Study On Reasonable Cross-Section Type Of Submerged Floating Tunnel Under Wave-Current Coupling

With the continuous development of China’s economy,the overall layout of the Belt and Road Initiative,it is imperative to build a fixed traffic channel across the strait.Traditional engineering solutions are facing many challenges in the straits that span complex sea environments.Therefore,SFT,a characteristic engineering form,came into being.How to maximize the reliability of SFT is the biggest factor restricting the practical application of SFT,and the choice of SFT structure type in different sea environments bears the brunt.Supported by the construction technology project "Deep Sea SFT Key Technology Preliminary Research(2013318740050)" of the Ministry of Transportation and Communications,this paper carried out the wave of key parameters of SFT’s dynamic response through the combination of theoretical analysis,model test and numerical simulation.Research on key parameters of SFT structure selection under the effect of flow coupling,the main results are as follows:1)The calculation method of the interaction between wave and ocean current is analyzed.According to the existing wave force calculation theory and the Morison equation,the wave force acting on the suspension tunnel is obtained.2)Under the action of waves,the pressure amplitudes of the three cross-section types SFT are positively correlated with the wave height and wave period,and the wave height factors have a more significant influence on the fluctuation characteristics of the pipe section.From the extreme pressure point,the circular cross-section is the largest,followed by octagon,and the ellipse is the smallest.The pressure extremes of circles and octagons are more sensitive to changes in wave height than ellipses.3)Under different wave elements,the elliptical and octagonal cross sections begin to appear local peaks or fluctuations as the wave height or period increases,and the number of local peaks of the octagon is more than the ellipse.The circumferential pressure distribution of the circular cross section is more uniform,but the pressure extreme value is the largest;the fluid linearity of the elliptical cross section is the best,and the pressure extreme value is the smallest,but the pressure amplitude difference between the front and back waves is large.Produce a certain amount of torsional fluctuations.The pressure extreme value of the octagonal section is in the middle,and the pressure amplitude difference is similar to that of the ellipse,but the fluctuation is more sensitive.This may be due to the reattachment of the vortex at the upper and lower inflection points of the back wave surface of the pipe section after the vortex is separated at the inflection point of the wave front under the action of the pressure field.4)The three types of SFT horizontal wave force and lift force are positively correlated with wave height.Among them,the horizontal force of circular section is 15%～ 24% greater than that of lift,the horizontal force of oval section is 49% ～ 53% greater than that of lift,and the horizontal force of octagonal section is 65% ～ 68% greater than that of lift.The three types of SFT horizontal force are greater than the lift force,and the sensitivity of elliptical and octagonal cross-sections affected by wave height factors is greater than that of circular cross-sections.5)Under the effect of wave-current coupling,with the increase of wave height,the maximum transverse displacement amplitudes of the three SFT types increase linearly.The amplitudes of the maximum heave displacement of SFT of different cross-section types all increase linearly,and the sensitivity of changes of elliptical cross-section is slightly smaller than that of other cross-sections.With the increase of flow velocity,the maximum lateral displacement amplitude of SFT of different cross-section types increases linearly with small amplitude,and the change trend of circular cross-section is more significant.The maximum heave displacement amplitude of the SFT of the circular cross-section type showed a nonlinear change,and the octagonal and elliptical cross-sections showed a limited amplitude increase.The effect of flow velocity on displacement parameters is significantly smaller than that of wave height.6)Eliminate the influence of the boundary effect at both ends of the pipe section.The junction of the circular section SFT pipe section and the anchor cable has a significant stress concentration phenomenon.The overall stress distribution of the upper structure of the pipe section is greater than that of the lower structure.The stress of the SFT pipe section with elliptical cross section is mainly distributed on the longitudinal line at the location where it is connected to the pipe section.There is no significant difference in the stress distribution between the upper structure and the lower structure.The stress of the octagonal cross-section SFT pipe section is mainly distributed on the longitudinal line of the location where the pipe section is connected.In addition,the stress distribution along the longitudinal section of the pipe section is also concentrated at the upper corners of the side walls on both sides,and the stresses of the upper structure and the lower structure There was no significant difference in the distribution.7)Under the action of waves,the amplitudes of the peak and valley values of the three types of SFT wave-front anchors are positively correlated with the wave height as a whole,and the tension time-history curves are periodically changed.The tension of the anchor cable of the three types of floating pipe sections on the wave-facing surface is greater than that on the back-wave surface,which is due to the rotation of the SFT pipe section under the torque.There is a small time difference in the formation of the peak tension of the front and back waves,which is caused by the phase difference of the wave sequence when it propagates at different points.For the extreme value of the peak tension difference between the front and back waves,the circular cross-section is the smallest,followed by the octagon,and the ellipse is the largest.Anchor cables with circular cross-section SFT are more prone to local deflection when irregular waves are applied.8)Based on the analysis of the above conclusions,in the sea area with a large water depth,a circular cross-section with a more uniform pressure distribution can be selected,and an elliptical cross-section with better hydraulic performance can be selected under the conditions of medium water depth or moderately small water depth.By controlling the pretension of the anchor cable when large,an octagonal cross section with better economical space and optimal cross section layout space can also be selected.