Research Of Second-order Air Gap Prediction And Near Tripping Of A Semi-submersible Platform

With the development of deep water oil exploiting, it is quite necessary to build deepwater platforms to carry out such tasks. FPSO, Semi-submersible, Spar and TLP are thefour main floating platform types used for deepwater oil and gas exploitation at present.Among them, Semi-submersible has priority of less total investment, more deck space、deck variable load and production ability. It also has multiple functions of drilling, wellrepairing, production and so on. And it could also be easily transformed according todifferent requirements. Especially its ability to adapt wide range of water depth and harshsea states make it has an extensive application prospect in the deepwater oil fielddevelopment in South China Sea.As a key issue in the designing process of Semi-submersible, air gap refers to thevertical distance from the bottom of lower deck to water surface. In the designing ofplatforms, lower deck is required to have a sufficient height, so as to ensure that the valueof air gap is not too small. A small value of air gap could result into slamming, whichwould cause structure failure, or even wreckage. On the other hand, an unreasonably largevalue of still air gap is not economically-wise, as the variable load reduces and platformheight increases correspondingly. So it is both practical and economical to have areasonable value of air gap, which makes the accurate prediction of air gap utterlyimportant. Platform’s air gap is relative to many factors, so it is very difficult to forecast it, then,there is not a sophisticated method to forecast it. What most widely used is the numericalprediction method based on the potential theory, but due to the linear approximation, theresult of it is bound to have considerable deviation compared to actual value, and manyresearcher confirmed that, the result would seriously underestimate the extreme values ofthe relative wave elevation. Therefore, engineers often use the result of the potentialtheory with experimental results together.Improved from the linear potential theory, the numerical method based onsecond-order scattering theory was proposed later, but its accuracy lack sufficient prooffrom model test, and some scholars found its results often overestimate the extreme valuesof the relative wave elevation.Firstly, a combination research between numerical calculations and model tests isdone with a deepwater semi-submersible drilling platform, then, the data measured frommodel tests used as a reference, a comparison analysis of the air gap forecast from thesecond-order and first-order potential theory is done.Furthermore, the platform’s air gap response depends mainly on the wave elevationwithin the platform, so near tripping phenomenon is also discussed here, which may causegreat wave’s run-up. Then the comparison between the results of the second-order andfirst-order potential theory is done again, also with the deepwater semi-submersibledrilling platform.