Feasibility Assessment for General Fastener Loosening Inspection
TWI Industrial Member Report 1203-2025 [pdf / 1,266KB]
By Stephen Grigg and Dandan Liu
Industrial Need
During installation of an offshore wind turbine, upwards of 1000 bolts are used to secure both the tower section and various other component sections in place. On operational wind turbines, approximately 10% of bolts at key bolted connections are checked annually to monitor for loss of tension. These checks require hydraulic tensioning/torque tools which are heavy and cumbersome to use, posing manual handling risks for personnel. Work locations also have access challenges including the need for marine transfers, working at height, and working in confined spaces. This costly and time-consuming process requires the turbine to be shut down and is the biggest scheduled maintenance task for operators. We estimate that the work required to perform this maintenance alone is in the order of a number of days for each turbine per year. Since the ~2500 offshore turbines in the UK, we estimate an annual saving estimated at £10m in labour costs alone. Factoring this across Europe indicates a market saving opportunity in the hundreds of millions of pounds.
Bolt failures are not automatically detected on offshore wind turbines. In the event of a bolt failure, all wind turbines affected by the bolt batch may have to be shut down until bolt checks can be completed or a root cause analysis is completed, this can also result in significant downtime.
We are seeing increasing enquiries from wind turbine supply chain in relation to failures and maintenance. We have also observed that the cost benefit opportunities from remote condition monitoring have yet to be fully realised.
Services are available for ultrasonic methods of bolt based on both pitch-catch and pulse-echo EMATS. These however do not eliminate, or even reduce the need to shut down the wind turbine for the inspection to be performed.
It would quite easily be possible to develop a system that implemented these methods in-situ, however it would unlikely to be economically feasible due to the cost of probes and hardware. We propose that there is an immediate and urgent opportunity to take up a market advantage in R&D and service delivery in this domain.
Key Findings
- It was demonstrated to be possible to determine if a bolt is tight or loose through the use of acousto-ultrasonics
- An effective test bed was developed which can be used for future development
Impact
The original objectives of the project were met and it has been demonstrated that this approach would be viable. Future work should be conducted to further demonstrate its rigidity, to optimise the process and develop a cost effective prototype to enable initial deployment of the system.