Dynamic Positioning Capability Analysis For Semi-submersible Platforms In Internal Solitary Waves

Abundant oil,natural gas and combustible ice resources have been found in the South China Sea.However,frequent internal solitary waves have become a serious threat to the developing engineering in the South China Sea.Latest generation semi-submersible drilling platforms mainly use dynamic positioning systems to carry out drilling operation in deep areas,but in the traditional dynamic positioning capability analysis only wind,current and wave loads are considered.However,if a semi-submersible drilling platform is going to work in the South China Sea,then the dynamic positioning capability analysis considering both internal solitary wave loads and wind-wave-current loads will need to be considered.To this end,a computational method for internal solitary loads exerted on semi-submersible platforms is established based on series model experiments,and then the dynamic positioning capability analysis considering both internal solitary wave and wind-wave-current inactions is carried out.Detailed work is as following:In chapter ?,two independent devices are designed to measure loads exerted on the columns and pontoons of the semi-submersible platform,and series experiments for internal solitary wave loads of such platforms with different wave angles are carried out in a large-scale gravity type density stratified tank.Results show that,for the columns of the platform,the horizontal forces exerted by internal solitary waves can be calculated by using Morison formula,and based on results of series experiments an empirical formula is established to determine the drag and inertia coefficients in Morison formula.The horizontal forces exerted on the pontoons can also be calculated by using Morison formula when the wave direction is not parallel to the central longitudinal section of the platform,and an empirical formula is established to determine the drag and inertia coefficients in Morison formula.When the wave direction is parallel to the central longitudinal section,the horizontal forces exerted on the pontoons can be calculated by using Froude-Krylov formula.Meanwhile,results also show that the vertical force exerted on the pontoons under different wave directions can also be calculated by using Froude-Krylov formula.In chapter ?,considering four kinds of ocean stratified conditions,internal solitary wave loads with different amplitudes are calculated based on the present computational method obtained in chapter ?.By combining internal solitary wave loads with wind-wave-current ones and writing the analysis program of dynamic positioning capability based on quadratic programming method,the dynamic positioning capability of a semisubmersible platform under the influence of different amplitude internal solitary waves and wind,wave,current with four kinds of ocean stratified conditions is carried out.Moreover,extreme safe amplitudes in four kinds of ocean stratified conditions are found out,and the dynamic positioning strategy when the semi-submersible platform counters internal solitary waves is proposed.