Experimental Study On The Hydrodynamic Characteristics Of The Floating Dual-pontoons With Tanks Under Incident Waves

Floating liquefied natural gas(FLNG)facility is a floating production device used for offshore natural gas field development.It is positioned at sea through a mooring system and has the functions of mining,processing,liquefying,storing,loading and unloading natural gas.Used in conjunction with liquefied natural gas(LNG)carriers to realize the exploitation of offshore natural gas fields and natural gas transportation.When berthing operations at sea,the gap formed between the hulls will cause the fluid to resonate.A similar phenomenon may also occur in the moonpool structure inside the drilling carrier.The wave force generated by the large oscillation of the fluid in the gap threatens the safety of structures and engineering operations.In the process of transporting natural gas on a liquefied natural gas(LNG)carrier,the sloshing force generated by the sloshing of liquid in the tank acts on the carrier and affects the movement of the carrier.At the same time,the movement of the carrier also stimulates the movement of the fluid in the tank.The coupling between the two also affects the safe navigation of carriers.In view of this,this paper studies the motion response of the adjacent floating dualpontoons under the excitation of incident waves and the change of the wave field in the gap due to the sloshing effect of the liquid tank.In the experiment,two rectangular pontoons of the same size are set.The pontoon on the weather side is provided with a tank,which can be moved with a single degree of freedom along the length of the wave flume by means of pulleys.The research contents include the analysis of the influence of the moving floating dual-pontoons on the wave field,the analysis of the motion response of the moving floating dual-pontoons,the analysis of the influence of the adjacent moving floating dual-pontoons with tanks on the wave field,and the analysis of the motion response of the moving floating dual-pontoons with tanks.First,this paper studies the analysis of the influence of the fixed floating dual-pontoons on the wave field.The parameters to be changed are the length of the pontoon and the width of the gap.The research content includes the change of wavelength and wave amplitude in the gap,the nonlinearity of the wave field in the gap,and the transmission and reflection coefficient of the wave field.The study found that under a long-wave period incident wave,a standing wave with a wavelength equal to the gap length is formed in the gap,which has nothing to do with the gap width and the length of the floating body.In a short-wave period,the wavelength of the standing wave is equal to the incident wavelength,regardless of the gap width and the length of the floating body.The amplitude of the standing wave in the gap decreases as the length of the pontoon decreases and the width of the gap increases.The nonlinearity in the wave field in the gap is affected by the gap width.With the increase of the gap width,the nonlinearity of the standing wave under the floating dual-pontoons lengths is more obvious in the case of lowfrequency incident waves.In the comparison of the transmission and reflection coefficient,when the length of the floating body is equal to the width of the tank,the change trend of the wave amplitude in the gap is opposite to the change trend of the reflection coefficient,and the same as the change trend of the transmission coefficient.Due to the superimposition of waves,the transmission coefficients of the other two floating body lengths are all greater than 1.Secondly,this paper studies the analysis of the influence of the moving floating dualpontoons on the wave field.By changing the width and spring stiffness of the gap,the changes of wavelength and wave amplitude in the gap and the nonlinearity of the wave field in the gap are studied.The study found that the wavelength of the standing wave is no longer equal to the incident wavelength when the low-frequency incident wave occurs,and the wavelength is equal to the length of the gap.The corresponding gap wave amplitude when the floating pontoon moves is greater than the wave amplitude when the floating pontoon is fixed.The amplitude of the standing wave in the gap decreases as the gap increases and the spring stiffness decreases,and as the gap width increases,the difference between the peaks and valleys of the amplitude curve corresponding to the moving floating pontoon is gradually smaller than the difference corresponding to the fixed floating pontoon value.For the nonlinearity of the wave field in the gap,the nonlinearity of the wave field in the gap when the floating pontoon moves is stronger than that of the fixed case.In the case of high-frequency incident waves,the standing wave has obvious nonlinearity and does not change with the gap width.In the case of low-frequency incident waves,the non-linearity of the standing wave is affected by the gap width.Under certain gap widths,the non-linearity is weakened and the waveform is closer to a sine wave.Subsequently,this paper studies the motion response of the moving floating dual-pontoons.After analyzing the free vibration curve of the pontoon,it is found that as the gap width increases,the attenuation speed of the curve first decreases and then increases.The decay speed of the curve corresponding to the hard spring is slower than that of the soft spring.Both the displacement curve and the wave response curve show an upward trend with the increase of the incident period.Changing the spring stiffness and gap width only changes the displacement of the floating pontoon and the amplitude of the wave response,and has no effect on the changing trend of the curve.Under the same incident wave,the narrower the gap,the smaller the spring stiffness corresponds to the larger displacement amplitude and wave response amplitude.After analyzing the average drift value of the displacement,it is found that the non-linear strength of the gap wave under each working condition is consistent with the average drift value of the pontoon displacement.With the increase of the gap width,the average drift amplitude tends to be stable and does not change greatly with the change of the incident wave period.The average drift amplitude corresponding to the soft spring is always greater than that of the hard spring.Then,this paper studies the influence of the moving floating dual-pontoons with tanks on the wave field.By changing the width of the gap,the spring stiffness and the depth of the tank,the variation of the wave amplitude in the gap,the variation of the wave amplitude in the tank,and the nonlinearity of the wave field in the gap are studied.The study found that under the same incident wave period,as the gap width,spring stiffness and tank depth increase,the wavelength of the standing wave decreases.In terms of wave amplitude,the gap wave amplitude corresponding to a floating pontoon with a liquid tank is larger than that of a floating pontoon without a liquid tank,which is larger than that of a fixed floating pontoon.The changing trend of wave amplitude in the gap is consistent with the changing trend of tank wave amplitude.The smaller the gap width,the smaller the spring stiffness,the greater the gap wave amplitude.The increase in tank depth makes the peak period of the amplitude curve transition to high frequency.The standing waves under different gap widths,spring stiffnesses and tank depths all show obvious nonlinearity under high-frequency incident waves.In the case of lowfrequency incident waves,it is affected by the gap width.As the gap width increases,the wave nonlinearity first weakens and then increases.Finally,this paper studies the motion response of the moving floating dual-pontoons with tanks.Analyzing the free vibration curve of the floating pontoon,it is found that with the increase of the gap width and the decrease of the tank depth,the damping ratio first increases and then decreases and then increases,and the decay speed of the curve corresponding to the soft spring is greater than the curve of the hard spring.The variation trend of the displacement amplitude curve and wave response curve with the period of the incident wave affected by the gap width,spring stiffness and tank depth is consistent with the corresponding gap wave amplitude curve and tank wave amplitude curve.After analyzing the average drift value of the displacement,it is found that the nonlinearity of the gap wave under each working condition is consistent with the average drift value of the pontoon displacement.The average drift amplitude corresponding to the soft spring is always greater than that of the hard spring.