Improved analysis method on extreme upwell of irregular waves for the airgap of floating platforms based on OTG-13

Abstract

Analysis on air gap is an essential job for the design of the semi-submersible platforms. According to OTG-13 by DET NORSKE VERITAS (DNV) in 2019, the extreme up well in severe irregular waves is the most dominant negative gain of the air gap and should be paid close attention to. Accordingly, in the present study, firstly the frequency-domain analysis method was introduced to predict the extremum of waves up well under a relatively severe sea state among the survival conditions. And the calculation results were found to be much larger than the experimental data of the model test (relative error can be 30%–75%). This leads to the extremely conservativeness of air gap evaluation according to OTG-13. Consequently, further studies were done to explore the deeper reason for such conservativeness. By analyzing the whole process of calculation, the misestimate of the two "details": the viscous effect of water and the wave asymmetry factors, are found to be the causes of the conservativeness of the OTG-13. Then, an improved analysis method of extreme upwell based on the computational fluid dynamics (CFD) solution of wave surface elevation was presented and found to be more practical. In fact, the relative error can be less than 10% in the case that those two "details" are evaluated more precisely. Next, the influence of those two "details" on the extreme upwell were further quantitatively discussed respectively by variable-controlling approach, and it is noticed that the accurate estimation of the wave asymmetry is dominated (relative error can thus reduce to 10%–23%). Finally, a more reasonable preliminary design method was proposed, which is by introducing the actual wave asymmetry factor to the calculation of the extreme up well and air gap based on the CFD solution.