The Tin Man in the Wizard of Oz fretted over his joints, and owners of wind turbines have the same angst. They need to be assembled offshore and maintenance of the joints is critical where a 20+ year life expectancy under the buffeting of winter seas and gusting storms is normal. The assembly is subject to prying forces, vibration and continuous temperature variation in a highly corrosive atmosphere of salt spray, rain and UV. Intellifast presents an illuminating despatch from the frontline of the wind energy arena…
As turbines move further offshore maintenance of these valuable assets and taking precautions to avoid failure gain in relative importance. Increases in unit size from 2 to 6 and now perhaps 10 or 12 MW unit size are favored by the economics and accentuate the problems. These units will be located in waters notorious for their weather and difficulty of access. The bolt sizes have increased from the familiar M30 and M36 to M42 and M64.
Studs or bolts are used to anchor the tower, assemble the tower and the nacelle. Many of these bolts are tightened close to yield. Each blade is attached to the hub with a large number of studs per blade.
Any engineer, who has experience with snapped bolts or sheared studs, knows the time lost in extraction and replacement, and the anxiety over what caused them, has an interest in revisiting these issues and what can be done to avoid failures in the future. It is not the cost of the bolt or stud; it is the time and expense of maintenance and the uncertainty of the system.
A relatively simple step to reduce the vulnerability and risk is to make sure the fastener clamp load has been measured, and documented from the day it was first tightened and at required intervals. It is in the owner’s interest and that of the insurers to ensure the best precautions are taken from the initial commissioning. The simple corollary is that at least in the most critical of joints there is baseline accurate data from which prediction and monitoring through life can begin. The ability to measure critical fastener loads at any time can be integrated into a true Condition Based Maintenance (CBM) system: directly measure the critical parameter then prioritize and optimize maintenance resources from analysis of that data.
