Freak Wave Impact On Semisubmersibles - Time-domain Analysis of Motions And Forces

For the design of safe and economic offshore structures and ships the knowledge of the extreme wave environment and the related wave/structure interactions is required. In general frequencydomain analysis has been regarded as an adequate tool for the assessment of motions and loads which are needed to derive stresses, fatigue and wear as well as operational limitations. However, the increasing number of reported rogue waves with unexpected large wave heights (Hmax/Hs > 2), crest heights (ζmax/Hmax > 0.6), wave steepness and group pattern (e.g. Three Sisters) may suggest a reconsideration of design codes. For investigating the response to specific extreme sea conditions this paper analyzes the motion behavior and resulting splitting forces of a semisubmersible of type GVA 4000 in a reported rogue wave, the Draupner New Year Wave embedded in irregular seas. Response evaluations in time-domain provide information of the significance of rogue wave impact. The numerical time-domain investigation using a panel-method and potential theory is compared to frequency-domain results. For time-domain analysis the commercial code TiMIT is used to provide the motions and forces on the wetted body of the semisubmersible in rogue waves as time series. Corresponding response amplitude operators are also calculated with WAMIT, a program system for zero-speed problems. The resulting response spectra, e.g. the splitting forces, are then transformed into time-domain, using Fourier transformation. For validation, the selected sea condition is generated in a physical wave tank, and the seakeeping behavior as well as the (splitting) forces on the semisubmersible are evaluated at model scale. In conclusion, WAMIT and TiMIT results compare quite well as far as significant and maximum motions and forces are evaluated by a frequency-domain analysis. Also the time-domain analysis of the dynamics of the selected semisubmersible agrees satisfactorily with experimental results. Thus, the effect of rogue wave sequences is adequately predicted by TiMIT evaluations. The specific freak wave itself excites motions and forces, i.e. which are directly correlated to the maximum wave heights, resonance phenomena are not observed in the investigated time-domain case. As higher responses are observed as compared to standard frequencydomain methods, design procedures and codes should be reconsidered if more comprehensive data on freak waves and their impact has been analyzed.