Study On Wave Climb And Near - Field Interference Effect Of Deep - Sea Semi - Submersible Platform

With the increasing demand of oil and gas, the exploration technology of oil and gas resources is developing, and the semi-submersible platform has been widely used in deep sea. The deterioration of the deep sea environment has an influence on the life of the workers and the structure safety of the platform. Air-gap performance is one of the important indexes to measure the safety performance of the platform, also directly related to the security of the platform. The numerical analysis for the wave run-up, near tripping and air-gap response of the semi-submersible platform is a frontier research topic and in the initial stage, furthermore it receive extensive attention of researchers in the field of marine engineering.Based on the knowledge of CFD theory, wave run-up,near tripping and air gap response, The research carry out seventy-two calculation conditions of the numerical simulation under the action of regular and irregular waves for the three modes(fixed、free、mooring) of platform by CFD software. Combined with self-programming the post-processing software which can shows the numerical prediction and analysis for the wave run-up, near tripping and air-gap response of the semi-submersible platform.Specific research work and conclusions are as follows:A comprehensive analysis for the influence about the wave run-up on forward face of column and around four columns under different wave height, period and wave direction angle;Numerical comparative analysis for wave run-up of amplitude and variation under three modes. The results show that: the wave run-up increases with the decrease of the distance of column, and the maximum value reached 3.0 near the column surface. which accompany the phenomenon of wave breaking and splashing. Wave run-up decreases with the increase of the wave forward face area.Wave direction angle β=0° is generally larger. wave run-up around the four columns is "W" shaped distribution. Movement has a great influence on wave run-up, and fixed is obvious, three vertical freedom degrees play a important role in wave run-up.Aimed at near tripping of the semi submersible platform, discussing the basic rules of the near tripping occurred between platform columns and floating body;studying on the different platform models and wave factors have influence on the near tripping around four columns. The further study on infield wavefront distribution characteristics when the near tripping occurred. The results show that: The incident,reflected, diffraction and radiation wave are superimposed more frequently in order to the larger wave elevation when the less period; The wave crest lines appear curved,and the phenomenon of asymmetry between peak and valley in the direction of wavepropagation. The near tripping phenomenon of wave angle β=45° is more obvious than others; The wave elevation by near tripping caused of the free and mooring modes of platform is bigger than fixed, and the distribution of wave surface is more complex; the maxima wave elevation are mainly concentrated in the inner side of rear column, the value reached 2.57 times of the height of relative incident wave.Focusing on the analysis of different wave direction angle and wave parameters effect on platform air-gap responses; Comparative statistics the change of negative air-gap of the same observation point on the lower deck under three modes of platform, Quantitative analysis for the mooring system effects on the negative air-gap size and distribution of semi-submersible platform. A brief analysis of the distribution of platform catenary tension; Detailed study on the influence on the nonlinear impact of the different platform modes, the wave direction angle and wave factors to the size of pop pressure. The results show that: the wave height have a great influence on negative air-gap. When the incident wave spectrum peak period is close to platform vertical natural period of three freedom degrees, which will produce larger negative air-gap. Under Platform fixed, the negative air-gap is sightly larger than the free and mooring mode, the maximum negative air-gap reached 8.64 m. Platform two catenary head wave surface tension is greatest, the maximum tension of up to 2695.9kN. The negative air-gap of platform occurred mainly in the column wave near the surface and under the edge of the deck, Only the middle of the deck under high sea condition, the region will appear negative air-gap With the larger pop pressure, the nonlinear phenomenon is more obvious.