Research On Vortex Induced Motions Of Deep Draft Semi-submersibles In Coupling Currents And Waves

With the increasing consumption of energy products and decreasing of the natural resource reserves, people today have a stronger interest in offshore oil and gas. As one of the most popular platforms, the semi-submersible has become the hot spot in the ocean engineering researches because of its advantages: the large deck area, better motion performance in waves, and good adaption in different work conditions and so on.The columns of Deep Draft Semi-submersibles(DDS) in certain flow conditions will be subjected to Vortex Induced Motions(VIM), which significantly influences the loads and life fatigue of the moorings and the risers. To investigate the VIM of a new Semi-submersible with four square section columns in uniform current coupled with regular waves, model tests and numerical studies through CFD method were conducted. The typical current in specific direction of 180 degree was chosen by coupling with the sinusoidal oscillatory flows, which is used to simulate the regular waves with different KC numbers and relative current numbers. The results of the model tests and numerical simulations show that the VIM response of DDS with four square section columns in current with regular waves is less than that in the current. In addition, numerical results of a traditional DDS with square section columns show the same conclusions at 135 degree current direction. And the wave height but not the wave steepness has significant effects on the VIM of DDS. However, in same evironments, the VIM response of DDS with four rectangular columns in current with regular waves was seen more significant than that in the current alone case.Although the regular waves have important effects on the VIM responses of the DDS, little effects were seen on the frequencies of the vortex-shedding.