The first monopile has been installed in Anholt Offshore Wind Farm
While others were about to enjoy their New Year’s Eve dinner, the installation vessel Svanen started the installation of the first monopile at Anholt Offshore Wind Farm, and the construction of Denmark’s largest offshore wind farm has now started.
 

After two hours of driving, the monopile had reached the final depth at 19:00 hours on 31 December 2011.

According to the original schedule, Svanen was supposed to start work on 1 January 2012, but the mobilisation of the installation vessel was completed a few days ahead of schedule, and Svanen was ready to leave the Port of Grenaa on 27 December; however, due to weather forecasts reporting stormy weather, Svanen did not leave the port until 29 December.

Svanen left the Port of Grenaa on 29 December.

The next day, on 30 December at 12:25 hours, the first monopile was lowered into the water at Bladt Industries in the Port of Aalborg and tied to a tug that headed for the wind farm. The trip from Aalborg to the offshore wind farm takes around 12 hours for a tug and upon arrival, the installation preparations could be made.

On 30 December at 12:25 hours, the first monopile was lowered into the water at Bladt Industries in Aalborg.

Concentration and focus
Before commencing such a major installation job, there is much focus on ensuring that all work procedures are followed and that the right lines of communication between the involved parties are established. Therefore, time was allocated to learn and optimise the procedures to ensure that the installation could be carried out in a safe and controlled manner.

Pile driving started on New Year’s Eve at 17:00 hours, and after two hours, the pile had reached its target depth.

 - Pleased with the good start
Of course, the project management is pleased with the good start, but Claus Bøjle Møller, Project Manager, is also aware that there is still a long way to go.

Claus Bøjle Møller, Project Manager: “The project has really taken off, and it’s a good thing, but we know that we’re facing a series of challenges. We know that construction start during the winter will lead to periods of bad weather conditions in the form of wind and waves. We have of course included it in our planning, and we’ll try to utilise all weather windows for installation.”

Already on 3 and 4 January, the work is suspended. The weather forecast from the Danish Meteorological Institute for East Jutland shows a wind speed of 16m/s, but at the site, the forecast shows 20m/s, ie storm.

What is a monopile?
The Anholt project is based on the well-known monopile concept. The concept entails that a single pile is driven into the seabed. The monopile foundation is a round steel pipe with a diameter of approx 5 metres and a wall thickness of 5-9cm. The length of the monopile is adjusted to the specific location and varies from 37 to 54 metres.

Monopile before departure from Aalborg.

In order to be able to float the approx 450-tonne monopile from Aalborg to the installation site, the pile is sealed by means of two end caps. The caps are removed when the pile is erected and placed at its final position.

The monopile is up ended and placed on it's final position.

By means of a large hydraulic hammer, the monopile is driven 18-36 metres into the seabed depending on water depth and seabed conditions.

See animation of the construction of Anholt Offshore Wind Farm.

Mines blasted away at Anholt Offshore Wind Farm
In 2010, DONG Energy performed detailed geophysical surveys of the seabed in the wind farm area. In connection with the surveys, a few objects with magnetic signs, indicating that it might be old bombs, were located..

  Detonation of approx 400kg undetonated explosives.

In connection with the surveys, a few objects with magnetic signs, indicating that it might be old bombs, were located. Further surveys confirmed the presumptions; the find turned out to be two old mines from World War II – each containing approx 400kg undetonated explosives.

One of the mines was situated close to the substation where the cables from the wind turbines will be connected, and the other mine was situated in the centre of the wind farm area. Both mines were visible on the seabed at a water depth of approx 16 metres.

DONG Energy reported the find to the authorities, and after a diving inspection, the mines were classified as high-risk mines. In such a situation, the Admiral Danish Fleet is responsible for removal of the mines and decided to send out the Frogman Corps to detonate the mines.