A Study Of The Large-amplitude Wave Resonance In Confined Space Between Floating Structures

When a FPSO(Floating Production Storage and Offloading)and a shuttle tanker are in side-by-side offloading operations at sea,a long and narrow(relative to ship beam)forms between the two vessels.The water in the narrow gap may show violent resonant motions under the excitations of incident waves with certain frequencies.This phenomenon is often referred to as the gap resonance.Similar resonance behaviors may also occur in a moonpool of drilling vessels.Due to the large-amplitude resonant fluid oscillations in narrow gaps/moonpools,the wave loads acting on the maritime structures could be significant,which therefore threatens the safety of the structures and engineering operations.So it is of engineering significance to establish an accurate and fast prediction tool for gap resonances.Conventional potential flow models generally over-predict resonant response amplitudes significantly due to an inherent drawback where the energy dissipations due to viscous effects cannot be taken into account.Efforts have been made to modify potential flow models by introducing an artificial damping mechanism(or coefficient),which can lead to accurate predictions while maintain the computational efficiency.But it is a challenge currently to find a reasonable approach to determine the damping coefficients in those modified potential flow models.And it is debatable whether a linear or quadratic damping should be used.To solve these problems,it requires a deep understanding of the mechanisms of the viscous damping in the fluid resonance in narrow gaps/moonpools.In this thesis,studies are carried out through theoretical analysis,physical model testing and numerical simulations.The causes and properties of viscous damping in the fluid resonance are clarified,and a quantification method for the viscous damping(coefficient)is proposed.First,considering inertial forces.restoring forces,damping forces and excitation forces acting on the trapped fluid bulk?a simplified equation of fluid motion in a narrow gap or moonpool is formulated.through which a formula of-resonant frequency as a fast and easy-to-use estimation tool is obtained.More importantly,a viscous damping model is proposed based on the fluid motion equation.The damping model takes into account the damping induced by both flow separation and wall friction through two damping coefficients,namely,the local and friction loss coefficients.In this manner,viscous damping is separated into two components based on the property(namely,linear damping and nonlinear damping):one is the damping due to flow separation.which is quadratically proportional to the velocity;the other one is the damping due to wall friction,which is linearly proportional to the velocity for laminar boundary layers,whereas the frictional damping is quadratically proportional to the velocity for turbulent boundary layers.The local loss coefficient is determined through the proposed method in this thesis,and the frictional loss coefficient is estimated through an empirical formula found in the literature.Next.the viscous damping model is implemented in the dynamic free-surface boundary condition in the gap of a linear frequency-domain modified potential flow model,in which the introduced damping coefficient can be determined automatically through iterative procedures.Through this approach,the uncertainties in the previous studies due to tuning damping coefficients against experimental data are avoided.In addition,a viscous fluid model is established,based on which numerical simulations are carried out to clarify the causes and deepen the understanding of the mechanisms of viscous damping in the fluid resonance.Then,the fluid resonance in the narrow gaps formed by two floating bodies or a floating body and a bottom-mounted vertical wall are investigated through a series of physical model tests.The influences of the parameters including the draft and corner shape of the boxes,gap width,geometry of the fins attached to the side walls in the gap and incident wave height on the resonant wave height and frequency.damping and phase differences between free-surface motions are studied.In addition,based on viscous fluid simulations,the characteristics of the flow separations at the sharp corners and the shear layers at the body surfaces are investigated by analyzing the evolution of the flow field in the narrow gap.The results confirm that it is reasonable to consider the two components of damping in the viscous damping model.Finally,the modified potential model implemented with the viscous damping model is applied to predict the wave resonance in confined space formed by fixed or floating bodies.The predictions are compared against the experimental data in this work and that available in the literature.The comparison results suggest that the modified potential flow model implemented with the viscous damping model works well in capturing both the resonant fluid and body responses under a wide range of damping conditions.In addition,the estimations made by the proposed approximation formula of resonant frequency of gap/moonpool resonance are validated against experimental and numerical results.The influences of the geometric parameters on the resonant frequency are clarified.