Developers planning the latest offshore wind power installations are using skills learnt by the oil and gas industry. Ed Owen reports on their application.
The Third Round of offshore wind farm developments proposed by the Crown Estate and Department for Energy and Climate Change in January has stimulated a rush to develop sites in the North and Irish Seas.
While the first two rounds of offshore wind were situated in shallow water up to 25m deep and relatively close to shore, the third round is in much deeper water of at least 40m, and is up to 200km from shore.
This is an order of magnitude of complexity beyond what has gone before. Ramboll project and technology director, offshore wind Henrik Carstens says: “If you look at the oil and gas industry worldwide, there have been 7,000-8,000 platforms built in the past 50 years. If we assume 40GW [of offshore wind generation] is to be installed in Europe by 2020, with an average of 5MW per turbine, then you are already looking at 8,000 structures in the next 10 years.”
Carstens has been instrumental in moving technology from the oil and gas sector into renewables and is considered to be the leading light in the field.
“When offshore wind started at the turn of the millennium, we basically took oil and gas knowledge into shallow water projects. We know about load from an 8m wave on a monopile, and about the strength of soil, and so on,” he says. “But now we think differently, and one area we need to focus on is fabrication of, say, 100 identical structures and their installation,” he says.
“One area we need to focus on is fabrication of, say, 100 identical structures and their installation”
Henrik Carstens Ramboll
Putting turbines in the water relies on what has up to now been the workhorse for the offshore wind industry - the driven monopile - but this is unsuitable for depths greater than 25m.
Unfortunately, no convenient alternative yet exists. The increased cost for the third round lies with the foundations, and several types could work, but there is no obvious winner yet.
Worse, the supply chain is not yet equipped to roll out new foundations with production-line quality and quantity. But potential solutions do exist, with much of the work already done by the offshore oil and gas industry.
Rather than a single monopile Burntisland Fabrications has for many years built lattice jacket structures supported by bored piles in the seabed at depths up to 80m for the oil and gas industry. The fabricator was involved in the pilot EU-funded scheme to test the concept for offshore wind that was run off the Beatrice oil platform and installed in 2006. The Upvind project proved the concept is technically workable.
30 turbine trial
Burntisland has since moved to a 30-turbine trial at Ormonde in the Irish Sea and launched a joint venture with Atkins to investigate process ideas to slash costs based on the existing technology.
Atkins oil and gas director Ramsay Fraser says: “We are looking at how to reduce costs for capital expenditure and simplify production.”
Their first collaboration has produced a much cheaper transition piece, cutting costs by around 40%. The transition piece is usually a highly complex and expensive piece of steel that straddles the foundation, linking to the turbine mast. The new designs allow the piece to be cast and mass produced, giving huge cost savings.
But adapting the transition piece is just the beginning. Bespoke oil and gas platforms must now be adapted for mass production for the renewables sector. Client Scottish and Southern Energy (SSE) has bought a 15% stake in Burntisland, and pledged orders of at least 50 and possibly up to 130 jackets and transition pieces per year between 2014 and 2026.
“For us, the utility must place orders - it’s all about scale and costs will come down,” says Burntisland managing director John Robertson.
The oil and gas industry has relied on expensive bespoke structures for rigs, accommodation, substations and utilities. But this model breaks down for offshore wind - the costs are simply too high.
Grontmij chief wind energy consultant Per Volund is involved in managing the Carbon Trust’s accelerator scheme to investigate new foundation designs for increased depths.
“As part of the Swedish Kriegers Flak windfarm we tested five new types of foundation in 35m depth, including gravity, jacket and monopile solutions. At 35m we found costs for each of these technologies to be the same,” Volund says.
The Carbon Trust is running a £30M project looking at cost-saving ideas for the third round of offshore wind and says its “accelerator programme” aims to deliver double the savings Carstens envisages. “We think installation costs for turbines 50km to 200km offshore could be 30% to 45% of the total cost, and that includes the costs of the foundation,” says Carbon Trust senior technology acceleration manager Ben Sykes. “We are hoping to make savings of some 20% here.”
Carbon Trust contenders
Seven leading designs are short-listed as part of a Carbon Trust competition to find wind turbine foundation designs suitable for deep sea water conditions (30m to 60m). These are:
- Gifford/BMT/Freyssinet (UK / France) - Large concrete gravity base structure, transported to site by a submersible transport and installation barge;
- Glosten Associates (US) - Floating wind turbine foundation based on tension leg platform (TLP) technology, comprising a buoyant hull, tendons, and novel anchors;
- Suction Pile Technology / Wood Group (Netherlands/UK) - Asymmetric suction bucket tripod foundation which can be transported from the quayside with the turbine already installed;
- Keystone Engineering (US) - Inward battered guide structure with three supporting legs angled around a central pile in a twisted jacket approach;
- Offshore Wind Power Systems of Europe (US) - Titan Platform floated into position then its three legs are lowered to the sea floor, similar to a jack-up drilling rig;
- Ballast Nedam (NL) - Drilled concrete monopile;
- MBD Offshore (DK) - Suction bucket monopile, primary material is steel, using buoyant installation and delivery.