Polyamide and mooring

Polyamide ropes: towards an increasingly reliable solution for floating wind turbine mooring lines

Among the options explored for the mooring of future floating wind turbines is the semi-tensioned configuration with polyamide lines to dampen the dynamics transmitted to the float and reduce the footprint on the ground. As the long-term behaviour of this type of synthetic fibre is not well known, its qualification for a time scale corresponding to the operating life of a wind farm is a major challenge. Indeed, polyamide 6 is a material characterised by a complex visco-elasto-plastic behaviour. Its viscous nature leads to increased deformation over time under constant loading: this is called creep. It is necessary to establish whether this deformation is totally or partially reversible to identify the point at which the phenomenon becomes critical. A critical deformation could lead to a relaxation of the tension of the mooring line, which would be compensated by a significant displacement of the float, which would then lead to a risk of damage to the electrical export cable. It is also important to know how the polyamide behaves in situations where the tension varies, such as during a storm.

This is the challenge of the unprecedented study carried out by Laure Civier and her co-authors in the framework of the POLYAMOOR and MONAMOOR projects and which was recently published in the journal Ocean Engineering. A specific test bench was developed specifically to answer these questions. It allows three sub-rope samples to be tested in parallel. Each mooring line sample is immersed in a water-filled tube, put under constant tension by suspended weights, and then monitored for almost two years: a world first. Three levels of tension were tested: one representative of what is expected in normal times and two others that are higher, but compatible with an average tension over several hours during a storm. The results are clear: the cumulative creep is limited, and its speed is stable for the three tension values. This information is crucial for the floating wind turbine industry, as it confirms that the service life of polyamide is compatible with that of a commercial wind farm. This opens the way for more widespread use of this material for mooring lines.

In addition, the same experimentation was carried out on a hydraulic bench, used for shorter-term characterisations, for a period of three hours. The results obtained follow the same trend as the long-term tests. The creep curve developed with the data collected over a year thus shows the validity of extrapolating results from tests lasting only a few hours over several orders of magnitude. This is another very important conclusion for manufacturers in the sector, as the characterisation of their product can now be carried out reliably and quickly.

Read the Civier et al., 2022 article

Photo credit: Onvictori0 / AdobeStock

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