Research On Moonpool Fluid Vibration Characters And The Effects On The Spar Platform Heave Motion

The Spar platform has center well which goes throughout the entire body from bottom to top, and in the center well, risers and important drilling facilities has special layout to structure the moonpool. According to the actual situation, the moonpool sometimes should be designed as half opening to let the seawater in and out. When the Spar platform has the top tension riser system, the mass of the water in moonpool will have the same order as the mass of the main body of Spar, and its effects on the Spar motion can not be neglected. In this paper, based on the open source software OpenFOAM, the moonpool fluid vibration charachters and its effects on the spar heave motion have been studied.(1)2D and 3D numerical wave tanks have been set up, and the grid has been tested to get the best mesh way according to the later caculations, as well as the wave asorbing way. The results show that the wave tanks have steady wave gegeration and can be used in later calculation.(2)The inherent characteristics of the moonpool fluid pistion motion and sloshing motion have been studied, and the effects of different opening areas on the fluid motion have been analysised. The results show that, when the wave period is near the period of the fluid pistion, the fluid in moonpool will have resonaces in heave direction and its amplitudes has a relationship with the opening areas. According to caculations in this paper, when the moonpool opening is 30%, the heave amplititude is the smallest. And if considering the Spar coupled with waves, the heave amplitudes of the moonpool water will be larger, so the coupling effects can not be neglected.(3)The damping of the Spar heave motion according to different KC numbers have been calculated with the consideration of the moonpool fluid effects. The results show that, the damping force has a relationship with different moonpool opening. Caculations in this paper show that, when the moonpool opening is 30%, the damping coefficient is the largest. And the coupled motion of wave-spar heave motion-fluid in moonpool have been calculated, and the effects of the moonpool opening on the Spar heave motion have been analysised. The results show that, when the wave period is near the Spar inherent heave period, the moonpool fluid will restrain the Spar heave motion and the restraining has a relationship with the moonpool opening. Caculations in this paper show that, the Spar heave motion RAO is the smallest when the moonpool opening is 30%. In actual design, the moonpool opening can be optimized to minish the Spar heave motion at the most extent.