Study On The Influence Of Column Spacing Ratio On Vortex-Induced Motion Of Semi-Submersible Platform

As one of the main floating platforms for the development of offshore oil and gas resources,semi-submersible platforms are prone to vortex-induced motion in complex marine environments,resulting in serious fatigue damage to themselves and their accessories.Therefore,vortex-induced motion of semi-submersible platforms has always been a hot issue in engineering and academic research.The column spacing ratio is one of the key structural characteristic parameters affecting the vortex-induced motion of the semi-submersible platform,but there are few research results on the influence of the spacing ratio.Therefore,this thesis establishes a numerical model of vortex-induced motion based on Reynoldsaveraged Navier-Stokes equation and SST κ-ω turbulence model,and uses the dynamic mesh technology in Fluent software to realize fluid-solid coupling.In the process of numerical simulation,four spacing ratios(2.5,3.0,3.5 and 4.0,respectively)and two incident angles(0° and 45°,respectively)were selected,and the reduced velocity range was 1.85 ～13.89(corresponding to Reynolds number Re =3600 ～ 27000).Starting from the response amplitude,response frequency,force characteristics and vortex shedding characteristics of the platform,the influence of incident angle and spacing ratio on the vortex-induced motion characteristics of the semi-submersible platform was discussed.The numerical simulation results show that the spacing ratio has a great influence on the platform response amplitude,response frequency,force characteristics,motion trajectory and vortex distribution mode.From the response amplitude,the platform sway and yaw response amplitudes are obviously affected by the spacing ratio at 0° incident angle.The surge response amplitude at 45° incident angle is greatly affected by the spacing ratio.From the response frequency and power spectral density,the platform sway with different spacing ratios has observed the frequency locking phenomenon,and the motion energy of the platform sway in the locking zone is more concentrated.There is no obvious frequency locking phenomenon observed in the surge and yaw.With the increase of the reduced velocity,the yaw response frequency gradually increases from the natural frequency of the platform sway to the natural frequency of the yaw.At 0° incident angle,the range of sway locking zone with spacing ratios of 2.5 and 3.0 is smaller than that with spacing ratios of 3.5and 4.0.At the 45° incident angle,the range of the platform sway locking zone with different spacing ratios is roughly the same,and the range of the locking zone is greater than the 0°incident angle.From the perspective of force characteristics,at 0° incident angle,the larger the spacing ratio,the more irregular the force coefficient curve;at 45° incident angle,the force coefficient curves of all spacing ratios are relatively regular.From the trajectory point of view,whether the incident angle is 0° or 45°,the ’ 8’shaped or elliptical trajectory can be observed in the locking area of the platform,but it is still affected by the spacing ratio.The trajectory is more regular when the spacing ratio is small,but with the increase of the spacing ratio,the regularity of the trajectory of the platform in the lock-in area gradually weakens.From the perspective of vortex flow patterns,the vortex flow patterns under 0° incident angle is more regular,and the ’ 2P ’ mode or skew wake mode can be observed in the lock-in area.At the 45° incident angle,the vortex flow patterns of the platform is more complicated,and it is difficult to observe obvious vortex streets in the wake region.