Nylon Ropes for FOWT taut and semi-taut moorings… a progress report

At the upcoming MIUG (Mooring Integrity User Group) event on June 17th, Wageningen, The Netherlands, Eduardo Dourado, Principal R&D Engineer, Lankhorst Euronete Portugal, is presenting the latest results from the company's nylon ropes test program for floating offshore wind turbine (FOWT) moorings, in a paper entitled: Progress towards adopting Nylon Ropes in taut and semi-taut moorings of FOWTs.

Lankhorst is a world leader in taut system offshore moorings exceeding 500m water depth based on polyester fiber ropes. For permanent moorings shallower than 500m, nylon ropes present a more viable option for taut leg and semi-taut systems.

Although, within limits, polyester ropes could be modified for shallower moorings by using several ropemaking techniques, this would be at the expense of rope efficiency and could significantly impair fatigue endurance.

In shallow water moorings, the key characteristic required to reduce wave frequency peak tensions is a lower stiffness compared to polyester ropes. Nylon ropes have one additional beneficial characteristic in shallow water mooring, in that the ropes have very low stiffness as loads approach to zero. This helps prevent slack conditions and snap loads in the leeward line(s).

In its research into de-risking the use of nylon ropes in permanent moorings, Lankhorst is looking at ways of managing, and where possible designing out, potential failure modes such as internal abrasion due to cyclic loading, penetration of marine growth reaching the load bearing sub-ropes, external abrasion due to contact with the seabed, external abrasion/cut due to interference from steel components (wire rope in particular), and long-term polymer degradation due to exposure to UV, water (fresh and sea) and temperature.

An essential part of the research is characterization of rope load-elongation behavior in realistic loading conditions, including an understanding of permanent elongation.

Eduardo Dourado, Principal R&D Engineer, will present the results of Lankhorst's efforts to characterize the behavior of its high efficiency nylon ropes, and outline planned on-going studies into failure modes.

“Successful development and manufacture of FOWT fiber ropes is the biggest station keeping challenge being faced by the floating offshore wind industry” says Cesar Del Vecchio, Synthetics Innovation Director- Energy, Lankhorst Euronete Portugal. “Our research indicates that we are well on the way to developing nylon ropes with the necessary performance characteristics to enable permanent moorings in shallow and intermediate water depths, that will be used by the Renewable Energy Industry and also by the Oil & Gas Industry.”

For more information on Lankhorst, FOWT moorings, and high efficiency low stiffness ropes email: cesardelvecchio@lankhorsteuronete.com