Session: Mooring

Paper Number: 165094

165094 - Modelling of Nylon 6 Mooring Lines for Offshore Floating Wind Turbines 

Abstract: 

Floating wind platforms developed in shallow waters (water depths between 70m and 100m) will be located in exposed areas where waves and wind are severe. The challenge for the floating systems is to find a robust and suitable floater and mooring solution insuring limited wave dynamic for long-term applications. Shallow water platforms can benefit from the Oil&Gas sector experience, but catenary mooring system is not optimized for floating wind turbines (FWT). The restoring force of a catenary solution is vertical, driven by the weight of the chain, whereas a FWT will be subjected to large horizontal motions due the turbine thrust. One proposed solution is a semi-taut mooring system with synthetic rope materials. Their high specific strength added to their flexibility will allow smaller line length, weight and line tension. Synthetic mooring lines will ease the handling and improve safety during installation. Polyamide 6 (or Nylon 6) is of high interest thanks to its competitive price, high elongation to failure (up to 20%) and low stiffness. Its visco-elasto-plastic behaviour is a source of damping, which is anticipated to allow floater structural optimisation.

Polyamide 6 ropes are characterized by a complex behaviour due  both to their fiber structure and properties, and to their hierarchical organization and construction. Synthetic fibers are characterized by a time-dependent load-elongation behaviour and a non-linear viscoelastic and viscoplastic behaviour (Chailleux and Davies, 2005). Currently in the offshore industry, recommended practices (DNV, ABS, NI) propose using a constant “dynamic stiffness”, calculated for each dynamic simulation under a stationary sea state. The correct estimation of the “dynamic stiffness” is crucial for the model to be accurate,  but the recommended practices assume the dynamic stiffness to be only dependent on the mean tension applied to the line. However, observations made both on sea trials and on experimental data showed that dynamic stiffness is not only dependent on the mean stiffness but also on the loading period (Ribault et al., 2024) and amplitude (Thuilliez et al., 2023). Finally, the time-dependent viscous behaviour of synthetic mooring lines is not represented by a constant dynamic stiffness model.

FEM’s POLYAMOOR, MONAMOOR and BAMOS (on-going) projects propose the development of a 1D rheological behaviour law (the POLYAMOOR law) based on an experimental characterization of the behaviour of a polyamide 6 sub-rope described in Civier et al. paper (Civier et al, 2024). The law is implemented in the in-house specific version of the design software Deeplines. The validation of the law has been performed both with laboratory experiments and with rare data of stress-strain behaviour of PA6 mooring lines obtained with at-sea trials also part of the MONAMOOR project (Ribault et al, 2024).

The law consists in 11 material parameters. Hence, a sensitivity analysis has been performed during the on-going BAMOS project to better understand and ease the use of the law. The use of optimization was also investigated to improve the predictions of the law with an optimized set of parameters. Furthermore, the mooring lines deployed during the at-sea trials have been tested so that a new identification of the material parameters of the POLYAMOOR law could be realized. This new identification allows to investigate the evolution of the parameters with a representative history of loading and to quantify its impact on the mechanical response of the rope.  These new investigations aimed at improving the accessibility and application of the law by the industry.

Presenting Author: Laure Civier France Energies Marines

Presenting Author Biography: I am a doctor and a research engineer at France Energies Marines. My PhD was part of the MONAMOOR FEM project aiming at developing our knowledge of polyamide 6 mooring lines for floating offshore wind turbines. The core of my work was to understand and model the contributions of their hierarchical multi-scale architecture, and their complex non-linear elasto-visco-plastic behaviour, to the dissipation and failure mechanisms of the rope, in order to predict lifetime in service. The project also included the development of new innovative sensors for the monitoring of these lines.

I am now a research engineer in innovative mooring lines for floating wind turbines. Part of my work is on the BAMOS project at FEM that aims at investigating the damage mechanisms of polyamide ropes and improving the modelling approaches.

Authors:

Laure Civier France Energies Marines
Romain Ribault France Energies Marines
Juliette Laurent France Energies Marines
Guillaume Damblans France Energies Marines
Jean-Sébastien Verjut France Energies Marines
Guilhem Bles ENSTA
Yann Marco ENSTA