Research On Vortex Induced Motion Of A Deep Draft Semi-submersible Platform

With rapid increase of energy consumption, the oil and gasdevelopment is becoming more and more important. Semi-submersibleplatform has become one of the most widely used facilities in deepwater oiland gas exploitation due to its good performance in waves, perfectdeepwater adaptability and large topside weight. However, its heave motionresponse is greater than other platforms like Spar and TLP because its heavenatural period is not far away from the wave energy region. The large heavemotion response has restricted its further development in deep sea oil andgas exploitation to some extent. The deep draft concept can be helpful toimprove the vertical motion performance. Thus a concept of Deep DraftSemi-submersible (DDS) comes up and will be one of the trends in the design of the semisubmersible platform.The Semi-submersible platform is a kind of typical multi-columnfloaters. Vortex shedding may happen around its columns in certain flowconditions, which will cause pulsatile pressure and Vortex Induced Motions(VIM). For a Deep Draft Semi-submersible platform (DDS), VIM is moreobvious because of longer excitation length comparing to a conventionalone.VIM of semi can cause fatigue damage to risers and mooring systemand even affect normal operation of the platform. So VIM of the DDScannot be neglected.This paper studies VIM of the new concept of DDSdeveloped by CNOOC consisting of four rectangular and four rectangularpontoons interconnecting the columns. Both experimental research andCFD numerical simulation are implemented in this paper and someimportant conclusions are drawn.The VIM model test is carried out with the scale of1:60in a towing tank and its6-DOF motions is measured under different current directionsand velocities. Further study is done in several aspects, including time series,response magnitude, trace, etc, to analyze the key characteristics and lock-inrange of VIM. The results show that VIM of a DDS mainly includes swayand surge, however，sway is predominant; The response amplitude is notonly dependent on current directions but also related to reduced velocity Urregularly; The response amplitude is larger under current direction of135deg and lock-in phenomenon is observed; It’s smaller under currentdirection of180deg and90deg, and no lock-in phenomenon happens.A simplified two-dimensional model is used in the CFD numericalsimulation. The model consists of the cross sections of four rectangularcolumns of the DDS. The numerical study includes two parts: the cross-flowaround four stationary columns and the VIM response of four columnsmoving freely. The numerical results can help explain the experimentalresults. And their general agreements on the response amplitudes indicatethat the numerical simulation is effective in predicting VIM characteristicsof a multi-column floater.This paper can offer valuable reference for the design of the DDS andit’s helpful for the later study on VIM of the multi-column floaters.Therefore the present research is not only of engineering importance butalso of great academic value.