The air gap performance of the semi-submersible is closely related to the safety of the production and personnel, which has significant meaning in the structural design. For many old platforms, the damage from slamming caused by the negative air gap is not considered in the structural design due to the limited means of the analysis in the early stage. However, the old platforms need to be reconstruction when using the existing methods for reevaluation, which is of high cost. The method in DNVGL-OTG-13 is adopted to analyze the air gap performance of an old platform. Two different approaches, Newman approximation and method proposed by Pinkster, are adopted to calculate the second-order force, which is intended to study the second-order effect. Besides, by selecting a series of environmental conditions in a sea area, the envelope curve of wave height to period under the positive air gap is given, so as to ensure that the working area can be selected without transforming the structure.


With the application of semi-submersible platform in offshore widely, the air gap and slamming have attracted more and more attention (Stansberg, 2008). When calculating the air gap, it is necessary to consider not only the first-order motion of the platform and the influence of the first-order wave elevation, but also the nonlinearity of the wave itself and the influence of the second-order nonlinear motion of the platform (Sweetman, 2002; Tian, 2017). For semi-submersible platform, the contribution of the mean drift force and low-frequency second order force should be considered. In 1974, Newman proposed that the off-diagonal elements could be approximated by the diagonal elements when solving the second-order transfer function (QTF), which is called Newman approximation (Newman, 1974). An approximate method to solve the full QTF was proposed by Pinkster (Pinkster, 1980), which was based on the assumption that the incident wave was undisturbed to solve the contribution of the second-order velocity potential to the wave force.

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