Design And Dynamic Response Analysis Of The Mooring Buoy

The mooring buoy system is widely used for the mooring of working ships in the offshore oil-gas exploitation project. Under combined wind, wave, current and the loads from mooring ship, the motion behaviors of moored ship are complicated. So, according to the requests of moored ship in the ocean, it has important significance for the safety of the mooring ships to design a safe and economical mooring system and to analyze the behaviors of dynamic response of the mooring system.The mooring buoy system is designed for the mooring of a 10000HP tug working in PengLai sea area. The mooring force is evaluated, and the parameter of chain, buoy and anchor are chosen. The diameter of buoy is 4.5m and the height 2.5m. The catenary's equation of chain is derived, in which the chain is not tangential to the horizontal at the seabed. According to the catenary's equation, the static stiffness curve of the mooring buoy system is calculated and the influence of the chain length and weight on the mooring stiffness is investigated. The loads acted on the buoy are evaluated according to the long-wave assumption and strip theory. The differential equations of the surge and heave motions of the buoy in survival case is established by strong nonlinear restore stiffness curve is poly-fitted by several subsections, and the motion of buoy and tension of chain are calculated by using four-order Runge-kuta numerical method, and the and the impact of the buoy on the chain is analyzed and the survival ability of the buoy under the critical environment is evaluated.The results and analyses show that the static stiffness is determined by the chain parameters. Longer and heavier is the chain, the better the elasticity of the mooring system is. Due to the lighter chain, the influence of chain on the restore stiffness of the buoy's heaving is small. As the result of the strong nonlinear restore stiffness around the balance position suffered from wind, current, the restore stiffness of surge motion is asymmetric and so the surge of buoy is asymmetric motion. Otherwise, the chain can be easily pulled to straight, while wave height is larger than 3m, the impact of the buoy on the chain will occur and the tension in the chain snap , and will become 5~10 times larger. The snap tension is dangerous but not to exceed the break limit of the chain, so the strength of mooring buoy designed in the paper satisfies the requirements of survival.