Session: 09-02-04 Wind Energy: Moorings II

Submission Number: 157112

Mechanical Properties of Nylon Yarn for Use in Wind Turbine Moorings at Various Water Temperatures 

The elasticity and flexibility of nylon mooring ropes offer many advantages, including a potential to reduce dynamic loads and by that fatigue damage and extreme loads in mooring elements. Thus, application of nylon mooring ropes has the potential reduce the need of heavy chains. Limited availability of steel chain for moorings has been identified as a constrain to the required development of offshore wind energy. In addition, the mentioned unique properties of nylon may open new possibilities in terms of wind park layouts with shorter mooring lines and shared anchors.

However, there is very limited experience with use of nylon ropes in permanent mooring of floating structures. To ensure their integrity during a lifetime of operation in wind and waves, it is important to study immediate and long-term stiffness, strength, and elongation of nylon ropes in varying sea temperatures.

Nylon ropes for moorings are made of high tenacity PA6 multifilament yarn. These fibres hold a high tensile strength to weight ratio and are known to have good abrasion resistance. However, they have complex and non-linear mechanical properties that are not fully investigated.

Mechanical testing of mooring ropes is expensive and time consuming due to their large dimensions and high strength, and limited access to relevant test equipment. Thus, when feasible, it will be beneficial to perform extensive testing and parameter studies on the nylon yarn used in rope production. This includes studying the effect of different water temperatures on stiffness and strength of nylon yarn. Then, mathematical models based on comparisons of sub-rope and yarn data will be applied to estimate equivalent mechanical properties of nylon mooring ropes.

This work is part of the NYMOOR project and represents important input in testing of two working hypothesis: Mechanical properties of ropes can be established based on limited sub-rope testing, extensive yarn testing and mathematical/numerical methods (H1). Ambient temperature and humidity significantly affect the properties and lifetime of nylon ropes (H2).

This paper presents a new test set-up and procedure for tensile testing of yarn in a water tank with varying temperatures. It discusses the importance of sample grips and sample end preparations to avoid premature and progressive yarn failure. Finally, results from tensile testing of two different batches of PA6 multifilament yarn submerged in water temperatures of 4, 15 and 25 °C water are given. Both (quasi-static) stiffness and strength properties are presented based on multiple tests replicates.

Results showed that mechanical properties of PA6 yarn are affected by water temperature. Yarn strength increased for colder temperatures, from an average of ~125 N at 25 °C to ~142 N at 4 °C (confirmed numbers will be included in paper). Yarn stiffness at small to intermediate strain was less affected by changes in water temperature.

Presenting Author: Heidi Moe Føre SINTEF Ocean

Presenting Author Biography: Heidi Moe Føre is a Senior Research Scientist at SINTEF Ocean in Norway, where she has been working since 2003. She is a Naval Architect and holds a PhD in Structural Engineering. Most of her research is within structural integrity and response of flexible polymer structures in the sea. This includes fish farming structures, fishing gear and moorings.