Onshore and Offshore Wind Turbines in Comparison

    While the generation and use of offshore wind energy is relatively new in Germany, the onshore generation of wind power started three decades ago. As a result of the service and maintenance conditions encountered by the offshore sector, these cannot be reasonably compared to those found onshore. Experts estimate that offshore service and maintenance can account for up to a quarter of total operating costs. In comparison, the equivalent proportion of total onshore costs are estimated to be in the single-digit percentage range at the most. If an offshore turbine suffers a system failure, this can result in enormous costs to multi-megawatt wind farms. Even the challenges for the whole logistics chain, from spare parts to personnel are highly complex.

    In bad weather and high seas, the transport of service teams to the turbine and the transfer from vessel to the offshore turbine is challenging. Helicopters cannot be used in fog and high winds. It is therefore essential that wind turbines work as smoothly as possible to ensure their cost-effective operation. To facilitate this, complex service and maintenance concepts are necessary – with more frequent service calls than for onshore wind turbines.

    In general, with these concepts, a differentiation is made between automatic maintenance, remote maintenance and on-site maintenance. In the case of automatic maintenance, an electronic Condition Monitoring System (CMS) is installed which monitors the condition of the turbine and all of its components in real time. The parameters monitored and transmitted include temperature, vibration and pressures as well as other physical variables.

    A CMS can identify malfunctions remotely because the system can sense when the system is overloaded and can then attempt rectification. Even electronic malfunctions and temperature and pressure deviations can be controlled and rectified to a certain extent. If the system cannot automatically rectify a problem, the wind turbine operator can remotely attempt to adjust the electronics via the control center located on land. If a malfunction cannot be resolved remotely by the system with remote adjustments, a service team has to go out to the wind turbine. And this is exactly the challenge in the offshore operations because to perform on-site maintenance to a turbine, the weather conditions must be suitable. Strong winds, fog or high waves can hinder such maintenance and the resulting shutdown may be prolonged with considerable economic implications.

    To prevent such situations, regular routine maintenance is a must. CMS systems do not offer one hundred percent protection especially for non-electronic turbine functions such as the rotor blades or the foundation. The German Federal Maritime and Hydrographic Agency (German: BSH) therefore prescribes regular on-site maintenance: The so-called periodic inspection (German: WKP) must be performed on 25 % of the wind turbines in an offshore wind farm once a year so that all turbines are inspected every four years. These inspections evaluate all the components of the turbines, with special attention being paid to cracks, corrosion and foundation and seabed erosion.