Field Study of Biofouling on the Surface of Synthetic Fiber Ropes in Japanese Coastal Waters and its Influence on the Engineering Characteristics of Hybrid Moorings

Abstract A hybrid mooring system with steel chains and synthetic fiber ropes for offshore floating wind turbines is attractive for several reasons, including lower material costs. However, the degradation of synthetic fiber ropes over long-term use has not been fully verified. In this study, the authors focus on the biofouling on the rope. The impact of biofouling on the rope surface includes (1) increasing the line weight of the rope and changing its initial catenary shape, (2) increasing the total diameter of the rope section and changing its dynamic behavior. The authors investigated the biofouling of ropes through a combination of field experiments and numerical simulations to assess its impact on the static and dynamic behavior of mooring lines. Two-year field experiments were conducted at three coastal stations in Japan to clarify the biofouling using different synthetic fiber ropes as specimens. The specification of the specimen was 56 mm in diameter and 1 m in length, made of nylon or polyester. The experiments recorded 83 mm as the maximum biofouling thickness and 100 N/m2 (17.7 N/m) as the weight of the biofouling. Based on the result, numerical simulations were also conducted to clarify the impact of the biofouling on the rope. The results showed that biofouling on the synthetic fiber ropes that make up the catenary mooring had virtually a little effect on the initial catenary shape or tension fluctuations.