'New species' of floating turbine

A new species of ultra-stable floating wind turbine foundation is taking shape in Germany, with engineers on track to moor a full-scale unit in the Baltic Sea next year.

Dresden consultancy Gicon’s tension leg platform (TLP) design is a latticework of braces and flotation cylinders that sits half-submerged in the water, like a semi-submersible, fixed to the seabed by stiff tendons and four diagonally splayed mooring lines anchored in place by micropiles.

A scale version of the SOF (Schwimmendes Offshore Fundament, German for “floating offshore foundation”) has just come through extensive wind and wave trials in the Netherlands. At the Marin research and development centre in Wageningen, the unit was put through hell and high water, with testing taking it from benign offshore conditions up to Beaufort 12, the nautical classification for hurricane-force winds with speeds of more than 118km/h or 33 metres per second (m/s) and waves cresting at 14 metres or higher. The SOF sailed through.

“The tests at Marin showed that the tower of an SOF would have the same, or even less movements and accelerations, than if it was fixed to a monopile foundation,” says Gicon chief executive Jochen Großmann.

“From the transition piece to the hub [the movements] are equal, but then you have the 20 metres from the water level down to the seabed, and here the tension on the tendons and the bracing [from the current and swell] creates less movement than it would on a monopile.”

Under the conditions simulated at Marin, the foundation created a stable base for the rotor. The unit rode out wind speeds equal to 36m/s and 10-metre waves. Its 100kW turbine saw accelerations — sudden, potentially damaging jumps in rotor speed caused by increased wind and wave loads — of less than 0.5m/s, only reaching cut-out in winds of 24m/s.

The Marin trials built on wave tests run in a basin at HSVA in Hamburg, Germany, in 2010 on a 1:25 scale model of the SOF, which suggested that the concept could handle 20-metre swells. The next stage is to install a full-scale version in the Baltic next year in 20-30 metres of water.

“We are confident, based on the model tests in Hamburg and the wind and wave tests carried out at Marin, that a full-scale prototype would work in the Baltic Sea,” says Großmann, left.

Success with the pilot — for which Gicon has partnered an unnamed developer — would give the stamp of approval to a design devised for the widest range of water depths.

The concept could challenge monopiles in the shallows but is also calculated to be up to the task of operating in water depths down to 700 metres, where the additional diagonal mooring spread would work with the TLP tendons to provide “the stability of a fixed structure foundation” on a wide variety of seabeds.

“We can install in water depths of less than 20 metres, like those of the monopile; we can compete with jacket depths of 40 or 50 metres; and we have calculated that it could be used down to around 700 metres,” says Großmann.

The flagship SOF will feature a 2-3MW turbine atop a 70-metre tower. Construction is slated to begin this year in northeast Germany, with steel fabrication at P+S Werften in Stralsund and final assembly at Nordic Yards in Rostock.

The unit — which uses a modularised design fine-tuned before the Marin test to whittle down the number of components, to improve “ease and speed” of construction — will be built quayside, complete with turbine, and towed to its operation site. Gicon is in “final discussions” with a shortlist of turbine suppliers.

“The design was modified to increase the stability, but we don’t see it adding any complexity to the concept,” says Großmann. “Because it is modular, it is in many ways less complex than some of the other designs being developed by our competitors, particularly when it comes to construction of the unit.”

Gicon has underwritten the first seven years of development on the SOF on its own. Where other pioneering floating wind turbine designs, such as Principle Power’s WindFloat, have lately had the sponsorship of a mentoring utility or — in the case of Statoil’s Hywind — the deep pockets of an oil major, Gicon has self-funded its concept through consultancy work such as ecological monitoring schemes at Germany’s Baltic 1 and Alpha Ventus wind farms.

“There are very few companies developing such constructions alone,” notes Großmann. “That we have been able to do this, we feel it’s something unique, not least because we are an engineering consultancy putting the money we make on consulting contracts into R&D. This is the ethos of Gicon.”

The company is already looking beyond the Baltic Sea project to a North Sea pilot in 2015 using a 5-6MW turbine “with partners either from Germany, France, Spain and/or the US”.