Driving forward
Jim Hodder, senior project manager at PMSS, discusses the key issues surrounding the delivery of subsea cable systems for UK Round Three offshore wind
Subsea cable systems are often underestimated in the design and installation of offshore wind farm projects, yet the majority of insurance claims from projects to date relate to subsea cables. Once installed, subsea cables represent a ‘single point of failure’ in the connection of the offshore wind farms to the grid. This has been evident recently where cable failures in offshore wind farms have resulted in the loss of several months (GWhrs) of generation, before repairs have taken place.
1. Cable supply
The reliability of a system will be directly affected by the quality of the subsea cable design and manufacture. The design life of the cable is assured through a qualification and type approval testing process. Manufacture quality is assured through a factory acceptance test process. The International Council on Large Electrical Systems publish failure statistics (number of failures per 100km per year) for high voltage (HV) systems.
Good contractor selection process, critique of the manufacture and installation process, and the application of a robust quality assurance process will increase the likelihood of good cable supply.
2. Cable installation
There has been a poor history of cable installation so far for offshore wind projects. Cable damage and/or subsequent cable problems or failures have occurred in the majority of projects. Even large interconnector projects such as Nor-Ned (Norway to Netherlands) and Basslink (Australia to Tasmania) have had delays due to cable installation related issues.
These issues can be mitigated by the following measures:
• Good cable route engineering, to ensure that the route avoids known key risk areas (such as ship anchorages and explosives dumping grounds), whilst not adding significantly to route length.
• Thorough ground investigation campaign(s) to inform the supply and installation specifications.
• Production of appropriate burial specifications based on an assessment of those external risks and ground conditions.
• Selection of contractors with good track record and sound financial standing.
• Ensuring that the proposed installation spread (vessel, handling and burial equipment) is ‘fit for purpose’ for the project in hand.
• Linking the cable supply and installation contracts to other construction contracts through robust interface management.
• Use of experienced client representatives during cable installation.
3. Technology changes
Export systems to date have been designed at either 33kV or 132kV using AC technology, for export cable distances of between 20 and 70km from the offshore substation to point of connection onshore. Some Round Three wind farms are expected to be located further offshore, so it is expected that high-voltage direct current (HVDC) technology will be necessary for those export systems, where high-voltage alternating current (HVAC) technology is uneconomic.
Voltage source converter (VSC), HVDC-based systems using cross-linked polyethylene (XLPE) cable technology will most likely be the best technical option, since this approach contains many advantages over the alternative Current Source Converter (CSC), including the potential of integrating large scale offshore wind developments into an offshore grid for export.
4. Supply chain
Whilst there is plenty of competition for the supply of 33kV (MV) cables (there are around eight European suppliers at present), there is very limited supply at 132kV or above (HV), almost all of which is concentrated in Europe or Japan. Production capacity in Europe is presently limited to around 800km of HVAC cables and a similar amount in HVDC. Lead times are long for HV cables (sometimes in excess of two years) and prices at present are high. This situation has been exacerbated by the demand from the oil and gas sector, due to recent high oil prices.
The outlook is not expected to change at HVAC or HVDC until more capacity or other suppliers come on stream. New players in the market would not be active for some time, due to the need to qualify and type test new cable products.
5. New arrangements for offshore transmission connection
BERR and Ofgem have set up a process to license offshore transmission owners, who will be responsible for connecting UK Round Three wind farms to the onshore GB transmission system. These companies are known in the sector as Offshore Transmission Owners (OFTOs) and they will ultimately own and maintain the export cable infrastructure for offshore wind projects.
There are a number of risks and unknowns with the OFTO process at the present time, largely related to the regulatory and charging regime, but also the tender process to be adopted. These issues need to be clarified before the system goes active in April 2009.
So, in summary, the subsea cable system exports all the value from generation plant to the point of sale, but at present its importance and technical complexity may be underestimated in the sector. There are many issues that wind farm developers face in the development and installation of subsea cable systems associated with the proposed expansion of UK offshore wind over the next 12 years. Many of these are project risks, which need to be managed at a project level, through proven risk management methods. Some are peculiar to the subsea cable industry and there is a need to engage with the present supply chain to reduce lead times, reduce prices of cable and also to ensure installation is carried out to the required specification. Others are more strategic, such as the need to coordinate the requirements of proposed wind farm export with the impending OFTO regime, both onshore and offshore.
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