A wind farm in the open seas

Aerial photograph of an offshore wind farm

Source: Fotolia.de/Halberg

The maintenance of wind turbines in an offshore wind farm is often time-consuming and cost-intensive. According to estimates by the Bundesverband Windenergie [German Wind Energy Association], maintenance costs of turbines in the sea account for up to 25 percent of the total costs.

But regular inspections are essential. If damage to a rotor blade is not discovered in time and the blade then needs replacing, the costs can be extensive. It can take time for a specialist ship required for assembly to be available and for the repaired turbine to be reconnected to the grid. Energy companies are therefore looking for cost-effective, practical and safe processes to be able to regularly inspect and service their turbines. The more wind farms are built along the coast, the greater the demand will be.

Using heat for non-destructive testing

BAM is currently developing a process to identify damage and thereby minimise the need for risky and time-consuming maintenance work on the open sea. Dr. Rainer Krankenhagen from BAM's Non-Destructive Testing Division has carried out thermographic studies to this end. Thermography is a remote imaging process that has already been tested by the Fraunhofer-Institut für Holzforschung [Fraunhofer Institute for Wood Research] on idle rotor blades on land. BAM is expanding this method to moving rotor blades – bringing the possibility of inspections being carried out from a passing helicopter a step closer. Infra-red-Camera [Kamera] technology for remote Analysis of Rotor blades under open sea conditions (IKARUS) is the name of the project concerned. Meanwhile the process, which is based on a direct comparison of the rotor blades with each other, has been patented.

Thermogram of a wind turbine. Clearly visible red spots indicate higher temperatures.

A photograph of a wind turbine compared with the corresponding thermogram

Source: BAM

In-flight testing

A possible scenario: as a helicopter flies past a wind farm, an infra-red camera on board identifies and visualises minimal differences in temperature on the surface of individual rotor blades. These are caused by normal weather influences and the transition from day to night. An unusual heat distribution is an indication that something is wrong. Around 80 percent of the time, these are undetected manufacturing faults, such as faulty bonding or insertion of hollow parts. "It is a problem of insufficient quality control, which could be solved during production using a thermographic process," says Rainer Krankenhagen. But wear and external factors like lightning strikes, ice deposits or damage during transportation could also be the cause.

Maintenance technicians are only deployed if irregularities are apparent that suggest damage. Wind turbines will be able to be repaired more efficiently and cost-effectively in future using BAM's thermographic testing process. BAM is thereby also contributing to the safety and serviceability of offshore wind turbines, which represent an important cornerstone of the energy transition.