Dynamic modeling of nylon mooring lines for a floating wind turbine

Abstract

As current attention of the offshore industry is drawn by developing pilot farms of Floating Wind Turbines (FWTs) in shallow-water between 50m and 100m, the application of nylon as a mooring component can provide a more cost-effective design. Indeed, nylon is a preferred candidate over polyester for FWT mooring mainly because of its lower stiffness and a corresponding capacity of reducing maximum tensions in the mooring system. However, the nonlinear behaviors of nylon ropes (e.g. load-elongation properties, fatigue characteristics, etc.) complicate the design and modeling of such structures. Although previous studies on the mechanical properties and modeling of polyester may be very good references, those can not be applied directly for nylon both on testing and modeling methods. In this study, first, an empirical expression to determine the dynamic stiffness of a nylon rope is drawn from the testing data in the literature. Secondly, a practical modeling procedure is suggested by the authors in order to cope with the numerical mooring analysis for a semi-submersible type FWT taking into account the dynamic axial stiffness of nylon ropes. Both the experimental and numerical results show that the tension amplitude has an important impact on the dynamic stiffness of nylon ropes and, as a consequence, the tension responses of mooring lines. This effect can be captured by the present modeling procedure. Finally, time domain mooring analysis for both Ultimate Limit State (ULS) and Fatigue Limit State (FLS) is performed to illustrate the advantages and conservativeness of the present approach for nylon mooring modeling.

Optimization study of a catenary mooring system for a spar floating wind turbine based on its hydrodynamic responses

“…Also, smaller dimensions of mesh elements increase accuracy and also computations, which becomes more important for calculating the force applied to the structure. 23 Independence of numerical solution from mesh. Figure 7 shows an overview of the meshed structure in the software.…”

“…Mooring system analysis was done for both ultimate limit state (ULS) and fatigue limit state (FLS) in the time domain. 23 The effects of the mooring configurations on the dynamic response of a submerged floating offshore wind turbine (SFOWT) were investigated in the work of Li et al In their study, the dynamic response of the platform, the tension in the mooring lines, and the bending moment in tower base and blade root were examined for a coupled dynamic system under four different mooring configurations. The results showed that the optimized mooring system presents a larger restoring force so that the structure has small movement in the harsh state of the sea.…”

“…Mooring system analysis was done for both ultimate limit state (ULS) and fatigue limit state (FLS) in the time domain. 23…”

Fatigue of improved polyamide mooring ropes for floating wind turbines

“…The idea of using nylon ropes for long-term mooring lines is relatively new as there is little available knowledge on the durability behaviour of such structures. Indeed, as Pham et al (2019) conclude in a very recent study "a comprehensive study on the critical fatigue damage mechanisms of nylon should be the topic of future work." It is therefore essential to generate further data on polyamide mooring ropes, in order to validate their use for this application.…”

Evaluating Mooring Line Test Procedures through the Application of a Round Robin Test Approach

“…The requirements for floating offshore wind are closer to those of the oil and gas industry, but the platforms are smaller and are subjected to larger movements. In this case, more mooring line compliance is needed to avoid generating high cyclic loads and materials such as polyester may be too stiff; therefore, there has been considerable recent interest in polyamide ropes for this application [7][8][9].…”