Probability Analysis And Numerical Investigation On Air Gap Response Of A Semi-submersible

Semi-submersible platforms have been widely used in oil and gas exploration and production,due to its advantage of small motion response and high ability to resist the wind and waves.It poses a serious challenge to the platform safety,with more severe sea environments being encountered in deep water.The performance of the air gap greatly affects the operation and safety of these platforms,and is a key factor in the design of a semi-submersible.The offshore industry popularly analyzes the relative wave crests measured in physical experiments to investigate and determine the performance of air gap.A semi-submersible operated in water depth of 1500 meter was investigated in this study.Firstly,through numerical calculation,the results of frequency domain and time domain were obtained to provide reliable basis for the model test.A model test research on the semi-submersible was conducted to obtain the hydrodynamic coefficients and the time history of relative wave elevations at typical locations.Then the numerical calculation was calibrated based on the hydrodynamic coefficients from the model test.Secondly,the time history data of relative wave elevations measured at different locations for different sea states were processed to obtain the relative wave crests to constitute an ordered sample data.Each ordered sample data was assumed to follow a two-parameter Weibull distribution and a three-parameter nonlinear Rayleigh distribution.L-moments were used to estimate the unknown parameters in both distribution models,respectively.According to the exceedance probability distributions,three-parameter Rayleigh distribution estimated by the method of L-moments captured the trend of relative wave crests better,but was not particularly reliable for relative wave crests with small exceedance probabilities.Then the method of LH-moments was introduced for further investigation and analysis.The results showed that,as the order of LH-moments increased,three-parameter Rayleigh distribution yielded better trend of the large relative wave crests under 1-year-return sea state,and clearly better fit of the large relative wave crests under 100-year-return sea state than L-moments.Finally,the statistic results of relative wave crests from time-domain numerical calculation were compared with the corresponding experimental results.The results showed that,the numerical calculation based on three-dimensional linear potential theory,could simulate weaker relative wave extremes,but deviated from high relative wave extremes.In addition,the similar standard deviations indicated that the numerical calculation could reflect the overall trend of relative wave elevations.