SPT's three-legged wind turbine foundation braced for next stage

The Netherlands’ SPT Offshore is gearing up to demonstrate its three-legged self-installing wind turbine (SIWT) foundation.

Discussions are being held with a Dutch developer and Scotland’s European Offshore Wind Deployment Centre to build a first full-scale unit for testing in UK waters.

The all-in-one “braced monopod” concept is turbine-agnostic. It is designed to make it possible to crane-assemble and commission a turbine’s rotor, nacelle, tower and substructure at port before fixing it to the corner of a cargo barge.

Once at the installation site, the SIWT would be lowered by three strand or chain jacks, and then hybrid gravity-base foundation suction piles would be anchored to the seabed using pre-installed pumps.

The design — one of four shortlisted in 2009 in the UK Carbon Trust’s next-generation foundation competition for the “challenging conditions” presented by UK Round 3 sites — is for use in water depths of 30-60 metres.

There is still a shortage of offshore wind installation vessels, says SPT Offshore chief executive Mark Riemers. “And even though a good number of newbuilds are being built... the trend is mainly towards traditional solutions such as jack-up barges, which are restricted to operating in water depths that are no greater than their legs are long [mostly 25-40 metres].”

Fabrication of an SPT Offshore barge tailored for installing 5-10MW turbine-jacket units is estimated at €50m ($67m) — roughly a quarter of the cost of a large jack-up vessel, which has a structure steel cost for a four-legged jacket of €5.50 per kg, compared with the SIWT’s €3.50/kg. Further savings come through the high-speed transit and installation time expected of the integrated turbine and jacket, which SPT calculates at 72 hours a unit.

“A big plus is that our offshore activity is really limited, so the time offshore is too. The positioning and piling of a jacket or monopile and TP [transition piece], the installation of the tower and turbine — all disappears. This is a lot of offshore activity that you would like to avoid.”

A similar model, a 9,000-tonne self-installing gas platform delivered last year for Centrica’s F3-FA oilfield, in 40 metres of water in the Dutch North Sea, proved the concept could be installed in 1.5-metre swells.

“For this job we had to deal with some seven-metre waves and hostile offshore conditions, and we still managed to install it in just over two days,” Riemers notes. “It was a useful test of the concept at large scale. The issues are all the same: it is all about stability and motions, and the suction-pile technology, of course.”

A full-scale model for depths of 45 metres would weigh about 3,500 tonnes, including 1,000 tonnes of concrete ballast built into the two corner suction piles to reduce tensile loads on the structure. The tripod’s monopile would have a diameter of four to five metres, with the 2.5-metre-wide suction piles braced 35 metres out in each direction.

The SIWT is designed to handle 50-year-storm waves and winds, with dynamic loads “dissipating into the steel” and no observable effect on the grip of the suction-pile foundations in most soil types.

“There has been a great deal of discussion about cyclic loading effects [repeated, regular loading] on suction piles, but from our analysis and experience, we have seen that cyclic loading hardly plays a role,” says Riemers. “Silty soil is the one soil where there may be some effect.”

From SPT Offshore’s study of the seabed geology off the UK and Ireland, he adds, 95% of sites will be “just fine” for the technology. The company has earmarked six ports on the east coast of the UK with “well-suited” fabrication facilities and the eight-metre draught needed to load the turbine on to the barge.

“The transport and installation process is so fast that we think we will need to operate out of three facilities at the same time [to keep up with a tow/installation cycle of three to five days],” Riemers adds.

SPT Offshore is fine-tuning the SIWT’s final design, via its collaboration with the Carbon Trust, with an eye on organising a demonstration project as soon as possible.

“This is not an easy process. There are plenty of developers with space [on a wind farm] for a demonstrator, but it is a case of one more cable and one more worry for them. So it is taking some convincing,” Riemers says.

Darius Snieckus, Bristol

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