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 caused by 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.
NYMOOR will provide knowledge on the mechanical properties, failure mechanisms and long-term behaviour 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.
During the first year 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. Three types of tests have been performed at SINTEF Ocean and SINTEF Industry, namely creep tests, tensile strength and stiffness tests, and dynamic stiffness tests.
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. Creep is the elongation, or strain, that occurs when the fibres slowly stretch under a constant load. The recovery behaviour when the loads is removed is also investigated. Different ambient temperatures and different loads were considered to assess the time-temperature-load dependency. The analysis of test data provided already interesting observations and conclusions, namely:
• RelationsThe relationship between load level, load duration, temperature and the creep has been identified.
• Creep is temperature sensitive and it the creep rate increases with higher temperatures.
• Nylon fibres recover part of the original length when unloaded. The recovery strain versus time was identified, and it the recovery rate increases with the temperature.
Tensile tests were performed to assess the material strength, while stiffness tests aim at identifying quasi-static stiffness and dynamic stiffness. Post processing of the results leads to the following conclusions:
• Mechanical properties of nylon yarn used to produce nylon mooring ropes are significantly affected by water temperature in the range of 4 to 25 °C. Stiffness and strength increase with reduced temperature.
• Nylon yarn stiffness is nonlinear and it increases with increased strain up to about 14% strain.
Quasi-static and dynamic stiffness tests have been performed during the second year to characterize the yarn stiffness properties at shorter timescales than the responses investigated in the creep experiments described above. Full tests had an approximate duration 58 hours to perform a full stiffness characterization like that done on the sub-rope level. The test procedure follows a complex sequence of quasi-static and dynamic loads. The test data is currently under analysis.
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. At the request of the project, Bridon InternationalBexco performed additional break load tests during the second year. The new test data is under analysis.
The project also established a reference mooring design for the floating wind turbine case study to be used within the project. The tension characteristics and time series obtained during the mooring analyses were used as input for the lab testing of nylon materials and for reliability studies within WP4. The mooring designs themselves will be base cases for further studies during the second part of the project.
The work within WP4, namely on reliability of nylon mooring lines, started during the second year. Based on the laboratory test data on nylon yarns and sub-ropes, relevant probabilistic models for the tension capacity of nylon ropes were established as a function of temperature conditions. Probabilistic models for the extreme tension were identified based on the available case study. Initial calculations of failure probability associated with the Ultimate Limit State (i.e. tension overload) have been performed. Probabilistic characterization of limit states related to strength degradation has been initiated, with focus on the Fatigue Limit State.
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.
