Soaring offshore wind dimensions accelerate HV cable investments

Offshore cabling specialists are combining higher voltage solutions with improved vessels and onshore industrialization to accommodate larger turbines and longer distances, experts told New Energy Update.

A trend towards higher voltage offshore wind cables has spurred investments in new vessels and onshore facilities. (Image credit: Transmission Investment)

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As offshore wind developers install higher capacity turbines and larger arrays, innovations in subsea cable technology are helping to drive down costs.

Larger wind farms are being constructed further offshore as developers seek to gain economies of scale. Hornsea Two, the world's largest offshore wind farm, is being installed some 90 km from the coast of Yorkshire, England. The 1.4 GW project, developed by Orsted, will use 165 Siemens Gamesa 8 MW wind turbines.

The offshore wind industry will need 46,470 km of subsea cables over the period 2018-2022, some 71% more than in 2013-2017, according to Westwood Global Energy Group’s Subsea Cable Tracker H1 report.

A key trend has been a shift towards higher-voltage cables, from shore to array and between turbines and substations. The offshore wind industry has recently introduced 220 kV cables and is moving towards 275kV cables, more than double the 132kV used on many operational assets.

Higher-voltage cables increase output capability and reduce the number of lines required, lowering line losses, Chris Veal, Managing Director of asset management group Transmission Investment, said.

The 275kV cables can reduce the combined cost of installation, losses and failures by 21% compared with the 132kV base case, according to a recent report by the Rina consultancy (formerly Edif ERA).

Higher-voltage cables can require larger vessel capacities due to wider dimensions. Edif ERA estimates that vessel costs are approximately 10% higher for cables larger than 132kV. A typical cable laying vessel can currently carry about 61 m of extra-large 275kV cable.

Cable specialist Nexans recently ordered a new 10,000-tonne cable-laying vessel, moving up from 7,000 t vessels.

Designed by Skipsteknisk, Nexans’ new Aurora vessel will have a length of 150 m is expected to be operational by Q3 2020. The vessel is designed for "worldwide installation of large volumes of HVDC and HVAC cable systems, even in the most severe weather conditions," Nexans said in a statement.

“We see a clear trend for using purpose-built cable installation vessels rather than just going with standard vessels/barges specifically upgraded for cable installation jobs,” Jan Jansen, senior project manager, electrical engineering, at K2 Management, told New Energy Update.

            Europe installed offshore wind capacity by country
                                                Cumulative, end of 2017

                                                 (Click image to enlarge)

Source: WindEurope

Inter-array cables

Offshore projects have also increased the maximum allowable voltage of inter-array cables from 33kV to 66kV to increase output and reduce losses. Most new projects now use 66kV inter-array cables, according to the UK's Carbon Trust.

“The benefits of transferring to a higher voltage are that less array cabling would be required and this can result in substantial capital cost savings, in terms of both cable purchase and installation,” Jansen said.

Higher-voltage cables allow more turbines to be connected to each string and also reduces the required number of substation transformers.

Using a voltage of 66kV reduces the average total length of inter array cable by around 25-30% for the same power output, Featherstone said.

“The net effect is to reduce costs per unit of energy generated,” he said.

Onshore optimization

Local supply chain buildout has been key to reducing European offshore wind costs and increasing economies of series.

Larger offshore wind farms spread over longer distances typically require factory jointing of power cores for individual subsea cables.

Increased jointing demand has spurred industrialisation projects. In Hartlepool, UK JDR has set up a dedicated team of onshore skilled, qualified jointers to join power cores on an "industrial scale," shifting away from occasional and project specific operations, Jeremy Featherstone, technology manager - HV and renewables at JDR Cable Systems, said.

Cable group Nexans has also looked to optimize onshore activities to increase installation efficiency.

The company has developed a junction frame which allows cables to be partly-installed to the wind farm platform while onshore, reducing offshore installation requirements.

As the U.S. prepares to industrialize its offshore wind sector, researchers have said that full-assembly onshore and single-operation crane vessel installs could achieve the largest cost reductions. The method could be scalable for turbines up to 20 MW, the research team led by the University of Delaware, said.

“The processes in our proposed method are amenable to industrialization of the offshore wind production process, with attendant increase in volume and lowering of cost,” the researchers said.

Fault detection

Supply chain and services innovations are set to slash the operational expenditure (opex) of offshore wind farms in the coming years.

Cable companies are improving monitoring capabilities to help predict faults and optimize maintenance schedules.

For example, the Transmission Investment group now uses Distributed Temperature Sensing (DTS) on some cable projects.

DTS detects issues by sending laser pulses down the export cable fibre core and measuring temperature across the cable.

This technology allows operators to perform condition monitoring of the load/temperature relationship to help prevent cables from overheating, and identify any fibre breaks or hotspots across the cable route.

In another example, acoustic sensor group OptaSense is currently trialling the use of Distributed Acoustic Sensing (DAS), already used in the oil and gas industry, to identify faults at an earlier stage.

This technology monitors vibrations and strains along the entire cable to estimate faults within 10 m accuracy.

More accurate condition monitoring will help inform decisions on whether to undertake predictive or preventative maintenance, a spokesperson for the Carbon Trust said.

“With more and greater capacity turbines, reduction in cable failure can have a significant impact on minimizing operational costs," the spokesperson said.

By Neil Ford