Deepwater challenge faces foundation providers

If it is true that any business needs a sound foundation then that is certainly the case for the offshore wind industry. But soon the sector will need to seek new foundations, because the ones it has at the moment will no longer work.

So far, the standard for offshore wind turbine foundations has been the monopile. And while the design has had its ups and downs, there is little doubt that the basic concept is sound. Up to a point, that is.
The monopile design has very definite limitations. For a start, it requires a soft seabed. And it only works up to a certain depth of water, with light-to-medium turbines.

“The limitation is water depth and weight,” said Mark Young, head of department for cleaner energy and utilities UK at DNV KEMA. “If you have a very heavy turbine, that will favour a non-monopile, a more expensive solution.”

Currently, he points out, these limitations are tending to drive turbine design, causing engineers to strive for turbines that are as light as possible while maximising rotor diameters.

“Every kilogram you can reduce top-head mass by saves you money in the foundation structure, in the vessel,” Mr Young explained. “The installation vessel does not need to be so large; you can install foundations more quickly.”

However, monopiles will only take the industry so far. Turbines delivering more than about 5 megawatts (MW) will almost certainly weigh too much for monopiles and, while the monopile design is currently used in water depths of up to 20-25m, its usefulness will not stretch much further. Mr Young believes the theoretical maximum may be about 30-40m, although this has yet to be proven in practice.

The current depth range is fine for the North Sea, where most offshore wind development has taken place until now. The North Sea’s gently sloping, muddy seabed has an average depth of 95m and sandbanks such as the 17,600km² Dogger Bank rise to up to 15m.

But the North Sea is fairly atypical. In the Mediterranean, for example, the coastline drops away steeply to an average depth of around 1,500m and, even in the North Sea, sub-30m depths are in increasingly short supply.

This means new types of foundation will undoubtedly be needed, for instance to cater for the nine Round 3 offshore wind zones that the Crown Estate in the UK auctioned off in January 2010.
According to a foundation consortium made up of Arup, Costain and Hochtief, water depths for the Round 3 windfarms will range from 15m to 75m, with 84 per cent of sites over 30m, ruling out monopiles for all but the shallowest locations.

“Of course they will build the nearer and shallower projects first, so I think there will still be some monopiles deployed in Round 3,” said Mr Young. “But when they build out the later projects in Round 3, and probably even in the German offshore rounds, they will have to be looking at jacket or some of these other innovative-type structures.”

Right now, the best bet for deepwater installations seems to be the jacket, which is essentially a steel lattice tower, or similar tripod design. These have already been used in a number of projects. The issue with them is how to reduce the amount of steel used in construction, in order to bring down the costs.

“The foundation typology is very much up in the air,” Mr Young explained. “It is a space where a lot of entrepreneurial activity is going on. In the near-term, the jackets, tripod systems and even monopiles are still going to carry the large majority of the wind turbines going offshore, but a more high-volume approach would be valuable, something that is less expensive.”

As he noted, Repower and other manufacturers are looking at using jacket structures with smaller members that are less expensive by the metre, and easier to assemble. Repower has, for example, a prototype substructure for its 5MW wind turbine which is suitable for water depths of up to 50m, and is significantly lighter than a standard jacket, using smaller members. Furthermore, it employs cast nodes that can be assembled using circumferential welding instead of more complicated fillet-weld processing.

Mr Young points out that this means it could be welded using automated techniques common to pipeline construction and other fields. “Building one, two or three might be more expensive,” he said. “But building three or four or five hundred could get quite inexpensive, and you can really start to achieve a continuous process, more like a high-volume automotive-type assembly process.”
Elsewhere, the Maritime Offshore Group from Bremen, Germany, together with IMS Ingenieursgesellschaft, from Hamburg, have developed a foundation called the Hexabase that is lighter than a three-leg jacket as less material is used.
The Hexabase is made of mass-produced, welded and coated pipes from continuous pipe construction. This reduces the cost of materials compared to rolled offshore pipes, and guarantees quicker fabrication.
Meanwhile, Keystone Engineering has demonstrated a ‘twisted jacket’ design that was one of four finalists from the UK Carbon Trust’s Offshore Wind Accelerator foundations competition. The ‘twisted jacket’ uses less steel than a conventional jacket and is easier to fabricate, using fewer components. Installation costs will also be lower because more units can be fitted onto an installation barge due to its small footprint.

Beyond these (and other) novel jacket designs, some experts see a role for gravity-based foundations at depths of more than 25m. Gravity bases are basically large ballast masses, traditionally made of concrete, which can be floated to where they are needed and then allowed to sink to the bottom.
Until now, gravity-based structures have traditionally been used in shallow water, but a British consortium led by independent engineering and environmental consultant Gifford has proposed a design that would work in water depths of up to 45m. The same team has also devised a transportation and installation system using submersible barges that are towed out to position before the offshore structures, which typically weigh more than 3,000 tonnes, are settled into place.

Arup, Costain and Hochtief also favour the use of gravity-based foundations for deep offshore projects, touting a conical design the consortium says can cater for depths of up to 60m and turbines of up to 8MW. Finally, Seatower, a Norwegian company, has a hollow 6,000-7,000 tonne design that it claims can be towed into position even in severe weather and can be installed without dredging or undersea work.

That gives plenty of options for deepwater turbine anchorage. But at some point the offshore wind industry is likely to encounter depths or conditions that even jacket or gravity-based foundations cannot serve. When that happens, there are a number of floating options currently on the drawing board, and at least one – the 2.3MW Siemens and StatoilHydro HyWind concept – in the water.
However, for now the key question for the industry is what to do when the monopile design reaches its functional limits. What seems likely is that no single new foundation type will take over.

“I think it is going to be horses for courses: whichever gives you the best ‘bang for your buck’,” said Fraser McLachlan, chief executive of the renewable energy underwriting agency GCube, which insured the installation of HyWind. “Wherever you are putting your turbine will probably dictate what turbine foundation you use. You will just have to build that into your overall project cost.”

However, foundation costs will undoubtedly be significant. DNV’s Mark Young said a rough rule of thumb for jacket foundations is to allow half a million euros per megawatt of turbine power, with other designs coming in at around plus or minus 20 per cent of that amount, although the figures are site dependent. Beyond that, there is also the issue of getting hold of vessels that can handle these new foundation designs.

“The stuff that has been used up till now has been largely standard jack-up barges with piling equipment on their decks that people would use for shallow-water jetty installations,” Mr McLachlan explained.

“When you go to tripods you probably are looking at a different type of vessel because you have got a much bigger lift that you have to do to get the tripod embedded into the site where the turbine is going to be.

So you are talking deeper water, you are talking much larger vessels, much bigger lifting capacity. There are going to be fewer of those vessels around and they are going to be more expensive.”