With offshore wind power set for major expansion in the UK and elsewhere, the choice of foundations will be crucial. What options are on the menu?

By Sam Phipps in Edinburgh

Rotating blades atop giant towers might be the image of wind farms that comes most readily to mind but intense research is under way into developing suitable substructures, or foundations, for turbines at sea.

Water depth, size of turbine and the nature of the seabed – rock, sand or clay, for instance – all determine which type of substructure is used in any given location.

Construction logistics, environmental constraints and magnitude of loads, including extreme events such as ship impact or seismic loading, are also factors for consideration.

As a rule, the foundation must satisfy four main criteria, including fatigue load, strength and dynamics and cost effectiveness per installed megawatt.

To date, the tried and tested methods include gravity, monopile and tripod foundations.

A gravity foundation consists of a large base  – essentially a concrete or steel raft – that rests on or just below the seabed. Such a structure is suitable for relatively shallow water or a rocky seabed, which would be awkward for piling.

MT Højgaard pioneered the use of gravity foundations in Copenhagen Harbour, where 20 2MW turbines have operated since 2001. Gravity foundations are still viewed as the most cost-effective solution in sheltered areas.

Further out to sea, at greater depths, the Danish company has favoured a monopile – a very large, single pile driven deep into the seabed. An example of this type of foundation can be found at the 160MW Horns Rev wind farm 14km out into the North Sea.

MT Højgaard is also involved in several more offshore projects in British waters: Rhyl Flats in Liverpool Bay, Robin Rigg in the Solway Firth and Gunfleet Sands outside the Thames Estuary.

A versatile option, monopiles can also be used in shallow water where the seabed is sand or clay or the layer of rock is thin. At Yttre Stengrund off Sweden, operated by Vattenfall AB, a socket was drilled into the seabed and a “mother” pile slotted in and grouted in place.

For much greater water depths, tripod, or jacket, designs are an option. These are structures below the turbine superstructure, which transfer loads into several foundation supports.

The Beatrice wind farm demonstrator, consisting of two 5MW turbines next to the Beatrice oil field, 25km off the east coast of Scotland, also uses jacket foundations. These were designed by Talisman Energy (UK) Limited, with co-operation from Scottish and Southern Energy.

To date, however, tripods have not been favoured by developers due to cost and technical factors, particularly difficulty of installation in certain geological conditions.

New foundations on the cards

With a range of other structures in the research and development phase, the current range of options may well be increased over coming years.

In the UK, the Carbon Trust has shortlisted seven new foundation designs that it says could potentially transform construction in the burgeoning industry. 

Dr Ramsay Fraser, head of structural integrity management for the oil and gas division at engineering consultancy Atkins, says the company has a joint patent in place with Scotland’s Burntisland Fabrications (BiFab) for a “very cost-effective” jacket structure to support offshore wind turbines.

The new design has evolved from Atkins’ knowledge of offshore structures combined with BiFab’s understanding of fabrication costs, he says.

This has resulted in a structure that meets the performance requirements of strength, fatigue durability and dynamic characteristics at a reduced cost compared with the most recently installed wind turbine substructures.

“We are putting forward projects that would utilise this design maybe a year or 18 months from now,” Fraser said.

“It’s an extremely narrow window that you’re trying to align your design with.” Not only must all the criteria of strength, fatigue load and dynamics be met but cost effectiveness is far from easy to achieve.

In the oil and gas sector the same issues of strength, fatigue and so on apply but the key difference is that once oil starts flowing, the revenue stream generated for the operator is much higher than for a utility company. “So the fourth constraint – [production] cost per barrel of oil – isn’t such a hurdle as it is for installed cost per megawatt,” Fraser says.

The floating option

Tension leg structures use multiple anchors or piles acting in tension, together with steel guys to support the superstructure.

Blue H Technologies, a Dutch company proposing an offshore scheme off Massachusetts, has launched an 80KW submerged deepwater platform (SDP) off the coast of Puglia in Italy to test the technology.

Last June Siemens and StatoilHydro installed the first megawatt-scale floating turbine in 700ft of water about 12km off Karmøy in Norway.

Tony O’Brien, head of offshore civil engineering for Mott MacDonald, says foundation costs are dominated by construction logistics and durations.

“If large offshore plants are delayed due to unforeseen problems, huge costs can be incurred. These problems can often be identified if experienced geotechnical input is used at an early stage to assess geohazards and help develop a robust project risk management strategy,” he explains.

The key, he says, is to identify the optimum foundation design and to link design to construction. Hence experts in materials, structure dynamics and logistics must play a vital role from the outset.

The seven proposals shortlisted by the Carbon Trust include UK, French, US, Dutch and Danish designs, ranging from gravity, floating and monopile to so-called bucket foundations, which are sunk below the seabed. 

Each design will receive up to £100,000 (€115,000; US$157,000) for concept development, engineering analysis, commercial feasibility and technical assistance. Up to three will be built and installed in large-scale demonstration projects before 2012.

It is safe to say, therefore, that considerable resources and expertise are being invested in the future of offshore foundations. 

To respond to this article, please write to:

Sam Phipps: samcphipps@yahoo.co.uk

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Rikki Stancich: rstancich@gmail.com