Offshore wind farms allow developers more freedom than their onshore counterparts, less restricted as they are by visual impact and noise concerns, and this has led the sector to push the boundaries of turbine design.

Most offshore developers deploy or are planning for turbines between 2.5 to 5MW, but deeper water developments, such as the UK’s Crown Estate Round 3, raise the possibility of 10MW turbines or bigger.

Two companies - UK-based Clipper Marine, and Norwegian state utility Enova in partnership with turbine developer Sway - are in the early stages of developing 10MW turbines, but does bigger necessarily mean better?

"It’s all about economies of scale," says Nicolas Fichaux, head of policy analysis at industry body the European Wind Energy Association. "The assumption is that it’s cheaper to install one 10MW turbine than two 5MW turbines. It’s been proven that when you increase the size of the turbines you decrease your costs per megawatt significantly."

Sway and Clipper have different approaches, and developers will be watching to see which proves more successful. Sway’s upwind turbine with 145-metre blades will be mounted initially on fixed foundations, anchored to the seabed using a single support and a suction anchor, or on a more traditional jacket foundation - a pylon-like structure within which the base of the mast sits. The construction of the prototype should be complete by 2012 and testing will continue for two to five years after that.

Sway is running a concurrent project focusing on foundation design. This is looking at mounting downwind 5MW turbines on a floating structure that can tilt at angles of up to six degrees, and when the wind direction changes can pivot via an undersea bearing. This floating unit will be loaded with ballast, with the mast supported by a single tension leg and anchored to the seabed by a flexible stay that allows the structure to withstand the fatigue load to the slender foundation. This approach, Sway claims, reduces stresses on the structure, meaning the mast can carry a larger turbine and enabling the whole unit to be deployed in deeper water. By 2015-2017, Sway is hoping to mount 10MW turbines on these floating structures.

"Our first market will be the UK Round 3 developments, in depths of 40 to 50 metres, so we are aiming for that initially," says Michal Forland, Sway’s chief financial officer. "But in the long run we think the floating market will be larger."

Floating turbines are not new. In 2007 Dutch company Blue H installed a large-scale prototype in Puglia, southern Italy, and construction of the first commercial turbine at the site is now underway. And Marine Innovation & Technology has developed Windfloat, a floating platform that can hold three turbines and is being considered for Principle Power projects in Portugal and the US.

It’s an attractive technology for developers as it allows turbines to be deployed in much deeper water than traditional monopile foundations, but whether it can stand the forces generated by a 10MW turbine remains to be seen.

Sway is also working on a gearless direct-drive generator, using technology similar to that currently being developed by both Siemens and GE.This system comprises generators with fewer moving parts than a geared system, similar to the single-stage generators used in the 5MW Areva Multibrid M5000 turbines, 80 of which have been ordered for the Global Tech 1 wind farm off the north German coast.

Conventional design

Clipper is looking at a more conventional design based on its existing 2.5MW Liberty turbine: a 144-metre-diameter machine, fixed to a solid foundation on the seabed, with head-on, rather than downwind blades and a geared generator. The company is planning a UK test for the prototype within two years. It is basing its design on a multi-stage gearbox similar to that used by REPower’s 5MW turbine so that existing installation vessels can be used, substantially reducing costs.

But regardless of the technology deployed, both ventures are taking a step into the unknown and must await the results from prototype testing. Fatigue, and therefore reliability, is one of the main challenges both companies face, as the larger the turbine, the more wind it captures and the greater the stress on the structure.

"This is pioneering stuff," says Feargal Brennan, head of offshore, process and energy engineering at Cranfield University in the UK. "I believe 10MW turbines are right on the limit of our knowledge; they may even prove to be over the limit. We may find that they work for several years and then start to develop problems. Will 10MW turbines still be working after 10, 15, 20 years of operation?"

Sway and Clipper are confident they will be.

How different turbine foundations work

Foundations are the critical yet unseen part of a wind farm’s construction, and as turbine deployment moves further offshore, the water gets deeper and the technological challenges mount.

There are a number of foundation types. The "monopile" comprises piles 4 metres or more in diameter and 20-35 metres long, drilled or driven into the seabed. A specialist jack-up barge is usually required for installation. Figures presented at EWEC 2010 indicate that monopile foundations represented 88%, of all offshore turbine capacity in 2009.

An alternative is gravity-based structures, currently used on most offshore projects, which consist of a large base made from concrete or steel resting on the seabed.

Towards floating foundations

There are variations such as "suction caisson" foundations - essentially large upturned buckets that are lowered to the seabed and have the water in them pumped out, which "sucks" the foundation to its final position. These are easier to install - if the caisson can be floated to site, little more than a pump is needed. In deeper water, a multiple-footed option- a tetra- or quadrapod (three- or four-footed) pile - can be used.

But the deeper the water gets, the more attractive floating foundations become. Several ways to tether a floating turbine are being considered: ballast stabilised (where the turbine is kept upright by ballast in the base), buoyancy stabilised (where the turbine floats on the surface) and mooring line stabilised (where it is anchored by three lines).

Getting the foundations wrong can have serious impacts - earlier this year wind farm owners including Dong Energy and Vattenfall investigated a potential design fault that was causing some monopile foundations to slip and has affected 10 windfarms to date. The problem is not thought to be terminal - the first successful remedying action was completed this June in the Netherlands on the 108MW OWEZ offshore farm, owned by Vattenfall and Royal Dutch Shell.

However, whether the techniques used there will work elsewhere remains to be seen and the industry could be facing costly repairs.