Sammendrag
For deep and ultra-deepwater applications, synthetic fibre ropes are considered an enabling technology due to their higher strength-to-weight ratio as compared to steel wire ropes and chains and due to their superior station-keeping performance. The advantages of synthetic fibre rope mooring systems include: • A higher floater payload and reduction in structural costs due to lower vertical load from mooring lines. • A reduction in vessel offsets and associated riser loads due to taut mooring system. • A potential reduction in installation costs due to lighter installation and handling equipment. • Superior endurance under cyclic loading compared to steel moorings. Synthetic fibre ropes have visco-elastic stiffness and stretch characteristics. The change-in-length response of a fibre rope is non-linear, load-path dependent (different unload-reload stiffness), and the length varies with the rate and duration of loading (due to elongation and contraction). The commonly accepted analysis approach is a simplification where a lower-bound and an upper-bound stiffness is used. This practice is primarily based on two factors: 1. The industry at large does not at present have a common, well-defined understanding of fibre-rope change-in-length performance. 2. There is a lack of commercially available mooring analysis programs with the capability to simulate the non-linear change-in-length response of the synthetic fibre rope. Individual designers may however have more advanced analysis procedures, but these are not commonly accepted yet. This paper presents results from the Syrope pilot study, Ref. /5/ and /6/, which has used rope testing to determine the characteristics of the elements in the spring-dashpot model. On this basis a strategy for software implementation in the frequency-domain has been proposed. A case study was performed for a semi-submersible production unit in deep water and harsh environment. The paper focuses on the differences between a commonly accepted, hereafter called traditional analysis approach and the proposed new frequency domain approach. The results show that there are large differences in extreme tensions and offsets as well as fatigue results. Hence, the new approach is considered to represent a significant improvement.