Taming the Baltic Sea for the Viking (Gallery)

Bladt is set to send off the last of 41 jacket foundations manufactured for the 350MW Wikinger offshore wind project, after the company loaded it on a barge yesterday. Spanish Navantia is in charge of the remaining 29 jackets for the 70-turbine wind farm in the German part of the Baltic Sea. 

The first jacket was installed in April, when Iberdrola, the company behind the project, said all the foundations are expected to be installed this fall.

Iberdrola awarded a EUR 135 million contract for the transport and installation of Wikinger’s seventy foundations and six offshore substation piles to Royal Boskalis and VolkerWessels in 2014. In the meantime, Boskalis acquired all offshore activities from VolkerWessels and has been executing the project on a 100% basis.

In a recent edition of its magazine, the company said the Wikinger offshore wind farm represented one of the biggest logistical challenges it has ever faced in the offshore wind sector, with some 20 vessels and 600 people working on the project that involves installing 70 jacket foundations and 286 piles, between 29 and 51 meters long, in just 90 days and with stringent underwater noise mitigation requirements.

The installation work encompassed pre-piling – the installation of four steel piles for each jacket, dredging and cleaning in and around the piles, installing the jackets with the Taklift 4 sheerleg crane and bonding the piles and jackets with high strength grout.

Because of Wikinger, Boskalis decided to convert its semi-submersible barge Giant 7 into a construction barge with an integrated pedestal for a permanent crane. The vessel has been equipped with a 1,000-tonne crane, an accommodation unit for around 70 people and an 8-point mooring system for flexible utilization.

Other preparations to customize the fleet for the Wikinger project included the design and construction of a pre-piling template, which has been deployed from the Giant 7.

Four towers on the template were used to position the pin piles to the client’s specifications – precisely to 0.5 degrees. The template is lifted overboard and positioned onto the seabed where it is leveled using cylinders. One after the other, the piles are upended then lifted over the side of the vessel and guided into the template and then hammered into the seabed.

However, driving the piles into the seabed took place only after the underwater noise mitigation measures were in place, van Gemert pointed out.

Protective measures were necessary to avoid harming rare species such as the harbor porpoise. Noise levels were capped at 160 decibels at 750 meters from the actual location where the piling took place. Van Gemert outlines the many steps that Boskalis had put into place.

“Firstly, we deployed a double Big Bubble Curtain from a platform supplier, which was fully loaded with compressors that blew air into submerged tubes,” van Gemert said. “However, this was not enough and we therefore took three additional measures. Apart from deploying acoustic deterrent devices, the barge was equipped with a hydro sound damper. In addition, a double layer of hoses expelling air was installed around the towers. With these measures we performed the project well within the noise limits.” 

The jackets were transported by two Dockwise vessels (Swan and Tern), which made seven journeys from Spain to Germany each, as well as various Boskalis anchor handling tugs and barges. Offshore, the sets of four piles were transferred onto the Giant 7 using barges.

Rob van Gemert, Boskalis Project Director for the Wikinger project, said: “We transported the materials from four different locations in Spain and Denmark to Sassnitz-Mukran in Germany, where they were stored in port on either barges or vessels.”

The jacket foundations will support Adwen’s 5MW turbines which are being constructed in Bremerhaven and Stade.

The EUR 1.4 billion Wikinger offshore wind farm is scheduled to be operational by the end of 2017.

Offshore WIND Staff; Source/Images: Boskalis