— The repair work was very technically complex and challenging due to the location of the fault and
the weather conditions
— The alternative transmission route via Elia Transmission Belgium's MOG (offshore power hub)
worked perfectly and ensured that all wind farms continued to generate electricity, despite the
fault
— The windy weather conditions in recent months hampered the repair work but did allow very high
levels of wind energy to be produced
— The cause of the fault is still being investigated
Belgian part of the North Sea | Approximately four months on from the failure of the cable which connects
the Rentel wind farm to the mainland, the repair work has now been completed. The fault was located just
below the Rentel transformer platform. A new section of cable, approximately 400 metres long, was
attached to the platform and connected to the existing cable under the seabed. The four connected wind
farms were still able to generate electricity throughout the repair work, because they are part of the
meshed offshore high-voltage grid (via Elia's MOG or offshore power hub). Their electricity output was
capped slightly during periods of very high winds to prevent the operational export cables from being
overloaded. Despite this, another offshore and onshore wind energy production record was broken in the
first few months of this year.
Complex repair operation
On 9 January, the MOG cable connecting the Rentel platform to the mainland suddenly failed. The fault was
located at the spot where the cable enters the high-voltage platform. Carrying out repair work at this location was
very challenging and complex. The high-voltage platform is 34 kilometres off the coast and the past few months
have been particularly windy. 400 metres of damaged cable were replaced. Special lifting techniques involving
ships were used to lift the cable from the seabed. However, such methods could not be adopted just below the
platform, so divers had to disconnect the cable.
Connecting the new and existing cables was also challenging. To do so, an offshore rigid joint was used that was
repositioned on the ground 30 metres below sea level. This operation could only be performed during an extended
period of low wind, so the repair work had to be postponed for some time. A platform joint also had to be installed
on the platform
We very quickly realised that this was going to be a challenging repair operation given the
timing and exact location of the fault. It is only due to the perseverance, expertise, flexibility and
knowledge of all staff and our partners that we were able to complete the repair work within just
a few months.
Geert Moerkerke, Head of Assets Offshore Elia Transmission Belgium
We are immensely grateful to Elia's staff for the way they handled the situation. The repair work
posed an unprecedented challenge to all teams involved, requiring them to work under quite
extreme conditions at critical times. Thanks to the MOG, all connected wind farms continued to
generate electricity throughout the repair work - something that makes a world of difference at
this (windy) time of year.
Mathias Verkest, CEO of Otary
The Rentel wind farm as part of a three-way connection
Each offshore wind farm has a transformer platform that collects the electricity generated by the wind turbines and
then steps up the voltage of this electricity before it can be efficiently transmitted back to shore. Upon its
commissioning in 2018, the Rentel wind farm (309 MW) only had one cable running from its platform to the mainland
(known as a ‘radial’ connection).
When Elia developed a meshed transmission grid for the next three wind farms (Mermaid, Northwester 2 and
Seastar), it integrated a Rentel export cable into this. A meshed grid saves on cables – which is more sustainable –
and enhances the wind farms' security of supply in the event of an incident.
Elia took over the Rentel cable that leads to the mainland in 2019 and laid a short connection cable between the
Rentel and Elia MOG platform which, along with the two export cables from the offshore power hub, formed a threeway connection (see map).
The usefulness of this set-up was clearly demonstrated during the repair work which had
to be undertaken.
Very high generation despite restriction
The MOG allowed Rentel's wind power to be injected into the transmission grid, despite the repair work. It was only
when it was very windy that the four wind farms connected to the MOG had to have their output reduced so that the
remaining cables were not overloaded. The four wind farms (Rentel, Seastar, Mermaid and Northwester 2) could
together generate a maximum of approximately 800 MW instead of 1,000 MW. Without the MOG, all 309 MW of
capacity from the Rentel wind farm would have been unavailable It looks like the first five months of this year will have
produced a record amount of wind. From Jan. 1 through May 24, 3304 GWh of offshore wind recorded. With one week
to go this month, it looks like the record of the first five months of last year will be broken. Onshore also recorded high
production figures (2,690 GWh).
