Research On Calculation Method Of Air-gap Of Semi-submersible Platform Based On OTG-13

With the area of our human beings’ development and utilization of resources gradually moving towards the deep ocean,the development of deep-sea floating marine equipment has attracted more and more attention from all over the world.At the same time,the extreme environment conditions of the deep ocean have become more and more common,and the structural damage and overall instability of offshore platforms caused by wave slamming caused by insufficient air-gaps has also been widely concerned in the academic and engineering fields.The classification societies have also put forward stricter air-gap requirements.In this context,this article analyzes and calculates the air-gap and slamming load of a semisubmersible platform based on the DNV-GL-OTG-13 specification updated by the Norwegian DNV in 2019.The results were compared with the relevant model test data and some of the existing problems were found out.After that,a more in-depth analysis and research on the existing problems were conducted through the potential flow and CFD methods.And some meaningful conclusions were summarized.The specific work content is as follows:(1)First,the DNV-GL-OTG-13 specification was studied thoroughly,and the calculation and analysis method and process of upwell and air-gap of the semi-submersible platform based on the frequency domain analysis method of the potential flow theory were summarized.(2)Analyzing and calculating the upwell and air-gap of a semi-submersible platform,and comparing the numerical results with the results of the model test.Finally,it is found that the extreme value of the upwell and air-gap obtained by the numerical method based on the OTG-13 is generally more conservative than the experimental results.The degree of conservativeness is quite high.(3)In order to further explore the reasons for the conservative results,it was decided to use the CFD method to conduct in-depth analysis and calculation of the above problems.Studies have found that the main reasons for the conservative upwell and air-gap results are the overestimation of the wave surface elevation amplitude caused by the neglection of the weakening effect of the water viscosity on the wave surface elevation amplitude and the overestimation of the wave asymmetry;To further explore the influence of viscosity and wave asymmetry on the upwell results respectively,two sets of numerical experimental control groups were designed,and finally it was found that the overestimation of wave asymmetry is the more major reason for the conservativeness of the upwell and air-gap results.In summary,according to the problems in the process of the calculation of the upwell based on the OTG-13,this paper successfully used the numerical experimental control group to initially verify the main cause of the problem.And it also innovatively proposed to use the CFD method to make certain improvements to the process of upwell calculation and analysis,so as to provide a more accurate and reliable reference basis for the air-gap performance of the semi-submersible platform and the prediction of slamming,in the future.