Floating foundations may have a buoyant future

WORLDWIDE: A poll conducted by renewable energy consultancy GL Garrad Hassan at the Husum 2012 Wind Energy trade fair in Germany found that 62% of participants believed floating wind turbines would take the lead from fixed-foundation installations in the offshore wind industry within 20 years.

Onshore assembly… Floating turbines, like this semi-submerged model, can be assembled in harbour or near shore and towed into position (pic: Principle Power)
Onshore assembly… Floating turbines, like this semi-submerged model, can be assembled in harbour or near shore and towed into position (pic: Principle Power)

This was a slightly surprising vote of confidence in a floating future for offshore wind because the number of plans and proposals for floating platforms stands in stark contrast to the number that have actually been built. Only a handful of working pilot projects currently exist — and most of those are scaled-down prototypes.

According to the Global Wind Energy Council, 5.4GW of offshore wind power had been installed by the end of 2012. By 2020, according to the more optimistic predictions, this figure could exceed 40GW. Only a tiny fraction of that will be supplied by floating turbines. If floating foundations are to overtake fixed models by 2032, they have a huge amount of ground to make up.

Fewer restrictions

However, floating platforms offer significant benefits that could make this a feasible prospect. They open the way to far-shore, deep-water projects with good wind conditions and fewer restrictions in terms of visual impact. They could also allow turbines to be assembled in harbour or near shore and then towed into position, removing the need for expensive jack-up vessels.

More importantly perhaps, floating foundations are the only real option for two of the world's great industrial powers, the US and Japan. Neither country has the advantage of sitting on a sizeable continental shelf, such as northern Europe enjoys, where there are plenty of sites that can be served by turbines mounted on conventional monopile foundations in relatively shallow water. But three fifths of the US's offshore wind resources and four fifths of Japan's are in deep water. Floating foundations are a must if those countries are to exploit offshore wind.

The US has shown little enthusiasm for offshore wind to date and although there are a number of floating projects under development it seems unlikely the country will be a driving force in the sector. The situation is very different in Japan, where the Fukushima nuclear disaster of March 2011 forced a total rethink of the country's energy generation policy.

Various Japanese companies were working on floating projects before the nuclear meltdown but they were speculative in conception and aimed at export markets. The powerful Japanese fishing and maritime logistics industries were not interested in sharing ocean space with wind power. Fukushima changed all that and the Japanese government is now investing heavily in deep water offshore projects, backing a number of floating foundation developments. This is not to say that Europe is steering clear of the field, but the serious money and technological research is currently in Japan.

Floating types

There are three main types of floating foundation: spar, tension leg platform (TLP) and semi-submersible. The spar is the most advanced in terms of development, having been successfully tested over a number of years by Statoil of Norway on its Hywind project.

The principles of the spar design are simple enough: a huge steel cylinder floats due to the large amount of air at the top of the structure but stays upright thanks to the weight of ballast at the bottom. It is secured to the seabed by mooring lines.

Sited ten kilometres off the coast of Norway in water depths of 200 metres and supporting a 2.3MW Siemens turbine, Hywind was installed in September 2009. The 8.3-metre diameter cylinder (six metres at the water line), constructed from 1,500 tonnes of steel and containing 3,600 tonnes of rock and water ballast, reaches 100 metres below the surface.

The main advantage of the spar design over other floating platforms is its small cross-section at the water's surface, which makes it less sensitive to wave motion. Hywind has endured wave heights of up to 11 metres, tilting no more than three degrees from the vertical and taking 20-30 seconds to sway from one extreme to another. The main disadvantage is the cost. The structure requires roughly five times as much steel as a standard monopile.

Adapted from a design developed for the oil industry, the TLP is a buoyant platform held underwater by stiff vertical tethers that can be secured to a large concrete ring on the seabed, or by individual piles or suction anchors. The Netherlands-based Blue H group tested a three-quarter scale prototype in Italian waters in 2007-09, but the most promising project currently under development is probably Glosten Associates' PelaStar.

A 1:50 scale model recently completed a three-week test in the offshore basin at the Maritime Research Institute Netherlands (Marin) and a full-scale prototype is being built by UK construction firm Harland and Wolff in Belfast in partnership with Alstom, which is supplying its 6MW Haliade turbine. It is scheduled to be deployed at the grid-connected wave hub test facility off the coast of Cornwall, UK, in early 2015.

Although TLPs offer a good degree of stability, the installation of the tethers often requires significant — and invariably expensive — seabed preparation. They are also limited to deep water areas free from wide fluctuation in tides and currents.

Semi-submersibles appear to be the most attractive option for floating wind power projects judging by the number now in development. Their principal advantage is the ease with which they can be installed, although they are seriously challenged in terms of sway, pitch and rolling. Clear leader in the field is Japan's Marubeni Corporation-led consortium, which has already installed a 2MW downwind turbine on a four-column semi-submersible platform off the coast of Fukushima in the first phase of the Fukushima floating offshore wind farm demonstration project. Phase 2, which is scheduled for commission in 2015, will see a 7MW Mitsubishi hydraulic turbine mounted on a three-column semi-submersible platform also built by Mitsubishi. Another 7MW turbine will be tested on a spar floater built by Japan Marine United. Ultimately, the site could be home to 132 turbines on floating foundations.

The future of floating offshore wind power rests to a large extent on how the foundations and turbines at the Fukushima project stand up to the region's notoriously challenging weather. They might not have to deal with an earthquake and tsunami over the next few years, but they will have to cope with the high winds and heavy seas generated by typhoons. If they pass that test the optimism of the respondents to the GL Garrad Hassan poll may well be justified.

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