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Larger wind turbines in offshore wind farms demand dynamic maintenance.
Blog January 12, 2018

Investing in offshore wind energy is no longer reserved for pioneers. Increasingly, pension and infrastructure funds also consider wind energy to be an acceptable investment. Since these investors have a long-term perspective, seeking predictable returns and choosing investments with limited risk, this takes us into the next phase of the wind market maturing process. In this blog, I will explain how to ensure grip on operational risks and asset integrity for the long term.

Operational risks

Operational risks relate to how the wind farm is performing in relation to expectations and budget. As a result, manufacturers focus on the development of larger offshore wind turbines. The reason behind this is not complicated. Proportionally, larger turbines make wind energy cheaper: they have a higher return than they cost and require less subsidy.

Nevertheless, this upscaling does not mean it’s all positive news, because larger turbines are to be associated with increased technological risks. While a wind turbine blade demonstrates a fast increase in size, it is among the highly stressed components. The boundaries of what is technologically feasible are repeatedly stretched. A few years ago, a large blade would be 45 metres long. The newest generation of offshore wind turbines features blades of 80 meters. On the other hand, gearboxes, generators and other mechanical parts are designed so that they are as light as possible. The combination increases the risk of stress-induced failure. Problems with the drivetrain or the blades cause significant production losses and, moreover, result in additional maintenance costs.

In the onshore wind energy market, wind turbines show a similar rapid growth, and the experiences are not always positive. There have been several onshore wind turbine models that required systematic replacement of blades or gearboxes. A costly affair, with negative consequences not only for the manufacturer, but for all concerned.

Asset integrity

Today, the majority of the offshore wind farms has been operational for less than ten years. While the expected lifetime is 20 – 25 years, it is unknown how these larger blades will function in the second phase of their service life. Like every machine, a wind turbine is subject to wear and ageing. Problems may start small but can result in huge consequential damages. A defect in one of the turbines is annoying, if it occurs in all 100 turbines, it leads to massive costs.

Now that we enter the next phase of the wind turbine service life, asset integrity demands a more long-term view. The financial value and risk profile of a wind farm for twenty to twenty-five years, drives the need for a long-term maintenance approach instead of the short / medium term maintenance focus.

Long-term maintenance approach

The way service and maintenance of wind farms are managed, commonly depends on the type of owner. The traditional utilities usually do make long-term plans. To manage the maintenance of their wind farm for the long term, they often invest in an in-house technical staff. For the initial period, the utilities agree on a short-term service contract (5 years) with the manufacturer which also covers the warranty period. During this period, the in-house technicians are working closely with the manufacturer’s technicians and get trained-on-the-job. After expiration of the contract, the utility takes over the maintenance coordination and implementation themselves. The involvement of the servicing party after that, is limited to technical support and annual maintenance.

Different from the energy companies, there are many other type of owners like investors, who sign a contract with the manufacturer for the long term (10 to 15 years) with full-service conditions. A situation where the owner is entirely depending on the manufacturer or servicing party. The downside of full-service maintenance contracts is however, that owners do not acquire experience themselves in assessing the risks. They gain limited insight into the technological problems and can therefore not control the profitability and maintenance costs for the long term. This becomes especially challenging when both the subsidy support scheme and service contract expire.

Traditional maintenance not the answer

One way or the other, I notice that maintenance management of offshore wind assets in general is still handled in a very traditional way: periodic checks based on calendar days or operating hours. Regardless of how the wind turbine is performing.

There are far better methods. Tools like Risk Based Inspection (RBI) and Condition Monitoring Systems (CMS) offer interesting opportunities to manage risks and secure performance at the same time, based on the condition of the assets.

In comparison: the maintenance of modern cars now is handled based on dynamic service intervals, based on the measured load and/or sensors that measure the oil quality. In the wind energy industry, both CMS and RBI enable condition based maintenance.

RBI has been developed in the oil and gas industry and is designed to determine where the potential risks are. For example, where is the greatest chance of degradation? An inspector will not just check whether something is still working at that time, but also assess the chances of this component continuing to function throughout its entire life. In the oil and gas industry, this methodology is primarily used for cost optimisation, by reducing the number of inspections. In the offshore wind energy market especially, it can also be used to monitor the asset integrity.

A CMS is based on sensors that determine the condition of critical components by measuring vibration, temperature, noise level and other parameters. Such systems will detect any failures prematurely, so that the consequential damages remain limited and the repair can be performed at the moment with the lowest costs. For offshore wind energy, CMS is particularly interesting because it involves unmanned installations at sea where a visit is accompanied by high logistical costs. When selecting wind turbines for a wind farm, optional CMS solutions are out of the scope to keep CAPEX to a minimum. This is a missed opportunity to reduce OPEX during the lifetime, as subsequent implementation of a CMS is often very hard and costly.

Dynamic maintenance is the future

The biggest challenge for investors and operators in wind energy is to balance the efficient execution of daily activities and at the same time, have maximum control over the long-term asset integrity. In the maintenance area, there are still many improvement opportunities to reduce the operational risks and costs. A different strategy based on dynamic maintenance is the key!