An investigation of dynamic stiffness changing with loading cycles on taut mooring analysis

摘要

Synthetic fiber ropes e.g. polyester, aramid and HMPE ropes, as the main component of the taut mooring systems, have totally different mechanical properties i.e. tension-elongation relationship compared with the steel mooring chain and wire ropes especially under cyclic loading conditions induced by the combined effects of wind, wave and current. Dynamic stiffness has to be modelled to represent the dynamic effect of the tension-elongation properties of the fiber ropes. Indeed, the dynamic stiffness is dependent not only on the mean load, strain amplitude or load amplitude, but also on loading cycles based on the observation of experiments. Unlike the classical formulation involving only the mean load, a new empirical expression which takes into account of all three parameters including mean load, strain amplitude and loading cycles is employed into the mooring analysis model. The iteration process is performed based on the update of the mean load for the calculation of the stiffness until it is converged. The mooring lines utilizing the three materials including polyester, aramid and HMPE fiber ropes are analyzed by the mooring analysis method. The effects of all the parameters especially the loading cycles are investigated and compared with other traditional empirical expressions of the dynamic stiffness. It is demonstrated that the loading cycles have obvious influence on the axial tension response at the fairlead due to the increase of more than 10% brought to the dynamic stiffness with the loading process.