Motion Responses Of A Moored Ship And Mooring Line Force In Open Pier

It is the key problem for the port designer and operation manger that moored ship’s motion response and mooring line force. Considering of the stability of wharf structure, vessel and cargo handing, it is important to determine accurately the motion of a moored ship exposed to the action of waves. Numerical simulations is performed to improve the understanding of the moored ship’s motions and predict the response and the mooring line force in an offshore environment condition in order to provide the scientific basis for manger and designer.In order to provide a quick and valid calculating approach, a numerical simulation program is developed, with mooring line force and displacements under the wind and current action. The program for calculating mooring line forces and movements of a moored ship under combined action of wind and current is presented based on static force equilibrium principle. The wind and current load is calculated by experimental formulate according to the ship type and tonnage. The non-linear effect of mooring lines and fenders is taken into account. Compared with the results of the experiment, Optimoor results and other experiment results, numerical model is proved to be correct and effectively. By using the program, the mooring line load of a moored ship under the wind and current action in different berth length and various arrangement cleats are calculated and optimized. In all, the computed results have been analyzed.Based on the potential theory, with three-dimensional source distribution approach, a time domain numerical simulation model of a moored ship is established by the approach of Cummins, transforming the added mass, damping and first-order wave force and second order mean wave force in frequency domain to time domain. By comparing to the floating semi-sphere, floating barge and experiment results, the time domain model is prove correctly and effectively. By the time domain model, the mooring line force and motion response are simulated under different initial tension, mooring line and current and wind speed. The mooing line forces and motion behaviors of moored ship are calculated under the combined action of wind, current and wave in different berth lengths, kinds of breast line lengths and three mooring line types. Based on the optimal principle of balancing the mooring line forces and minimizing the motion responses of moored ship, the more suitable case is pointed out about berth length and breast line pier arrangement.Groupiness is an important features of ocean wave field, and waves in quick succession are regarded as wave groups, which is greatly affect the floating body motion responses and breakwater safety. Basing on the previous research work, regard the wave spectrum and envelope spectrum as target, a mathematical model of a moored ship under the action of wave groups is presented in this thesis.Waves with the same parameters, such as significant wave height and period, but with different wave groupiness factors are simulated, to study the motion behavior of a moored ship under the action of wave groups with different groupiness factors height (GFH) and group length factors (GLF). The numerical results indicate that compared with the same significant wave height and period, same spectrum energy, the motion behavior of moored ship is greatly influenced by the wave groups. When GFH increases, the wave grouping becomes more and more pronounced. The sway motion increases with the increase of GFH. In contrast, the sway motion marginally increases with the increase of GLF. At the same time, the heave motion increases with the increase of GFH, while decreases marginally with the increase of GLF. The spectrum peak frequency of the surge motion under the wave groups action moves to the low frequency area, the surge motion spectrum peak frequency so close to the natural frequency of surge that it may lead to the moored ship resonant responses.Last, it is analyzed that the relationship of groupiness and statistic wave heights in detail. In all, taking account of the second order difference frequency wave force based on the second order mean wave force, the motion responses of tanker has been simulated.