Advances Towards Replacing Top Chain with Fiber Rope for Offshore Moorings: A DeepStar Study

Abstract

Abstract Previous studies suggest that chain failures may contribute to about half of the mooring failures sustained by permanent mooring systems [Ma et al. 2013. OTC-24025]. DeepStar commissioned a study to generate data to prevent failures in fiber ropes if they were to replace top chain. The effect of fairlead sheaves was not considered in the present study. For fiber ropes brought close to water surface, or even above water, the key degradation processes identified as possible causes of failure were marine growth, and the effects of temperature and Ultra-Violet (UV) radiation. Samples of current production, supplied by six ropemakers, containing both polyester and High Molecular Weight Polyethylene (HMPE) ropes were tested, as the study scope of DeepStar 21403 and 22403 projects. Samples were exposed to marine growth for two yearly growth seasons, followed by dissection to investigate penetration of organisms in the load-bearing core. The capacity of the rope cover and filter to screen UV getting to the core and the effect of UV eventually reaching the core fibers was measured. The effect of temperature on the polyester and HMPE fiber of the rope cores was also investigated. The braided cover, combined with the soil ingress protection layer of ropes tested, provided effective screening of ultra-violet radiation, with very little UV reaching the load-bearing fibers of the rope core. Small diameter ropes with thinner cover and filter layers tend to allow more radiation to pass. The effect of UV eventually reaching the core fibers was measured. After being attenuated by the screening effect of cover and filter, UV is not expected to be an issue for fiber rope performance, provided that cover and filter maintain their original screening characteristics. For the range of temperature tested, significant degradation was not identified in the polyester yarns (tested in immersion). Testing of the HMPE yarn suggested that further studies may be required if temperature reaches the upper part of the range of temperatures tested. Penetration of marine growth through cover and filter layers is also reported. Results can be used to better define candidate materials and rope construction to be used if fiber rope is used to replace top chain, provided mooring lines are connected to the hull without passing through deviating devices, such as fairlead sheaves. The results should be useful for floating production units of oil and gas and wind energy. The use of fiber rope in shallow water can be particularly beneficial in the moorings of floating offshore wind turbines (FOWTs).