Study On The Motion Responses Of Floating Structure Under Bimodal Spectrum Waves

The sea areas of the South my country Sea are affected by tropical cyclones and typhoons in the western Pacific with a high frequency.After tropical cyclones and typhoons,most of the sea conditions appear in the form of mixed waves,and their spectrum structure is mostly bimodal,which poses a potential threat to the safety of marine floating structures.In this paper,a comparative study on the motion response of floating structures under the action of bimodal wave and conventional unimodal wave is carried out by numerical simulation method.Clarify the potential damage of bimodal spectrum waves to floating structures compared to conventional waves.It can provide a reliable reference basis for the design of marine engineering equipment in the South my country Seas Islands and reefs.First,a wave numerical water tank was established based on STAR-CCM+,grid independence verification and time step independence verification.The regular waves are generated by the boundary velocity entrance method and the push-plate wave-making method respectively.The numerical calculation results are in good agreement with the theoretical values.The six-parameter spectral model proposed by Ochi-Hubble was used to numerically generate three typical bimodal spectral waves.The numerical simulation wave spectrum is in good agreement with the target wave spectrum.Secondly,a numerical calculation model of the motion response of the two-dimensional pontoon under the action of waves is established.Calculated the motion response of the pontoon under the action of regular waves.The numerical results are in good agreement with the experimental results in the literature.The motion response of the two-dimensional pontoon under the action of the bimodal spectrum wave with three different low-frequency spectrum peak periods(three types of mainly wind waves,Wind waves and swells are equal,mainly swells)are calculated.It was compared and analyzed with the single-peak JONSWAP spectrum waves(wind waves and long-period waves)with the same total energy.the result shows: For the rolling motion of the two-dimensional pontoon,When the wave action is dominated by wind waves,the floating body roll motion amplitude is the largest,approximately 1.19?1.44 times of swells and about 1.25?1.91 times of wind waves.For the lateral and heave motion of the floating body,the extreme value of the swell motion is the largest,and the extreme value of the wind wave is the smallest.The motion extremes of the three types of bimodal spectra are all in between.Finally,a moored semi-submersible platform was analyzed in the frequency domain and time domain based on the AQWA hydrodynamic calculation software,and compared with the literature test results,the reliability of the calculation model was verified.The motion response of the platform and the change of mooring line tension under the action of different energy ratios(mainly wind waves,equivalent wind waves and swells,and main swells)and bimodal spectrum waves with different peak periods are studied.Compare the pure wind waves(shortperiod irregular waves)and pure swells(long-period irregular waves)with the same total energy.The results shows: For the semi-submersible platform,the bimodal wave is larger than the single-peak wind wave and the single-peak swell in terms of heave motion,pitch motion and the maximum tension of the mooring line.The amplitude of swelling motion can reach up to3.78 times that of swells,the amplitude of pitching motion can reach up to 1.85 times that of swells,and the maximum mooring line tension can reach up to 1.44 times that of swells.The amplitude of swaying motion is up to 1.17 times that of wind waves,and the amplitude of swaying motion is up to 1.09 times that of wind waves.The maximum tension value of the mooring line can be up to 1.12 times that under the action of wind waves.