IN DEPTH: Floating shape-changer

The first floating wind turbines braving the waves off various coasts around the world owe a clear design debt to the huge, ultra-stable structures of the offshore oil industry.

But in the drive to reduce the cost of offshore wind energy below the magic £100 ($167) per MWh mark, engineers are now looking to break away from reworking moored oil platforms to create lightweight giants for the richer gusts offshore.

TetraFloat is one of these. Recent wave-tank testing of the pyramidic concept, which is backed by seedcorn funding from the UK’s Department of Energy and Climate Change (Decc), suggests an initial cost of energy (CoE) of about £120/MWh, which could be further cut when scaling up for 10MW turbines.

“Offshore wind is still relatively immature — there is a great deal of development in virtually every aspect of the field,” says Nottingham University professor Seamus Garvey, who is heading a consortium of British companies developing TetraFloat.

“The methods part of technology — how you design, build, deploy — is contributing a large part to the current CoE [around £140/MWh] and this figure will come down in the future through learning by doing, as the cost of materials goes up with demand.

“This is where we need to depart from the oil and gas industry, where a project is costed chiefly on material and then multiplied by a factor based on scale, environment and so on.

“This is at the root of the TetraFloat concept — very low materials costs without sacrificing stability. We already compete quite favourably with monopiles, which can weigh upward of 1,200 tonnes and more when you look the XL designs for deeper water depths.”

The 13% CoE savings over existing fixed-bottom foundations promised by TetraFloat is magnified when compared to other floating concepts — a 10MW TetraFloat would tip the scales at around 3,000 tonnes all-in. By comparison, Principle Power’s 2MW Windfloat off Portugal weighs some 2,000 tonnes; Statoil’s first-generation 2.3MW Hywind well over 5,000 tonnes; and the 2MW Fukushima Forward off Japan is heavier still.

“You look at these platforms — and to be sure they are each first of a type — and you see that scaling them up to 10MW, if they stuck to the same design, they would become very heavy, and so very expensive structures indeed,” Garvey says.

Computer modelling shows that TetraFloat would slash costs across the board. Support structure, construction and installation would be cut by 25-50% compared to seabed-fixed turbines, and O&M by 20%. Capital expenditure for a unit would be just over £3m, with annual operating spend under £90,000.

Being a lightweight poses challenges, however, when it comes to stability in the water. TetraFloat works against this by having a broadly spread base — the triangular hull will measure 120 metres by 140 metres front to back in a 10MW model, with a hub height of 100 metres and a below-water depth of 40 metres.

“There are two things a floating platform has to do,” says Garvey. “One, it has be strong enough to react to structural loads: the TetraFloat has a pretty wide spread of geometry for the troublesomely heavy horizontal downwind loads and, key to it all, it has no component put under the stress of pure bending — like a tower — because the loads are distributed through the whole structure as direct compression or extension.

“The other is to stay stable and here it is about keeping the water plane area as small as you can, that is as little breaking the surface as possible.”

TetraFloat counterbalances for the fact that — like every other floater — it has a centre of gravity above the water line by fitting its three legs with heave plates many metres below the waves — think lifting a dinner plate vertically out of a sink full of water.

“The TetraFloat is light and so it should bob around like a cork but the broad footprint and the heave plates both provide buoyancy and grip the water to effectively add weight to the structure as it floats, making it very stable,” explains Garvey.

Fully assembled with turbine quayside and towed out by a spread of ocean-going tugs in a “folded flat” position, the whole structure would be erected “like a barn frame” and then moored to the seabed by a single anchor point with a two-chain harness. This way it can free-yaw, like a weathervane, into the prevailing wind, using the sea as a near-zero-friction plane bearing.

As Garvey notes: “At sea, unlike on land where the wind is always shifting, you can make sure that the big forces always come from the same direction relative to the structure.

“When most people first encounter the TetraFloat concept, their first reaction is that it will not yaw quickly enough. In short, it does. The actual rates of yaw required for very large wind turbines are very low anyway but mainly it is because even if the passive weathervaning is not quite quick enough, there are four other things that you can do.”

The main one involves a sensor-driven system that would winch the lengths of harness chain out of the front of the platform to steer the turbine nose to be more accurately aligned to face the wind. As an array, the turbines would be laid out “like a Chinese chequers board” with the units moving in unison while keeping a safe distance from one another.

The Decc technology development grant awarded last year has covered trials of a one-metre-high version in the wave tank at the Ocean Systems Test Laboratory at the UK’s Cranfield University, as well as a short series of tests on a four-metre axis-height structure at the Haslar marine R&D centre in Portsmouth, southern England. 

“The stability of the platform is verified now. When the heave-plate buoys are set correctly, the structure has very low stiffness over a small range of pitch and roll angles, but that stiffness rises sharply when it is needed to prevent capsize,” says Garvey.

Using money from the EU’s Marinet scheme, TetraFloat will next undertake a programme of tests at Ifremer’s wave-basin in Brest, France, early in November, with follow-on plans to have a 20-metre-tall model afloat off the Scottish Orkney Islands by next spring.

“The main priority right now is to get everything into good shape for the testing at Ifremer and subsequently at sea. After completion of these, we will be in very good shape to bid for prototype demonstration contracts,” Garvey adds.

The TetraFloat consortium expects to have a full-scale version of the platform in the North Sea or the Baltic Sea kitted out with a “dummy” turbine by mid-2016 to overwinter.

After that, it would be equipped with a real-life 6MW machine for a battery of commercialisation testing the following year.

The small flotilla of floater prototypes now turning offshore will probably one day look rather archaic, given their oil industry design DNA. For Garvey, floating wind turbine designers need to view it in the light of Newton’s maxim about seeing further by standing on the shoulders of giants.

“There are certainly parallels with what happened in oil and gas. In that industry, floating platforms took over in many places and came in cheaper. And certainly, floating platforms are the only option in very deep water,” states Garvey. “I hope that we would take all the wisdom of the offshore oil industry but broaden our horizons with it.

“As time progresses, I believe [floating wind turbines] will penetrate into shallower water and ultimately floaters such as TetraFloat will compete and win against seabed-fixed supports — with the only limitation here being that the water must be deep enough.

“The big question mark is how we bring down costs still faster because the margins for offshore wind are so slender and floating turbines have some way to go to prove themselves to be the best and most economic option for developments potentially even as shallow as 30 metres of water, as more and more in this wind-power industry are beginning to feel.”

Two years ago, when Garvey was first knocking on doors with his TetraFloat idea, he “could hardly get an appointment”, he relates. “The attitude toward the concept since has changed radically. Momentum in floating wind is gathering.”