To understand the evolution of dynamic stiffness of damaged synthetic fiber mooring ropes, experimental investigations of polyester and high modulus polyethylene (HMPE) ropes are systematically performed utilizing a specially designed experimental system. An experimental procedure is proposed and test results show that the dynamic stiffness increases with increasing mean load and loading cycles, while decreases with increasing strain amplitude and damage level. The similarity criterion of dynamic stiffness is derived from the dimensional analysis for damaged ropes and verified by experiments. An empirical expression that accounts for the damage, mean load, strain amplitude, and loading cycles is proposed to describe the damage effect upon dynamic stiffness of synthetic fiber mooring ropes.

In this study, effects of damage levels of fiber ropes on the performance of a hybrid taut-wire mooring system are investigated. The analysis is performed using a numerical floating production storage and offloading (FPSO) model with a hybrid mooring system installed in 3000 m of water depth. An in-depth study was conducted using the numerical model, the dynamic stiffness equation of damaged fiber ropes, the time-domain dynamic theory, the rainflow cycle counting method, and the linear damage accumulation rule of Palmgren-Miner. Results indicate that, in a mooring line with an increasing damage level, the maximum tension decreases, while the offset of the FPSO increases. Particularly, when a windward mooring line failure occurs, in addition to the significant increase in the offset of the FPSO, the maximum tension, tension range, and annual fatigue damage levels of the remaining lines adjacent to the failed also increase significantly. The present work can be of great benefit to the evaluation of the offset of the floating platform, the tension response, and the service life of the hybrid mooring systems.

Creep tests of high modulus polyethylene (HMPE) samples are performed to obtain the corresponding creep and creep-rupture curves. The results show that the secondary creep stage is especially pronounced, and its creep rate is nearly constant. Based on this fact, the creep rate at the secondary stage has been utilized as the representative value of each case. Therefore, the similarity criterion of creep rate at secondary stage is proposed to analyze the corresponding relationship between engineering creep rate and that in laboratory tests. Besides, an empirical expression of HMPE yarns is proposed to account for the effects of both loading level and test temperature on the creep rate at the secondary stage. Moreover, an equation for calculating the creep lifetime of HMPE samples is derived and verified by test data. This study will help to improve the understanding of creep behaviors of HMPE ropes and provide a significant reference for the application of HMPE ropes in mooring engineering.