Lessons Learned: The first case of heavy maintenance on floating wind

The Kincardine floating wind farm, located off the east coast of Scotland, was a landmark development: the first commercial-scale project of its kind in the UK sector. Therefore, it has been closely watched by the industry throughout its installation. With two of the turbines now having gone through heavy maintenance, it has also provided valuable lessons into the O&M processes of floating wind projects. 

In late May, the second floating wind turbine from the five-turbine development arrived in the port of Massvlakte, Rotterdam, for maintenance. An Anchor Handling Tug Supply (AHTS)

vessel was used to deliver the KIN-02 turbine two weeks after a Platform Supply Vessel (PSV) and AHTS had worked to disconnect the turbine from the wind farm site. The towing vessel became the third vessel used in the operation.

This is not the first turbine disconnected from the site and towed for maintenance. In the summer of 2022, KIN-03 became the world’s first-ever floating wind turbine that required heavy maintenance (i.e. being disconnected and towed for repair). It was also towed from Scotland to Massvlakte. 

Each of these operations has provided valuable lessons for the ever-watchful industry in how to navigate the complexities of heavy maintenance in floating wind as the market segment grows. 

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The heavy maintenance process

When one of Kincardine’s five floating 9.5 MW turbines (KIN-03) suffered a technical failure in May 2022, a major technical component needed to be replaced. The heavy maintenance strategy selected by the developer and the offshore contractors consisted in 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. (Note, the following analysis only covers KIN-03, as details of the second turbine operation are not yet available). 

Comparing the net vessel days for both the maintenance and the installation campaigns at this project highlights how using a dedicated marine spread can positively impact operations. 

For this first-ever operation, a total of 17.2 net vessel days were required during turbine reconnection—only a slight increase on the 14.6 net vessel days that were required for the first hook-up operation performed during the initial installation in 2021. However, it exceeds the average of eight net vessel days during installation. The marine spread used in the heavy maintenance operation differed from that used during installation. Due to this, it did not benefit from the learning curve and experience gained throughout the initial installation, which ultimately led to the lower average vessel days.

The array cable re-connection operation encountered a similar effect. The process was performed by one AHTS that spent 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 a cable layer.

Overall, the turbine shutdown duration can be broken up as 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. 


What developers should keep in mind for heavy maintenance operations

This analysis has uncovered two main lessons developers should consider when planning a floating wind project: the need to identify an appropriate O&M port, and to guarantee that a secure fleet is available. ‍

  • Identification of the O&M port

Floating wind O&M operations require a port with both sufficient room and a deep-water quay. The port must also be equipped with a heavy crane with sufficient tip height to accommodate large floaters and reach turbine elevation. Distance to the wind farm should also be taken into account, as shorter distances will reduce towing time and, therefore, minimize transit and non-productive turbine time. 

During the heavy maintenance period for KIN-03 and KIN-02, 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 North Sea projects. 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 maintenance that may be needed on short notice. ‍

  • A secure fleet

At the time of the first turbine’s maintenance program (June 2022), the North Sea AHTS market was in an exceptional situation: the largest bollard pull AHTS units contracted at over $200,000 a day, the highest rate in over a decade. 

During this time, the spot market was close to selling out due to medium-term commitments, alongside the demand for high bollard pull vessels for the installation phase at a Norwegian floating wind farm project. The Norwegian project required the use of four AHTS above a 200t bollard pull. With spot rates ranging from $63,000 to $210,000 for the vessels contracted for Kincardine’s maintenance, the total cost of the marine spread used in the first repair campaign was more than $4 million.

Developers should therefore consider the 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 as the market tightens in the future.

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While these lessons are relevant for floating wind developers now, new players are looking towards alternative heavy O&M maintenance options for the future. 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, as is currently seen on onshore turbines. This method is already employed in onshore turbines and could be applicable for offshore. The second method is self-elevating cranes with several such solutions already in development.

The heavy maintenance operations conducted on floating turbines at the Kincardine wind farm have provided invaluable insights for industry players, especially developers. The complex process of disconnecting and towing turbines for repairs highlights the need for meticulous planning and exploration of alternative maintenance strategies, some of which are already in the pipeline. As the industry evolves, careful consideration of ports, and securing fleet contracts, will be crucial in driving efficient and cost-effective O&M practices for the floating wind market. 


Sarah McLean is Market Research Analyst at Spinergie, a maritime technology company specializing in emission, vessel performance, and operation optimization.

Spinergie | www.spinergie.com

Author: Sarah Mclean
Volume: 2023 September/October