In late May, the second floating wind turbine from Pilot Offshore’s five-turbine Kincardine development off the east coast of Scotland arrived in port for maintenance. The KIN-02 turbine was delivered by JD Contractor’s AHTS Assister to the quayside at the port of Massvlakte, Rotterdam. This was two weeks after PSV Coco and AHTS BB Ocean had disconnected the turbine from the wind farm site.
The first floating wind turbine from the Kincardine wind farm, KIN-03, had also been disconnected and towed for repair in the summer of 2022. This marked the first incidence of a floating wind turbine requiring heavy maintenance (i.e. being disconnected and towed for repair) anywhere in the world. KIN-03 was also towed from Scotland to Massvlakte. Bourbon Subsea Services was responsible for the scope of work for the maintenance (including towing and hook-up)
With maintenance of the second turbine ongoing, we can take the opportunity to examine what lessons can be learned from the world’s first example of heavy maintenance of a floating wind turbine.
Kincardine Case Study: Installation vs Maintenance
Kincardine is the first commercial-scale floating wind farm in the UK sector and, as such, has been watched closely by the industry through each stage of its development. When one of the five floating Vestas V164-9.5 MW turbines, KIN-03, suffered technical failure last May, a major technical component needed to be replaced. The heavy maintenance strategy selected by the developer, Cobra Instalaciones y Servicios, S.A and the senior management team of Flotation Energy, and the offshore contractors (JD-Contractor and Busker og Berging), consisted of disconnecting and towing the turbine and its floater to Rotterdam for maintenance, followed by a return tow and re-connection. All of the infrastructure, such as crane and tower access, remained at the quay following the construction phase.
A comparison of net vessel days for the maintenance and installation campaigns at this operation highlights the potential upsides of using a dedicated marine spread to allow for easy maintenance operations.
Turbine reconnection during maintenance stands at 17.2 net vessel days, similar to the first hook-up operation performed during installation in 2021 (14.6 net vessel days). However, this exceeds the installation average of eight net vessel days per turbine. The new marine spread used during the heavy maintenance operation differed from the one used during installation, therefore it did not benefit from the learning curve and experience from the initial installation campaign.
A similar effect was encountered during the array cable re-connection part of the maintenance programme. It was performed by one AHTS which spent a total of 10 net vessel days on the operation. This compares to the installation campaign where the array cable second-end pull-in lasted a maximum of 23.7 hours using the cable layer Maersk Connector.
The full breakdown of the turbine shutdown duration was 14 days at the quay for maintenance, 52 days from turbine disconnection to turbine reconnection and 94 days from disconnection to the end of post-reconnection activities.
Key considerations for heavy maintenance in floating wind
Two main lessons emerged from the first heavy maintenance operation at Kincardine: the need to identify an appropriate O&M port and to ensure a secure fleet is available.
Identification of the O&M port
A port requires sufficient room and a deep water quay in order to be suitable for O&M in floating offshore wind. It needs to be equipped with a heavy crane with sufficient tip height to accommodate large floaters and reach turbine elevation. Furthermore, the distance to the wind farm is a key element in reducing towing time to minimize transit and non-productive turbine time.
During the heavy maintenance period for KIN-03, the selected quay (which had also been utilized in the initial installation phase of the wind farm project) was already busy as a marshalling area for other projects (HKZ and Dogger Bank A). This complicated the schedule significantly as the availability of the quay and its facilities had to be navigated alongside these other projects. This highlights the importance of abundant quay availability both for installation (long-term planning) and also for maintenance sometimes at short, unexpected notice.
A secure fleet
At the time of KIN-03’s maintenance (June 2022) the North Sea AHTS market was in an exceptional situation with the largest bollard pull AHTS units contracted at over $200k a day: the highest rate in over a decade. The spot market was nearly sold-out due to medium-term commitments alongside the demand for high bollard pull vessels for the installation phase at the Norwegian Hywind Tampen floating wind farm project. Hywind Tampen’s installation required the use of four (out of 41 trading in the North Sea market) AHTS above a 200t bollard pull.
With spot dayrates ranging from $63k to $210k contracted for Kincardine’s maintenance, the total cost of the marine spread used in the first repair campaign was more than $4 million.
The key takeaway from this is that developers will need to structure maintenance contracts with AHTS companies either through frame agreements or long-term charters to decrease their exposure to spot market day rates.
Alternatives for the future
While these lessons are key for floating wind developers in the immediate future, new players are looking towards alternative heavy O&M maintenance. Two crane concepts are especially relevant in this instance. The first method is for a crane to be included in the turbine nacelle to be able to directly lift the component which requires repair from the floater. This is what can currently be seen on onshore turbines.
The second method is self-elevating cranes. Several such solutions are already in development including Mammoet’s SK6,000 crane; WindSpider, which is supported by RWE; and Liftra ApS which is supported by DEME. The Mammoet solution, which is capable of lifting over 4,000t to a height of 175m with a maximum reach of 144m, allows developers more flexibility when utilizing onshore facilities as part of the installation and O&M programmes. Meanwhile, WindSpider is self-erecting and uses the tower of a wind turbine as part of the crane when performing installation, maintenance, re-powering, and decommissioning of wind turbines. Finally, Liftra is also self-erecting.
To sum up, the heavy maintenance of floating turbines at the Kincardine wind farm has provided valuable lessons for the industry. The requirement to disconnect and tow turbines for repair proved to be a complex and time-consuming process, highlighting the need for careful planning and consideration of alternative maintenance strategies, some of which are already in development.
Analysis of the installation compared to the maintenance campaign revealed that the lack of a consistent marine spread for maintenance resulted in significantly more vessel days. As such, two important lessons have been learned: the identification of an appropriate O&M port with sufficient facilities and availability, and the establishment of secure fleet contracts to mitigate exposure to high spot market dayrates.
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