With the imminent deployment of three pilot wind farms in the Mediterranean Sea and one in the French Atlantic waters, and the soon to arrive call for tenders for commercial farms, the French FOWT (Floating Offshore Wind Turbine) industry is on track for a significant contribution to the French ambition to reach 32% of energy production based on renewable sources by 2030.
France Energies Marines (FEM) and its members identified surface-bottom connection items (mooring lines, dynamic electric cables) as critical components in marine renewable floating systems. This is a particular concern in shallow (<150m) water depth.
The present investigations will be done in the frame of three ongoing projects: MHM-EMR, OMDYN2 and POLYAMOOR so generating input data for such models. In particular, dynamic cable properties but also a dedicated numerical models of nylon rope behavior may be used.
Description du poste
Design of Semi-taut mooring system for a 10MW FOWT with a reliable power cable configuration
When designing FOWT, the floater and the turbine will tend to be standard. However, mooring system and export cable configuration will be adapted to each specific location. The global FOWT numerical model has to be adapted to a specific site and associated meteo-ocean data. FEM already designed a simple chain catenary mooring system for a 10MW wind turbine. It was qualified against 50 years storm conditions (Ultimate Limit State) for site conditions similar to Groix-Belle-Ile French pilot project.
The next step is to develop an innovative mooring design, based on semi-taut philosophy. It is identified that for water depth ranging between 50 and 150m semi-taut mooring design would reduce mooring cost and environmental impact.
The objective of this task is to perform a mooring design exercise and propose a semi-taut mooring arrangement which would be qualified against, typically 50-year storm conditions in preserving the export cable integrity.
• Selection of mooring components: mooring chains, synthetic mooring lines, buoyancy modules
• Modelling of the components: while chains and buoyancy modules use standard modelling, the synthetic mooring lines follow complex behavior laws which are under development within FEM projects.
• Proposition of an export cable configuration (lazy-wave, steep-wave…) satisfying the conditions imposed by the mooring pattern;
• Simulation and validation of mooring design against ULS load cases.
Impact study of the creeping of synthetic lines:
When permanent synthetic mooring lines are used, it is expected that a permanent elongation of the lines occurs. If the elongation is too long, the high related floater offsets may induce power cable breaking. Obviously, this shall be evaluated and anticipated. The objective of this task is to:
• Define appropriate initial mean tension;
• Study the maximum expected permanent-elongation on synthetic mooring lines and the related maximum floater offsets.
• Propose a power cable configuration satisfying these constraints and the presence of biofouling
• Perform sensitivity simulations to evaluate the impact on the mooring system functionality.
• Investigate if a monitoring of the movements of the platform can provide sufficient information to assess the creeping of the mooring line.
• Assess the number of re-tensioning operations that would be required during the life of the FOWT.