View of wind turbine including its foundation set in concrete

Foundations bear the brunt of offshore wind's growing stature

With the new breed of offshore wind turbines weighing in excess of 1000 tonnes the challenge is to develop innovative and cost effective foundations to support these massive structures as E&T discovers.

The granting of licenses for the nine round three offshore zones has set the clock ticking for the engineering community to solve several pressing challenges to make the schemes a success.

One of these challenges is the sheer size of the structures that will be built. From the root of their foundations to the top of the nacelle the wind turbines will stretch a stunning 250m three times the height of the statue of Liberty. This presents some unique difficulties not least in designing and constructing a cost effective foundation that can support the massive weight in a 1.5MW turbine the nacelle weighs 56 tonnes and the blade assembly 36 tonnes, so the proposed 5 - 7MW offshore turbines will weigh considerably more than that.

Almost all of the wind farms that operate or are in construction around the coast of the UK use monopole foundations. The prime advantage of these is that they are a tried and tested technology that has been used for some time in marine construction, both by the offshore wind industry and before them the oil and gas sector. At their most basic these are simple steel tubes that are hammered into the seabed.

However as we move into deeper and deeper waters these structures have reached the end of their useful life. Monopiles cannot be used in water that is deeper than 30m with turbines that are rated at MW or greater.

In addition to this the diameter are limited to 6m which means that they are uneconomical beyond 20m water depth for the current 5MW turbines and will continue to be so unless there is a significant reduction in the power to weight ration of the devices.

In mind of these constraints the wind industry has been considering its options. Ideas such as concrete gravity based structures, adaptations of monopiles such as tripods and tripiles and jacket structures as used successfully by the oil and gas industries. If the water depth exceeds 60m then floating structures are being considered.

The Carbon Trust performed some analysis that was published back in 2008 that suggested that it was possible to reduce the estimated 75bn cost of offshore wind by almost 20 per cent with two parallel strategies. The first was to choose optimal sites, which has been addressed by the Crown Estates in the round 3 sites, and the second was to perform RD&D to reduce the cost of the technology.

'Normally when you install new technology you find that it gets cheaper because of the experience effect,' Phil de Villiers, innovations manager at the Carbon trust, says. 'When we started to install offshore wind farms in the UK there was a nice trend of the cost coming down. Unfortunately after 2006 the costs of offshore wind farms started to increase due to a number of different factors, such as a rise in commodity prices, bottlenecks in the supply chain as suppliers were trying to meet the market in onshore rather than offshore as well as the complexity of the sites.'

Given that scenario there is a clear challenge to reduce the cost of offshore wind farms if we are going to meet our CO2 targets. There are generally considered to be three levers that you can use to improve the economics of wind farm: CAPEX the cost of manufacturing and installing the wind farms; OPEX the operating and maintenance costs; and Yield the output of the wind farm, ensuring that you get a high load factor.

In October 2008 the Carbon Trust set up the Offshore Wind Accelerator, with the objective to reduce the cost of offshore wind by ten per cent. The 30m RD&D programme is being delivered in collaboration with five major offshore wind developers Airtricity, Dong Energy, RWE, Scottish Power Renewables and Statoil.

The Carbon Trust carried out a six-month due diligence process to work out in what elements of offshore wind the cost best could be reduced. From the 50 technology areas that were initially considered they identified four extremely promising areas to focus on; foundations, access, electrical connections and wake effects.

Of these areas the work on foundations is considered to be of prime importance. 'The reason that we were so interested in foundations is that it is almost half the capital costs,' de Villiers adds. 'In the summer of 2009 we ran a competition to bring in novel foundation designs from outside the current industry. We received over 100 entries from players, many of these are established players from the oil and gas sectors, this has led to seven projects being short listed by our partners.'

From those seven contenders there are four projects that are considered to be the most likely to offer an advantage and these are a twisted jacket, a floating turbine, a self installing turbine and a concrete base.

The first, Keystone, is based on a twisted jacket which uses significantly less steel than a typical monopole design and therefore reduces the cost. 'This is a structure that has come out of the oil and gas industry, the concept has already been installed in the shallow Gulf of Mexico for an Exxon Mobil production platform,' de Villiers says. 'It had Hurricane Katrina pass over it so it certainly passed its storm test. It uses considerably less steel that a standard jacket and has significant benefits around the installation methodology that drive the costs down compared to a standard jacket.'

Next up is the Glosten design, a floating turbine that allows deployment in deeper water than can be achieved with existing monopiles. 'This has a long way to go before it is deployed, there is a lot of subscale testing required,' de Villiers explains. 'This is designed for depths that are greater than 60m. Interestingly this is based on TLP (Tension Leg Platform) technology from oil and gas.'

The novelty of this solution is in the anchoring system that for obvious commercial reasons the company are keeping under wraps. 'It's a highly sensitive IP that offers potentially a much cheaper and faster solution for installing anchors for a TLP,' de Villiers says.

The third design is collaboration between Suction Pile Technology and The Wood Group and is an intriguing self-installing turbine. 'This introduces an interesting aspect into wind turbine foundations in that it is asymmetric, which obviously has some challenges given that wind can blow from any direction,' de Villiers says. 'But it has a huge advantage because it allows you to mount the turbine and the blades on the structure before you leave the shore. It incorporates suction buckets which mean that you don't have to drill into the seabed to attach the turbine.

'You can mass produce the turbine at the dock, the nacelle and blades assembled in benign conditions and then the whole assembly is dragged out to sea using conventional tug boats rather than special vessels. It is effectively sunk and suction buckets attach it to the seabed. It is a pretty radical change of technology to what is used today and so it is not going to be rolled out soon.'

The final design is the Gifford design. The team's design centres on a concrete foundation rather than steel, with a large base allowing the structure to self-stabilise under forces of gravity in water depths of 20 to 45m. The team has also devised a cost-effective transportation and installation system using submersible barges, avoiding the use of heavy crane barges and offshore jack-up platforms. The offshore structures which typically weigh over 3,000 tonnes are then settled into place.

The system proposed allows for production line onshore construction to achieve substantial reductions in the cost of deep water wind farms, an important consideration in the design competition. Next year up to three designs will be taken on to the second phase of large-scale demonstration.

The Carbon Trust expect two or three of the devices to be demonstrated by the partners over the next two or three years. 'We think that the short listed concepts can reduce the cost of the foundations by 20% so that is a material improvement, ' de Villiers concludes.

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