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Planning for the unplanned: Operations and maintenance strategies for offshore wind

The progression of offshore wind development in the United States is an exciting story: new wind energy areas (WEAs) are being determined, auctions are being held, and the first commercial projects are under construction or even delivering energy. However, the story does not end once the wind farm is fully installed and the big machinery moves on to the next project. The end of development and construction phase marks the next chapter: the actual operational lifetime of the wind farm, which can last between 25 and 35 years.

Offshore wind is still new to North America, and the technical and financial risk is magnitudes higher than onshore wind projects. At DNV, we've helped customers around the world manage these risks in some of the toughest operating environments. I'm excited to bring lessons from these offshore wind projects to North America.

Design the responsibility structure

The operations and maintenance (O&M) strategy should already be defined when the project enters commercial operations, and responsibility can be summarized as three different approaches: hands-on, hands-off, or hybrid.

The chosen approach determines the contract term of the operational agreement with the original equipment manufacturer (OEM) and whether post-warranty responsibilities are taken internally or externally.

With a “hands-on” approach, the owner/operator takes direct responsibility for a wider range of activities (in some cases the WTGs and the balance of plant assets), while with a “hands-off” approach the project owner relies on a few key contractors or on a third party asset manager to look after the project. The hybrid of these approaches is a mix of both. For example, the owner/operator may handle the O&M function in-house but retains some specialist support from the OEM. Larger and more experienced offshore wind players may aim for a hands-on approach, whereas financial institutions may prefer to be hands-off. Regardless of the approach, a well-planned O&M strategy that adjusts to unforeseen events will ensure the project operates at its maximum capacity.

Know your vessel and crane needs

Preventive maintenance and minor corrective maintenance is handled by technician crews accessing the WTGs via crew transfer vessel (CTV), service operation vessel (SOV), helicopter, or a combination of these. These vessels are well suited for day-to-day activities. However, special crane vessels may be necessary to replace blades, gearboxes, or generators. Replacing these components involves complex logistics due to the remote and challenging offshore environment. For bottom-fixed projects, a crane vessel is typically deployed.

Floating projects have more flexibility, as each individual WTG can be disconnected and towed to a port, where the components can be replaced. For example, the five WTGs operating within the Hywind Scotland floating wind farm will be towed to Norway for maintenance. Alternative solutions for offshore component replacements are currently under development, but the motion of a floating WTG makes the lift of major components particularly complex.

Understand your infrastructure risks

Consideration must also be given to the potential failure of the offshore balance of plant (BoP) infrastructure consisting of foundations, offshore substations (OSS), and subsea cables. Failures of BOP infrastructure could result in downtime and have a tremendous impact on operational expenditures (OpEx).


The owner/operator may manage the above-water inspections and preventive maintenance of the structures using several subcontractors or one contract agreement. Often, in-house resources support a big part of these works. Contracts for below-water inspections and surveys are typically awarded to a specialized contractor. Corrective maintenance tasks on seabed, structures, or associated infrastructure that do not have an impact on the operation of the wind farm and do not pose risk to the environment, people, and property can be scheduled when the time is optimal, determined by a combination of environmental conditions, economic factors, component life cycles, and regulatory compliance.

Offshore substations

Since the OSS is a single point of failure, it requires expertise and a quick response when issues arise, even though most vital systems are redundant. For this reason, O&M agreements with specialized contractors are usually executed, but in some cases the owner/operator handles work in-house, depending on their strategy and available expertise.

Subsea cables

Subsea cable failures can shut down single WTGs, WTG strings, or even the entire wind farm if the export cable is faulted. Repairs require complex logistical coordination to get specialized vessels, (un)burial spreads and crews, and faultless spare parts on site as fast as possible. A subsea cable repair can take several months to complete, especially when vessel procurement and engineering needs to be started from scratch. Therefore, many owners/operators have cable repair framework agreements in place to minimize downtime once a cable fault occurs.

Stage your spare parts

Positioning spare parts in key locations, fully maintained and ready for use, can make maintenance and repairs easier and faster. This is especially important for items with long procurement lead times. Typical strategic parts seen in different projects from DNV’s experience include:
Subsea spare cable lengths for inter array and export cables with matching jointing equipment
A set of blades, generator, transformer, switchgear, and gearbox (if applicable).

Monitor conditions

The further the wind farm is located from shore, the harsher the environment may be. This can lead to higher failure rates and longer response times that result in prolonged downtime, reducing overall energy production and revenue generation. Remote condition monitoring systems are crucial for the early detection of issues in offshore wind farms. They also help determine the optimal time for replacements or repairs when the impact on the wind farm is at its lowest.

Commit to O&M planning

Choosing an O&M strategy depends, among other factors, on the intentions of the owner/operator, design particulars of the offshore wind farm, the project location, synergies with other offshore wind farms, and the environment, as well as local contractor and supplier availability. All factors should be carefully considered early in the project development stage to identify feasible and cost-effective options.

Even though the outage of WTGs presents a high financial risk, risks associated with the failure of the BoP infrastructure should not be underestimated and carefully assessed and mitigated. DNV’s extensive experience with all areas of offshore wind enables us to help developers find a tailored O&M strategy for their projects.

2/12/2024 2:00:00 PM

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Philipp Koehler

Philipp Koehler

Senior Project Engineer, Offshore Wind Advisory

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