Dynamic cable: crucial role and cause for concern
Any offshore power generation system requires a cable network to export power to shore. Fixed systems such as fixed wind turbines or some tidal turbine systems use a static cable laid or buried on the seabed. The cable called in this case static is subject to few mechanical loadings. However, for floating systems, it is necessary to have a link between the floating structure and the main export cable on the seabed which is the dynamic cable also called umbilical.
This component is subject to high mechanical loadings due to floating structure movements, swell and current, as well as the risk of wear due to friction or scraping. This is why its design is different from that of a static cable with, in particular, a double armour. The high voltages that pass through these cables (> 66 kV) combined with the high mechanical loads make it a complex and expensive multi-material component. As no redundancy is envisaged, its robustness and longevity are crucial elements for the viability of a farm.
Feedback from bottom-fixed wind farms in Northern Europe indicates that 80% of legal remedies are associated with malfunctions in the export cable of offshore wind turbines. Given the dynamic aspect of cables in floating systems, the risk of production loss from a farm is considered high by insurers. This can be a source of concern for investors and penalise the sector if a proper control of this component is not quickly demonstrated.
The challenge of dynamic configuration
The dynamic cable must be able to withstand the maximum offsets of the floating structure under the load of centennial current, swell and wind. This implies deploying an overlength capable of absorbing these decreases or increases in distance at the ends. This over-length, which includes buoyancy modules, allows the umbilical to remain far enough from the surface to maintain safe navigation, but also from the bottom to avoid premature wear by abrasion. The dynamic cable also has to withstand changes in its environment over its 20 to 25 years of service, including the development of biofouling which can multiply its weight by three. The influence of this phenomenon on the umbilical is being studied in two collaborative R&D projects: one on the thermal effect (ABIOP+ project) and the other on the hydromechanical behaviour (OMDYN2 project).
Coupled behaviour over the long term
Dynamic cables are confronted with mechanical loadings dominated by bending due to the moderate floating structure size which induces strong dynamics, but also due to the shallow depth which implies less mechanical tension. This is a notable difference from oil umbilicals, which are dominated by the mechanical tension imposed by the suspended weight at great depths (> 1000 m). The dynamic cable of ORE is therefore subject to significant and regular bending loadings, coupled with high electrical and thermal loadings. It is therefore necessary to identify whether the decoupled loads used in current design methods are satisfactory and to refine our understanding of the specific degradation modes induced by this coupling (OMDYN2 project).
Technology strategies and solutions for in-service monitoring
The multi-physical coupling and the use of many different materials make the dynamic cable a complex object to observe. Indeed, the thick steel armour limits external measurements by constituting an almost impassable barrier. However, given the stakes surrounding this component, it is essential to set up a real monitoring strategy and to develop technological solutions to ensure its in-service monitoring (DYNAMO project). Today, interrogation techniques are primarily concerned with electrical functionality tests. The identification of secondary degradations could make it possible to prevent and act accordingly before the complete loss of functionality. If the use of the internal optical fibre in the umbilical is already explored, an effort should be maintained to develop innovative sensors that could meet the need, as degradation modes are not yet completely mastered.
Photo credit: France Energies Marines
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