Siemens' 6MW put to the test

In the shallows of the northern Thames Estuary, off the coast of southeast England, two of the mightiest wind turbines yet devised by the industry are being prepared for switch-on.

The machines — a pair of Siemens’ 6MW SWT-6.0-120s — are the first designed by the German company specifically for the extreme rigours of operation in the rough seas of northwest Europe.

Standing 148 metres tall and topped by 120-metre-diameter rotors, the giant turbines are at the centre of a pioneering pilot project being run by Denmark’s Dong Energy alongside its three-year-old Gunfleet Sands wind farm, a UK Round 2 development currently flowing enough electricity for 125,000 homes.

Great store is being put by these next-generation Siemens units, which are on track to be generating this spring. Pitched as a workhorse model for the vast Round 3 zone developments planned off the UK — a 300GW programme that GL Garrad Hassan expects to cost in the region of £100bn ($157bn) — the machines were prototype-tested for over a year at Høvsøre, Denmark, before being erected for Gunfleet Sands 3.

“Even though you make prototypes to verify your engineering and weed out any issues that are not identifiable during the design phase, it is something very different having turbines at sea,” explains Siemens chief technology officer Henrik Stiesdal.

“Getting them offshore, it becomes a ‘real life’ scenario: getting them installed in hostile conditions, getting them hooked up to offshore infrastructure.

“The first of our pilot series now [at Gunfleet Sands] includes a number of upgrades and improvements that were the result of building these prototypes as well as of the testing programme on land.”

Michael Hannibal, Siemens’ head of offshore wind for Europe, the Middle East and Africa, calls the newly installed machines “true game-changers” for offshore wind.

Installation of the two prototypes, which were mounted on conventional Bladt monopile foundations in 15 metres of water by A2Sea’s Sea Installerjack-up crane vessel in its fir

st assignment, took place in January in an “entirely successful fast-paced” one-day operation, says Stiesdal.

“For a company to go out with a new vessel and do the job they did in the time they did — from the management side we were extremely happy with the speed and efficiency with which the turbines were installed.

“You can plan as much as you want, but in the end you also need the right people. In this case, their qualities were clearly demonstrated.”

Siemens’ 6MW direct-drive concept — dubbed “turbina sapiens” for its intelligent operations system that self-corrects to adjust to changing wind loads and conditions — eliminates two thirds of the conventional geared drivetrain arrangement and 50% of the moving parts to create a streamlined design with a tower-top weight of under 350 tonnes.

“Gunfleet Sands is not the most severe of locations,” Stiesdal adds.

“These turbines have been in more challenging sites onshore but this will give us a chance to study them closely offshore before we put them in really harsh locations in the North Sea.”

Beyond the testing of the turbines at Gunfleet Sands, the pilot — which Dong views as “a big step” towards cutting the cost of energy from offshore wind — has been carrying out tests into monopile scour protection, use of remotely operated underwater vehicles for cable installation, and a new cost-reducing export-line protection system.

Set to be trialled for two years in winds averaging 8.5 metres per second, the 6MW Gunfleet Sands units are a bellwether for a souped-up 154-metre-diameter rotor version of the turbine currently being put through its paces onshore at Denmark’s Østerild test centre.

Flying Siemens’ record-breaking 75-metre B75 Quantum blades, one SWT-6.0-154 will be able to churn out 25GWh a year offshore — enough to supply 6,000 households.

Thirty-five of the new turbines were recently chosen for Dong for a maiden commercial outing for its €1bn ($1.36bn) 210MW Westermost Rough wind farm in the UK North Sea, under a 300-machine framework deal with Siemens inked last year.

The Danish company is also sizing up the turbine for its “small” UK Round 3 sites — the 1GW Njord and 2GW Heron projects.

The Gunfleet Sands machines, according to Stiesdal, will contribute to “closing the loop” on testing of the 154-metre-diameter rotor version. “They will give us a very good picture of the ‘transfer function’ between the two — onshore the 120 behaves like this, offshore like that, so based on its onshore behaviour the 154 will likely behave like that offshore: it will help us calibrate the bigger turbine so we can carry out any final fine-tuning before sending it out to sea.”