Nylon ropes for mooring of floating wind turbines

NYMOOR contributes to the implementation of safe, sustainable, and cost effective offshore floating wind energy. Floating wind turbines are kept in place by mooring systems, and the goal of NYMOOR is to enable design and use of nylon ropes in moorings. Nylon ropes are flexible, and this property reduces dynamic tensile line loads in wind and waves and by that reduces the amount of material needed in moorings. Application of nylon ropes has the potential to very much reduce the need of heavy chains, or even remove the need of chains. However, there is no experience with use of this material in permanent moorings. Of concern is the long-term endurance and behaviour of nylon in the ocean environment. NYMOOR will provide knowledge on the long-term properties and failure mechanisms of nylon moorings. Time, load, humidity, and temperature dependent mechanical properties of nylon ropes will be found through development and use of new laboratory testing equipment and procedures, and mathematical models developed in the project. The results will be applied to study the effect of using nylon ropes in novel mooring solutions and the design opportunities this brings. During the first 9 months of the project, a significant effort has been devoted to acquiring and setting up laboratory equipment and procedures for testing of small samples from nylon ropes, namely for testing PA6 nylon yarns. The samples have been provided by our project partner Bridon International. Two types of tests have been initiated, namely creep tests at SINTEF Industry and tensile strength and stiffness tests at SINTEF Ocean. The aim of the creep tests is to assess how the strain evolves over time when the fully wet material is subjected to constant load. Different ambient temperatures and different loads are considered to assess the time-temperature-load dependency. Tensile tests are performed to assess the material strength, while stiffness tests aim at identifying quasi-static stiffness and dynamic stiffness. The new laboratory equipment includes pneumatic grips to hold the samples, a load cell for measurement of the load level, an actuator to apply the load and precise measurement of the actuator position. The sample is fully submerged in water. A conditioning machine controls the water temperature. Initial testing has been performed successfully and the system and procedures are ready for systematic testing. An accurate characterization of the nylon properties described above is essential for optimal and safe design of the mooring system based on nylon ropes. One other activity consisted of post-processing nylon sub-rope laboratory data acquired by Bridon International. The post-processed results include creep behaviour, break load, static stiffness and dynamic stiffness. Sub-rope fatigue properties were also provided to the project, namely points on the T-N curve. One should note that the project methodology consists of performing extensive laboratory testing with small samples (yarn level), which is very efficient in terms of time and cost, and relate the identified mechanical properties at the yarn level with the same properties at the sub-rope and rope level. The sub-rope data mentioned in the previous paragraph will be used to establish the relations between yarn and sub-rope. In parallel with the activities described above, the project established a reference mooring design for the moored floating wind turbine case study to be used within the project. This consists of the INO WINDMOOR platform located at the Gulf of Maine, with a report under review. A reference mooring design is also being prepared for Buchan Deep, which is characterized by more severe environmental conditions. The tension characteristics and time series obtained during the mooring analyses will be used as input for the lab testing of nylon materials. The mooring designs themselves will be base cases for further studies during the second part of the project.

The goal of NYMOOR is to enable design and use of nylon mooring systems for floating wind turbines (FTWs) with high durability in ocean environments. Availability of steel chain for mooring lines has been identified as a constrain to the required development of offshore wind energy. Application of flexible nylon mooring ropes has the potential to solve this problem, but there is no experience with their use in permanent moorings. Of particular concern is the long-term endurance and behaviour of nylon, while design analysis methods require new models for the mechanical properties of nylon ropes. NYMOOR will provide knowledge on possible long-term degradation and failure mechanisms of nylon moorings. Time, load, humidity, and temperature dependent mechanical properties of nylon ropes will be found using new tensile testing equipment and procedures, and mathematical models developed in the project. This is based on the hypothesis that mechanical properties of ropes can be established based on limited sub-rope testing, extensive yarn testing and mathematical/numerical methods. These results will be used to establish numerical models for load history dependent stiffness and elongation of nylon ropes, which will be applied in design analysis tools. Results will be applied to study the effect of using nylon ropes in novel mooring solutions and the design opportunities this brings. Finally, methods for determining the long-term reliability and behaviour of nylon ropes, including variations in mechanical properties and statistical models of mooring line loads, will be developed and applied in case studies. An interdisciplinary approach combines experimental research, material science, numerical structural analysis and structural reliability. The first two disciplines are used to investigate, model and identify the nylon rope complex properties, which are combined with last two disciplines to determine structural loadings and probabilities of failure.