With confidence to Tahkoluoto

After Nobelwind, the Vole au vent sailed to Tahkoluoto in Finland to install Finland’s first offshore wind farm, a challenging sequel for the vessel and her crew.

"We are very proud that we played a key part in this project"

This job for the Finnish wind energy producer Suomen Hyötytuuli Oy entailed the installation of ten wind turbines with foundations, also called gravity-based foundations (GBFs). Assistant Project Manager Arnaud Eeman explains: “The project was broken down into two campaigns during summer 2016 and 2017, due to the risk that the sea freezes over in winter.” 

“In 2016, we carried out preparatory works: the backhoe dredger Mimar Sinan dredged ten large pits at a water depth of 10 to 18 metres. The soil conditions were challenging: the soil consists of hard rock with a layer of moraine clay on it and numerous boulders, large rocks that can have a diameter of a few metres. The surface was so hard that we even had to use explosives for one pit. Afterwards, we connected the pits to each other and to the mainland by dredging six kilometres of cable trenches.”

“The excavators on board of our split hopper barge Tiger poured a layer of coarse gravel in the pits as a solid subsoil for the foundations. That gravel was then compacted with a falling weight. Quite a demanding phase, but our crew on board delivered a great and very accurate job.”

Strict tolerances

In 2017, the focus was on the installation of the foundations and wind turbines. “We used the winter break to do the engineering”, adds Arnaud. In April 2017, the teams went back to work in Finland. The pits that had been partly filled in 2016 were now given a final gravel layer which had to be perfectly level.

Subsequently, the Vole au vent installed the ten Gravity-Based Foundations on the levelled gravel beds. The crane of the Vole au vent lifted the foundation from the quay onto its deck and then to the correct position in the pit. "The big difference from Nobelwind was that we were working on a hard surface this time," says Rick Van Hellemondt, Offshore Installation Manager.  “For jacking the Vole au vent, this harder surface was a great advantage.” 

Once the foundation was in place, it was important to ballast the foundation with stones as soon as possible. Arnaud Eeman: “An empty Gravity-Based Foundation is not resistant to bad weather. We therefore filled each of them with coarse gravel”. Once the ten foundations were placed and ballasted, the Vole au vent could install the wind turbines. To that end, an entire Siemens team came on board to support the lifting operations for the installation of the wind turbines.

Key part

The project was completed in August 2017, after having installed a rock layer around the turbines to protect them against erosion. Arnaud looks back:  “It was a complicated project with a tight schedule. We completed the lifting operations for all wind turbines one week ahead of schedule, which was a key success factor in this project. This is the very first offshore wind farm using Gravity-Based Foundations in Finland and at the same time also the first that has been installed in such demanding icy conditions. We are very proud that we played a key part in this project.”

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