TenneT's HKZ Transformer Platforms in Dutch North Sea Hit Major Milestones
Bartolomej Tomic
March 30, 2022
Dutch grid operator TenneT has this week installed the topside of the second offshore transformer platform built to serve Vattenfall's Hollandse Kust Zuid offshore wind farm in the Dutch North Sea.
Following the installation of the first transformer platform, the Hollandse Kust Zuid Alpha, in December 2021, Tennet had Allseas install the 3,950 t Hollandse Kust Zuid Beta topside on a previously installed jacket.
On what was a rather foggy Monday in the North Sea off Scheveningen, Allseas used its giant Pioneering Spirit installation vessel to lift the topside, which recently arrived from Dubai, and install it on the jacket.
Hollandse Kust Beta Topside Installation on Monday - Credit: TenneT
Interestingly, back in February, the project faced a scare when a drifting bulker hit the jacket in stormy weather, just a month ahead of the planned topside installation.
However, subsequent inspection by TenneT proved that the jacket suffered only minor damage, and that it was safe for the topside to be installed, thus avoiding any potential delays. A total loss, which luckily didn't occur, would have caused a lengthy project delay and hefty additional costs, Offshore Engineer understands.
The HKZ Beta High Voltage Alternating Current (HVAC) offshore transformer platform with a 700MW capacity, and built by Petrofac and Drydocks World Dubai, will now be prepared for the connection to Vattenfall's wind turbines.
According to TenneT, the Hollandse Kust Zuid Alpha transformer platform, to be used for a connection for offshore wind farms Hollandse Kust (zuid) I and II is now ready for use. 
Hollandse Kust Zuid Alpha ready to connect - Photo by Bartolomej Tomić, Offshore Engineer Magazine
"TenneT completed the installation and energization of the offshore grid connection. Wind farm developer Vattenfall can now connect the wind turbines to the high voltage grid." TenneT said in a statement sent to Offshore Engineer.
Marco Kuijpers, Director of Large Projects Offshore: "With the energization of the Hollandse Kust (zuid) Alpha connection, the offshore grid that TenneT is building in the North Sea is starting to take shape. Hollandse Kust (zuid) Alpha is the first connection of an offshore wind farm to the Maasvlakte and has a capacity of 700 MW. The energization of Hollandse Kust (zuid) Beta later this year will add another 700 MW from wind farms Hollandse Kust (zuid) III and IV.”
The platforms form part of the grid connection that connects the offshore wind farm zone to the Dutch mainland at the Maasvlakte. Located 20 kilometers off the Dutch province of Zuid-Holland, the Hollandse Kust Zuid (HKZ) wind farm zone covers an area of 235.8 km2.
The platforms will convert voltage of the electricity produced at the wind turbines from 66 kV into 220 kV, and will then transport the electricity is shore via subsea cables, to an electrical substation at the Maasvlakte. At this substation, TenneT said, the voltage is converted into 380 kV. Finally, the electricity is transported to the high voltage grid via high voltage station Randstad 380 kV Zuidring, TenneT explained.
The turbine installation for what will be a 1,4 GW offshore wind farm project, is expected to start in the coming weeks, Offshore Engineer understands.
In a press tour hosted by TenneT this week, where members of the media, including Offshore Engineer, had the opportunity to sail around the two platforms aboard Chevalier Floatel's DP Galyna vessel, one could also see multiple transition pieces installed on monopile foundations and ready to host Siemens Gamesa's powerful 11MW wind turbines.
The construction on what is expected to be the world's largest wind farm once operational is being done in phases.
According to Vattenfall's annual report, the first phase ran in 2021 and saw the installation of the first 34 monopiles. The second phase started in March 2022, with the installation of the remaining 106 monopiles.
Transition pieces awaiting their respective wind turbines - Photo: Bartolomej Tomić - Offshore Enginer Magazine
As mentioned earlier, Petrofac was responsible for the engineering, procurement, construction, and offshore installation of the HKZ Alpha and HKZ Beta platforms.
Commenting this week, Sami Iskander, Petrofac CEO said: “The safe installation of the Beta topside is a significant milestone in the delivery of this important energy transition project. Everyone involved should be proud of this achievement. ”
TenneT's Kuijpers added: “I am very pleased that the project was installed safely and I compliment Petrofac and Allseas on this. Wind energy plays a crucial role in the energy transition, also in industry, and I am proud that TenneT, together with our partners, can contribute to a sustainable future in this way. "
Once fully operational in 2023, Hollandse Kust Zuid, known as the first subsidy-free wind farm area in the world, is expected to generate enough electricity to power more than two million households, TenneT said.
Pioneering Spirit leaving the HKZ Beta Substation - Credit: Bartolomej Tomić / Offshore Engineer Magazine
