Two essential criteria for mooring: reliability and robustness
Floating ORE systems have several advantages: higher wind and wave resources, technically unlimited water depth, better adaptability to soil types… However, they must limit their dynamics for a number of reasons:
- efficiency (e.g.: reduction in wind turbine performance if the floating structure movements are too severe),
- safety (e.g. disruption of vessel traffic),
- robustness and technical and economic viability (e.g. pulling out the export electric cable, overdesigning costs).
Developing reliable and robust mooring lines is therefore essential. To do this, two criteria must be taken into account: limiting the amplitude of the forces and resisting maximum loads over the life of a farm.
Learning from offshore oil and gas and adapting to the ORE sector
It is difficult to transpose the learning from offshore oil and gas experience to the ORE sector as it stands, as certain constraints are radically different: limitation of the footprint on the ground, strong dynamics of the floating structures in the swell, reduction of the footprint on the public maritime domain… Floating ORE systems are deployed in relatively shallow water depths (50 to 150 m) where the wind and wave resource allows for a better load factor. However, the shallower the depth, the more complex the design of the mooring. The mooring proposed for floating systems, particularly wind turbines, therefore differ significantly from those used in the offshore oil and gas sector. Standards and design tools need to be adapted and validated. Feedback from test sites or pilot farms obtained in parallel with specific studies on mooring will help reduce uncertainties in predicting their lifespan and optimise the solutions selected.
Characterisation of synthetic mooring
Synthetic mooring allow dynamic damping to be introduced into the system, thanks in particular to a high elongation capacity, similar to that of nylon. These semi-tensioned devices are particularly suitable for water depths between 60 and 150 m. Nylon has the advantage of being less expensive than conventional mooring chain, which contributes to reducing the system’s LCOE and limiting the overall length and therefore the footprint on the ground. France Energies Marines and its partners led two R&D projects on this theme:
- The first focused the fine characterisation of the dynamic and long-term behaviour of nylon mooring lines (POLYAMOOR project). This project has demonstrated that nylon can have a fatigue life for permanent mooring of the order of 25 years. A law representative of the complex visco-elasto-plastic behaviour of this material and a methodology for accelerated life assessment by self-heating were also proposed.
- The second aims to develop means of monitoring the mechanical behaviour of nylon anchor lines in service. It includes a deployment at sea, the development of dedicated sensors and methodology for monitoring these mooring lines as well as an installation procedure including pre-tensioning (MONAMOOR project).
In-service monitoring of mooring lines
The design of offshore systems is necessarily based on meteorological-oceanic statistics. It therefore involves uncertainties to which are added the integration of new materials. It is therefore essential to have methods and means to continuously monitor the state of the mooring components, to warn in case of high risks of damage and to anticipate repair or replacement operations (MHM-EMR, SUBSEE 4D and DIONYSOS projects).
Influence of biofouling on mooring
Biofouling can strongly affect the weight and the hydromechanical behaviour of the floating structure and mooring lines, especially synthetic ones. This is particularly true for sites at shallow water depths (< 150 m) where most of the water column contains living organisms. An collaborative project completed in 2022 aimed to develop protocols for measuring and characterising biofouling, taking into account the specificities of sites and components (ABIOP+ project).
Photo credit: Wanfahmy / AdobeStock
Consideration of biofouling using quantification protocols useful for engineering
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Accounting for biofouling through established protocols of quantification
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Monitoring of polyamide mooring lines
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Mooring Systems and Offshore Monitoring Research Engineer
Mooring Systems and Marine Operations Research Engineer