The target to install 25GW of offshore wind power by 2020 has been discussed many times in the press and at wind energy conferences. Ideas are coming to light about how to make this possible, looking at supply-chain issues and installation methods, for example.
Recently, Dr Gordon Edge, the British Wind Energy Association’s (BWEA’s) chief economist, warned that the projected 20 per cent reduction in offshore costs to about £2.5m per installed megawatt was dependent on the development of a dedicated offshore turbine supply chain. He added that costs will only fall if the sector can bring an end to the scenario whereby developers compete for parts with onshore wind farms and for installation vessels with the offshore oil-and-gas industry.
This highlights the need to reduce costs and the requirement for a competitive supply chain. What is not clear is how this can be achieved. One wind turbine generator (WTG) manufacturer has been quoted as saying: ‘The biggest benefits the industry could award itself would be the ability to build economic foundations at a water depth of more than 30m.’
Clearly, piles are limited to water depths of less than 30m or thereabouts and therefore other solutions are required for deeper water. However, many of the proposed alternative solutions require complex, costly structural joints between steel tubes, typically seen in jacket, tetrapod and tripod designs. In addition, the wave loading on these structures is severe and requires very lowly stressed designs that use more material and lead to heavier, more costly structures that are difficult to handle. In fact, cost reduction for these types of foundations seems quite limited using current technology since cost is linked to material content and manufacturing complexity.
Turning to the installation process, all the current methods require a minimum of two —and often more — stages: piling followed by offshore assembly of the tower, nacelle and blades. The pre-assembly of blades and nacelle has been used, but this still requires a two-stage installation process. Even more important are the needs for specialised piling and heavy-lift vessels. It was reported recently that it took almost four months to install 25 mono-pile foundations for the Rhyl Flats wind farm off the coast of North Wales. This required the use of a specialist vessel, HLV Svanen, with delays reported because of bad weather. If this is typical, this approach adds approximately £350,000 to each offshore WTG based on prices for vessel daily rental of £75,000 (DTI Study of the Costs of Offshore Wind Generation, 2007).
The use of such specialised vessels will always lead to high costs as, even in a market where they are readily available, the cost of building such vessels is at least £100m, which needs to be recovered by their owners through the rental fees. There is another cost that could be missed. This two-stage build process takes a long time — up to two years. In the case of Rhyl Flats, the engineering works started in early 2008 with foundations installed from April to July. However, the turbines were not installed until April through June 2009. During this period, capital was tied up in foundations, onshore works, cables and WTGs. This means the wind farm owners are already paying interest on the loans. The cost of this delay is compounded by the fact that electricity is not produced during this build period, so loss of production means loss of income.
However, there is another way. Imagine a wind farm that could be erected in one summer, with no need for specialised vessels, using a single-stage process. Such an approach would reduce installation costs and bring offshore wind farms online much faster, speeding up the investment work.
This vision is the driver for Xanthus Energy’s SeaBreeze: Fixed system, a self-installing concrete gravity base foundation structure designed so that the whole system, including nacelle and blades, is assembled and tested in shore, then towed out to site using its own buoyancy by means of readily available tugs and installed without the requirement for specialist or heavy-lift vessels in a single ballast operation.
This eliminates the cost of specialist vessels and the high-risk offshore assembly process that also uses higher-cost manpower. It also allows a wind farm to be installed much faster, potentially in one season. One WTG manufacturer said that it would put the WTG through a 500-hour test, perhaps not at full load, before towing it away from the shore, which would help with reducing costly offshore work if components needed changing or repairing during the settling-down period.
The concept of using self buoyancy to move and install offshore structures is not new. In the 1970s and 1980s, the 656,000-tonne Condeep structures were built and installed using the same principle and Ocean Resource’s SeaNova and SeaProducer buoy structures were installed in the same way more recently. In all these cases, basic tugs have been used to tow the structures to site and seawater has been used to ballast them, rather than expensive rock or sand.
Another cost saving can be made by using a two-bladed WTG. Nordic Windpower has developed a two-bladed teeter hub low-weight and low-cost WTG. Its 1MW onshore N1000 WTG costs around $2m (£1.2m) installed. Dr Charles Gamble, chief technical officer for Nordic Windpower, predicts that an offshore WTG using the same technology could achieve a 20 per cent reduction in the WTG cost.
An independent assessment of SeaBreeze: Fixed based on a 5MW WTG scale suggests that the savings from the foundation alone would be in the region of £1m per turbine or £150m for a modest-size wind farm of 150 WTGs. Using a financial model with a 12 per cent cost of capital and incorporating the lower-cost two-bladed WTG combined with a single-step installation process using SeaBreeze: Fixed foundations, the saving on the cost of electricity was computed as €18/MWh. This brings the cost of offshore wind power to the same level as gas and coal, when considering CO2 cost and a 9m/s mean wind speed, which is typical for offshore locations.
Gamble has spoken about the need for support from the government for a UK-based offshore wind industry and all the jobs that it would create.
As we have seen, the solution lies in new thinking and new methods, with real cost savings. However, unless the funding is there to support new UK suppliers, industry will miss out on this major opportunity.
Dr Lewis Lack is the managing director of UK offshore wind specialist Xanthus Energy