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By locating floating wind farms in deeper water areas further from the coast (50 to 300 m deep), it will be possible to operate offshore sites where the available wind potential is up to four times higher than for fixed turbines. This is accompanied by a scramble to optimize the ability of offshore farms in order to withstand severe marine conditions and the economic performance of this blue electricity.
Researchers in mechanic engineering at ENSTA Bretagne's IRDL (Dupuy de Lôme Research Institute) lab, are participating in research programs initiated by France Energies Marines as part of future investments "France 2030", with multiple goals such as specifying the magnitude of the dynamic stresses submerged systems are subjected to or predicting their in-service durability. The studies focus on the anchor lines that maintain the wind turbine's position at sea, the dynamic cables that conduct the electricity to the shore and the strength of masts subjected to the repeated impacts of breaking waves.
Characterize the performance of twisted nylon fibers
Measure and monitor their ability to withstand severe conditions
Characterize the ability to withstand the impacts of breaking waves
France Energies Marines*: "Anchoring systems used for floating wind turbines differ significantly from those used in the oil & gas sector due to elevated dynamics in shallow waters. The challenge is to be able to maintain a floating foundation in an extreme environment by limiting the forces transmitted by the mooring lines as well as limiting the maximum offset of the foundation to which the export cable is connected. One envisaged solution is to use nylon mooring lines capable of absorbing the involved forces, but whose behavior is highly non-linear and poorly understood in cases of long service life."
This research was carried out as part of Yoan Chevillote's thesis defended in 2020: "Characterization of the long-term mechanical behavior and the durability of polyamide mooring ropes for floating wind turbines (MRE)".
This is the subject of Laure Civier's thesis started at the end of 2020: "Monitoring of polyamide mooring lines for offshore wind turbines".
France Energies Marines*: "Dynamic submarine cables, required to export the electricity generated by all floating ORE systems, are critical components subject to different and much more varied constraints than their static counterparts. Their in-service monitoring is therefore of paramount importance. Some technologies are used for monitoring subsea cables (DTS, DAS, Partial Discharge, etc.), but they have limitations and may not be the most suitable for in-operation monitoring of dynamic cables."
As part of a larger national program aimed at sizing and monitoring submerged power cables, a consortium has been formed (DYNAMO project, 2020-2022) and is focusing on techniques for monitoring dynamic cables, with the aim of eventually proposing a set of tools and methods for the preventive maintenance of these cables.
The research project has two main scientific goals:
ENSTA Bretagne-IRDL is involved in identifying and prioritizing the failure modes of dynamic cables, induced by mechanical or thermal loads.
Mechanical tests are carried out to assess the suitability of cable fault detection techniques. These involve fatigue tests on complete cables subjected to a 3-point bending load. Various measurement techniques have been used to account for the occurrence and propagation of defects in cables (based on acoustic emission, fiber optic measurement and reflectometry). In addition, fracture surface analysis work is to be carried out to identify the failure scenarios of the cables tested.
This work is part of Issam BENCHEIKH's post-doctorate on "Failure modes and instrumentation of dynamic submerged cables" started in February 2021, over a period of 24 months.
France Energies Marines* : "When designing offshore wind turbines for a specific site, the industry has to analyze the Ultimate Limit State (ULS) of the structure, i.e. the maximum expected response that the offshore wind turbines will experience over its lifetime. In the assessment of the ULS, the effects of energetic steep or breaking waves (hereinafter ESBW) are thought to be responsible for considerable uncertainties. As well documented in the oil and gas or fixed offshore wind turbines literature, ESBW may excite the first structural modes (the so-called ringing in presence of steep non-breaking waves), cause damages due to vertical water excursion (run-up) or submersion of the platform (green water) or local deteriorations due to slamming loads. Numerical modelling of a full-scale floating offshore wind turbines (FOWT) even showed that the blade tips could hit the water in presence of realistic ESBW."
* The background information on the industrial goals is taken from the France Energies Marines (FEM) website: france-energies-marines.org
Further information on the industrial goals and partners of these projects: