Designing to withstand Arctic conditions

FINLAND: Much is at stake for Finland's first commercial offshore wind farm, the €120-million 40MW Tahkoluoto project in the Gulf of Bothnia, in the north of the Baltic Sea, west of Finland.

Frozen… The 2.3MW pilot turbine (right) is sited off Finland, testing the arctic weather conditions on the turbine and steel cassion foundations (left) ahead of installing the country’s first commercial offshore project (top)

Scheduled for commissioning in the third quarter of 2017, the project will be custom-designed and built to withstand the local icy conditions.

The consortium behind the project is led by Finnish wind power developer and owner Suomen Hyotytuuli (SHO). As well as operating several onshore projects in Finland, SHO has been running an offshore pilot in Pori Tahkoluoto - the site of the new project — since 2010, with a Siemens 2.3MW turbine on a new type of steel caisson foundation.

The Tahkoluoto project will comprise ten 4MW Siemens SWT-4.0-130 turbines. Private and state sources will provide financing, with around 85% of the capital provided by SHO's eight Finnish utility shareholders.

In these complex conditions, SHO faces the challenge of delivering the project in time and on budget. "Right now Tahkoluoto's construction costs are twice those of an onshore wind farm, due mainly to the need to install special foundations and sea cables," said Tahkoluoto's project manager, Arto Huhmarkangas. "The project will look to find the most cost-efficient ways to build offshore wind power for Finnish conditions."


Dredging work is scheduled to start from April, followed by installation of the onshore underground cable and substations, offshore foundations and the undersea cable. ABB Finland is responsible for the project's onshore electrical contracting work, including the substation and main transformers. Installation of the 4MW turbines is expected to start from June 2017, with grid connection and operation to follow in the third quarter of 2017.

Ramboll Finland, the local subsidiary of the Danish engineering group Ramboll, is consulting on the design and construction elements of the project.

The project research will utilise data from a growing pool of cold-climate Arctic wind-power technology research conducted by leading Finnish institutes.

Much of this research, including advanced testing of wind drag, as well as materials and techniques to circumvent potential dangers relating to the locking of moving parts, has been carried out by the Lappeenranta University of Technology and the VTT Technical Research Centre of Finland.

Turbines operating in cold climates require special steels to achieve sufficient strength, while optimising steel structures can help to cut costs and save on materials, said Toni Sulameri, chief executive of SHO.

Good conditions

"The conditions for offshore wind power are excellent in Finland: we have a long coastline, windy conditions, shallow waters and a hard seafloor, as well as harbours and industrial infrastructure all along the coastline. Offshore wind power produces roughly one and a half times the energy of onshore wind power," Sulameri said.

The country's first offshore wind farm is also expected to deliver valuable data for Finnish government energy planners. The project, which will receive €20 million in state funding, is expected to provide significant engineering, environmental and turbine performance data over the medium to long-term. This will be used to determine government policy and promote investment in constructing offshore wind farms along the country's northern and southern archipelago-dotted coastlines.

The crescent shaped site for the Tahkoluoto project is between 0.5km and 3km west of the south-west port of Pori. The ten 4MW Arctic-class Siemens turbines will be built on special rock-filled steel gravity-base foundations in water depths of up to 15 metres. Turbine installation is scheduled to start around June next year.

Ice solutions

The gravity-based steel foundations, modelled on those used for the pilot turbine, will have a conical top to withstand heavy ice loading. Technip Offshore Finland, a subsidiary of French construction group Technip, will provide the foundation work.

The conical top foundation design can withstanding ice ridges, formed from pressed drifting ice, to a height of 25 metres. Such ridges are common in the Gulf of Bothnia in peak winter months. The turbines will have a rotor hub height of 90 metres and a rotor diameter 130 metres.

Siemens has not specified what de-icing solution will be used in the rotor blades supplied to the Tahkoluoto project. The German manufacturer recently patented a de-icing solution that uses a heating mat integrated into the blade. Installed close to the blade surface, the mat contains no wiring, but is electrically conductive and can be activated as required to de-ice and enable normal operation of the turbine.