Research On Motion Performance Of A Deepwater Dry Tree Semi-submersible

Dry tree system can be applied to either drilling or well completion production,whose maintenance and management is much more convenient and economic efficiency is much higher than those of wet tree.Recently,dry tree system is mainly applied in TLP and Spar which is restricted by the sea depth and variable loads respectively while Semi-submersible platform dose not have these restrictions.The top tensioned riser(TTR),the total stroke of which is limited to 10 m or less,is mainly used in the dry tree system.Therefore,dry tree system is sensitive with the relative motion between platform and riser,especially the heave motion.In this paper,the motion response of a dry tree semi-submersible in irregular waves and vortex-induced motion(VIM)are mainly investigated,specific content is as follows:The dry tree semi-submersible’s global performance was carried out through model test with the model scale ratio 1:70.Firstly,free rolling and pitching decay test were performed respectively without mooring system in the static water,obtaining the natural periods,damping coefficient and some other important parameters.Then,model test was carried out under the condition of white noise irregular waves to get six-degree-of-freedom response amplitude operator(RAO)in different incident angles.The numerical model of dry tree semi-submersible was established by the hydrodynamic software AQWA,calculating the platform’s six-degree-of-freedom RAO and natural period in different incident angles based on the damping coefficients obtained by model test.The numerical results were validated by comparing with the model test results.Motion response model test of the dry tree semi-submersible in irregular waves was carried out with horizontal mooring system,the environmental condition of which only considered about the wave condition including one year return sea state and one hundred extreme sea state,obtaining the heel,trim and heave motions of the platform in 135° and 180° incident angle.AQWA software was applied to simulate the mooring system,calculating the motion responses of the platform with the incident angle of 135° and 180° in irregular wave condition.The numerical results were compared with the experimental data,which indicated the accuracy of the choice of damping coefficients.Then,dry tree semi-submersible with mooring system was numerically simulated to check whether the motion response under different operating conditions meet the design requirements.The vortex-induced motion experiment of the dry tree semi-submersible was carried out through a towing tank test.The model was dragged by a carriage with constant speed to obtain a stable flow field.The cross-flow and in-line motion were measured and recorded,and the in-plane trajectories in 180° incident angle with different flow velocities were draw to analysis the influence of the vortex shedding to the platform’s motion.