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 themain component of the taut mooring systems, have totally differentmechanical properties i.e. tension-elongation relationship comparedwith the steel mooring chain and wire ropes especially under cyclicloading conditions induced by the combined effects of wind, wave andcurrent. Dynamic stiffness has to be modelled to represent the dynamiceffect of the tension-elongation properties of the fiber ropes. Indeed,the dynamic stiffness is dependent not only on the mean load, strainamplitude or load amplitude, but also on loading cycles based on theobservation of experiments. Unlike the classical formulation involvingonly the mean load, a new empirical expression which takes intoaccount of all three parameters including mean load, strain amplitudeand loading cycles is employed into the mooring analysis model. Theiteration process is performed based on the update of the mean load forthe calculation of the stiffness until it is converged. The mooring linesutilizing the three materials including polyester, aramid and HMPEfiber ropes are analyzed by the mooring analysis method. The effects ofall the parameters especially the loading cycles are investigated andcompared with other traditional empirical expressions of the dynamicstiffness. It is demonstrated that the loading cycles have obviousinfluence on the axial tension response at the fairlead due to theincrease of more than 10% brought to the dynamic stiffness with theloading process.