Dynamic Characteristics And Motion Responses Of Deepwater Mooring System With Submerged Buoy

The mooring positioning system is popularly acknowledged as the most common way of passive positioning for offshore floating structures.Its working principle is to use mooring lines to connect the floating structure with the anchor foundation fixed on the seabed,restrain the motion response of floating structures within the operation requirements and ensure the strength design of mooring system itself to prevent the excessive tension simultaneously.According to the line shape of mooring lines,it can be divided into two types:catenary mooring system and taut mooring system.The catenary mooring system is the traditional and most commonly used positioning system for floating structures,and it provides the restoring forces for the floating structures through changes in its own gravity.For deepwater or ultra-deepwater floating structures,the gravity of traditional catenary mooring line consisting of chain and wire would increase rapidly as the operating water depth increases,leading to the inappropriate augment in economic cost.Meanwhile,the horizontal stiffness would also decrease,resulting in a decrease in positioning capability and vertical bearing capacity of floating structures.Nevertheless,it has a high safety reserve performance due to the existence of lying chain in the catenary mooring line.The taut mooring system consisting of synthetic fiber has the advantages of light weight and high restoring stiffness.However,the complex mechanical properties of synthetic fiber materials,such as dynamic stiffness,creep,and hysteresis make the damage form of taut mooring system very complicated.Based on the above research background,the design concept of deepwater mooring system with submerged buoys is proposed in this dissertation,and the size and position of the buoys are optimized.The results demonstrate that the proposed system can retain the advantages of traditional catenary mooring system(including the high safety reserve,low requirements for anchor,and low cost)while reduce the vertical load of mooring system on the platform,improve the motion responses of floating platform,and weaken the risk of line breakage,which can expand the traditional catenary mooring system into deep water or ultra-deep water.The specific research contents are described as follows.(1)Experimental and numerical study of the hydrodynamic characteristics of buoy.Firstly,the calculation method of hydrodynamic coefficient is established for the fluid force acting on the buoy under unsteady motion excitation.Then,the hydrodynamic performance of spherical buoy under sinusoidal forced excitation is investigated experimentally.Finally,the hydrodynamic performance of spherical buoy under the low-frequency and wave-frequency motion is further explored by computational fluid dynamics software,and the variation law of the hydrodynamic coefficient of buoy is obtained.(2)Calculation method and corresponding experimental verification of the static and dynamic response of mooring lines.Firstly,the numerical solution methods for the static and dynamic response of mooring lines are established.Then,an improved mooring damping calculation method is proposed.Finally,the physical model experiment of the single mooring line is performed at the State Key Laboratory of Coastal and Offshore Engineering of Dalian University of Technology to further verify the correctness and effectiveness of the proposed method.(3)Static characteristics and optimal design of the mooring system with submerged buoys.Firstly,the influence of attached buoy on the static characteristics of mooring system is analyzed when the parameters and arrangement of the catenary mooring line are unchanged.Then,the optimization design principle of the mooring system with buoy is proposed based on the engineering requirements,and the corresponding optimization design steps are provided.Finally,the static characteristics of the optimized mooring system with buoys are investigated,and the advantages of the optimal design are illustrated.(4)Dynamic characteristics of the mooring system with submerged buoys.Firstly,the action mechanism of attached buoy on the tension amplification of mooring line is explored.Then,the influence of attached buoy on the mooring damping is analyzed.Finally,the influence mechanism of attached buoy on the mooring damping is deeply revealed by combining the improved calculation method of mooring damping proposed in(2).(5)Influences of the attached buoy on the motion response of floating platform and mooring lines.Firstly,a coupled time-domain analysis model of the motion response of floating platform and its mooring system is established.Then,the influences of optimized mooring system with buoys on the motion response of platform and mooring system are investigated using a semi-submersible platform and its mooring system as the research object.Finally,a physical model experiment of the semi-submersible platform and its mooring system is conducted to confirm the superiority of the proposed optimized design of mooring system with buoys.