lag between successive high waves, are adopted to calculate the hydrodynamic performance of the semisubmersible with mooring systems in time-domain. The paper shows that the maximum motion responses not only depend on the largest wave crest amplitude but also the time lags between successive giant waves. This paper will provide an important reference for future designs which could consider the most dangerous wave environment.Copyright

Surge motion of a semi-submersible in freak waves

The surge responses of a semi-submersible under freak wave sequences are studied in time domain. In the present study, a measured New Year Wave sequence is applied to the reference semi-submersible, and its surge responses are analysed to reveal the interaction with freak waves. To investigate the influences of the relative phases, crest amplitudes and the three sister waves, a series of freak wave sequences with different wave parameters have been generated by embedding a freak wave sequence into a random wave sequence. Focuses are paid to the magnitudes of the surge responses, which are directly related to the forces of mooring lines. The surge motion amplitude is observed to be significantly influenced by the freak waves. Main conclusions are drawn regarding the influence of freak waves and their wave parameters on the surge motion responses.