Numerical Study On Gap Resonance Between Side-By-Side Marine Structures Under Wave Actions

In the side–by–side operation between large offshore structures,there are often long and thin gaps between them,which are much smaller than the size of the structure itself.Besides,when ships dock in front of the wharf,there will be similar gap between them.Relevant engineering practice and research work show that under the wave action with the specific frequency,the fluid in the gap between floating marine structures will move violently,which is generally called "gap resonance".When the gap resonance occurs,the height of the water in the gap will increase sharply,and the wave force acting on the structure will also increase sharply,and the motion response of the floating structure under the wave action will be greatly enhanced,which directly threatens the safety of the anchoring system,fender device and the workers and engineering facilities on the structure or wharf.In this thesis,a viscous numerical wave flume based on OpenFOAM is used to study the gap resonance between a ship–wharf system and a two-structure system.At the beginning of this dissertation,the research background and significance of gap resonance are introduced,and the previous study concerning this subject is systematically reviewed.Subsequently,the numerical model used in this thesis,OpenFOAM,is introduced.The governing equations,boundary conditions,the numerical discretization dynamic mesh motion and numerical wave generation validation.And then the ability of the numerical model used for simulating the gap resonance phenomenon is varified by reproducing laboratory and numerical experiments presented in the previous literatures.On these bases,the following problems are further investigated:For the gap resonance of the ship–wharf system,the fluid resonant frequency and the frequencies at which the maximum horizontal wave force,the maximum vertical wave force and the maximum moment occur decrease with the increase of the slope of the topography,which indicates that the fluid resonance in the gap plays a leading role in the wave loads acting on the structure.The variation of maximum horizontal wave force and maximum moment with the slope are in good agreement with the variation of resonance wave height height with the slope.For different incident wave heights,the maximum vertical wave force increases first and then decreases slightly with the increase of slope and reaches the maximum value when the slope is 0.100.(1)For the fixed two–structure system,the frequencies at which the maximum vertical wave forces on the two boxes and the maximum horizontal wave force on the upstream box occur deviate from the fluid resonant frequency obviously,and larger incident wave height will lead to more obvious difference between them.The frequency at which the maximum horizontal force on the downstream structure occur is equal to or very close to the fluid resonant frequency.The minimum reflection coefficient and the maximum energy loss coefficient always appear at or very close to the fluid resonant frequency,while the frequency at which the maximum transmission coefficient appears is obvious less than the resonant frequency.Both the amplification curve method and the direct envelope-fitting method are able to accurately evaluate the response time and the damping time of the resonant free-surface elevation in the gap,and it is shown that the damping time is always significantly larger than the corresponding response time.(2)Compared with the fixed two–structure system,releasing the heave motion of the upstream structure will increase the fluid resonant frequency and decrease the resonant wave height in the gap.The dimensionless fluid resonant wave height in the gap will decrease with the increase of the incident wave height.The maximum and minimum heave displacement of the upstream box decrease and increase with the increase of the incident wave height,respectively.The incident wave frequencies at which the maximum and minimum heave displacement occur are obviously different from the fluid resonant frequency.The influence of incident wave height on them is completely different.For all the incident wave heights considered,the minimum reflection coefficient and the minimum energy dissipation coefficient appear exactly at or very close to the fluid resonant frequency.(3)For all the incident wave heights considered,due to the heave motion of the upstream box,the frequencies at which the maximum horizontal wave force,the maximum vertical wave force and the maximum moment on the two structures occur are greater than the corresponding frequencies for the fixed structure system,and they are different from the fluid resonance frequency.In addition,the heave motion of the upstream structure will make the dimensionless maximum wave forces(including the horizontal force,the vertical force and the moment)of the two structures smaller than that of the fixed structure system under the considered incident wave height.