Vattenfall readying for HKZ construction with mock-up trials

Vattenfall has been using a mock-up foundation installed at Sif’s Maasvlakte site earlier this year to gain better insight in what awaits its teams once the Hollandse Kust Zuid foundation installation starts in the summer of 2021.

The 140 monopile foundations at Hollandse Kust Zuid will not have transition pieces (TPs), bringing the offshore works to a different level.

With the TP-less monopile foundations, on which the work platforms and ladder will be attached directly, Vattenfall aims to minimise the project costs.

However, as the offshore construction stage is nearing, the company and its suppliers are preparing by using the mock-up.

“We will soon be performing the same installation work 140 times, so we see this test phase as a final rehearsal”, said Bart Buisman, who managed this test phase for Vattenfall.

“Naturally, as Vattenfall, we have a lot of experience with the construction of offshore wind farms, but never before have we built a wind turbine without a transition piece”, Buisman said.

In May 2020, Vattenfall started the mock-up trials of the foundations with the scale model at Maasvlakte. The model is a 22 metres high large hollow steel tube with a seven-metre diameter, weighing 220 tonnes. The actual height of the foundations will be between 68 and 85 metres, depending on location.

Vattenfall and its suppliers for the HKZ wind farm each tested their offshore installation activities on the test foundation. In total, the test period lasted about two months.

At the end of October, all feedback points from the mock-up trial were submitted and will be implemented in the design.

The second and final mock-up trial is scheduled for March 2021, when the verification for the offshore installation processes and tools of the offshore installers will be verified.

The now completed trials brought many lessons valuable for the installation phase.

For Bart Buisman, two lessons learned stand out: “Once the wind farm is operational, we will have to carry out maintenance work. To do this, the wind turbines must be easily and safely accessible. That is why we attach a ladder to the turbines, which consists of two parts. During the test it appeared that the installation method for lowering one of the ladders is not yet optimal. That is now being adjusted”.

“The second insight is about how we pull the electricity cables through the steel pipes. That is labor-intensive and heavy work. Small adjustments allow us to make a good improvement in terms of safety and ergonomics”, Buisman said.

The conditions at sea are very different from those on the Maasvlakte, where the mock-up trial was carried out, Vattenfall said, adding the installation conditions as they would be at sea were simulated, such as deliberately hanging the external work platform on land a little tilted in the crane.

The Hollandse Kust Zuid zone comprises four offshore wind farms that will be connected to the HKZ Alpha and HKZ Beta substations. 

The six-legged jacket foundation was recently installed for the Alpha offshore substation.