RENEWABLE ENERGY Measurement technology in wind energy


With manufacturers, operators and service providers looking for greater reliability and safety, less maintenance and downtime, and a reduction in operational and build costs, RotaBolt asks whether the wind energy sector could save millions of euros by simply adopting a basic engineering principle when it comes to fastener technology - accurately measure what the operator is doing.


W


ind turbine assemblies are complex structures. A typical 2.3MW turbine will have nine major bolted joints in its construction with approximately 1,000 bolts in the complete assembly. The most obvious and visible bolted


joints are the main turbine blades connecting to the blade flange and the blade flange to the main hub, but other critical joints include the bearing housing, base frame, gear stays, the main yaw bearing, as well as tower flanges and foundations. Many of these joints are subject to complex loads and forces


with fatigue and vibration cited as the most common factors in failures. Understandably, maintenance regimes are both intensive and costly. It will take 140 man hours to carry out a complete bolt check on the same 2.3MW turbine. In offshore locations these costs become multiplied. Three basic factors


will assure the reliability of a bolted joint – the right design, the right component quality and the correct tension across the joint. Design and quality are factors that are both measured, but the tension across the joint rarely is. What is measured in traditional practice is the force being applied to tighten; the tension or clamping force that is being achieved remains completely unmeasured and unknown. As a result, it’s hardly surprising that approximately 90%


of all bolted joint failures are caused by insufficient tension at installation. One simple solution is to employ fasteners with measurement technology built in. This way the correct design tension can be achieved and simply maintained throughout the life of the bolted joint. Currently there are four operators in wind energy using measurement technology in the construction of their


RotaBolt®


turbine assemblies, but there is ‘parallel learning’ from another sector altogether which adds weight to the argument in favour of using measurement technology. The slew bearing of a pedestal crane is a bolted joint which


almost directly parallels the dynamics of key joints in a wind turbine. It is the single most important connection in the entire crane assembly and the performance of the crane depends on


38 Fastener + Fixing Technology • Issue 5 Apr - Jun 2012


the critical load path through this bearing. After many years of learning, every pedestal crane that’s on


an offshore platform in the British section of the North Sea is now using RotaBolt measurement technology in the main slew bearing, and quite often on the boom assembly as well. Robert de Rijcke from Kenz cranes is just one of the


beneficiaries: “We can accurately determine pre-tension at installation and with RotaBolts we don’t have to loosen and re-tighten in our annual checks.” Because each RotaBolt can be instantly finger-tip checked for correct tension, it has been estimated that the maintenance savings alone on each crane range between £10,000 (12,000 euros) and £15,000 (18,000 euros) every year. In addition, the assurance and increased performance


provided by the technology also means that Kenz has in special circumstances been able to increase the load bearing performance provided by their slew bearings. Back in wind energy, if the same technology were employed on a typical 2.3MW turbine, the maintenance checking of 1,000 bolts would reduce from 140 hours down to 2 hours. In addition, because of the increased performance in bolted joint integrity delivered by the technology, the number of bolts being used in the design could be substantially reduced with further major savings in build and maintenance costs.


Two of the four


turbine manufacturers currently using RotaBolt technology are Suzlon and Vergnet. Suzlon Energy fits the fasteners on all its 1.25MW and 2.1MW


wind turbines. The tension fasteners are standard fit on the wind turbine blade connections. Vergnet is now using them on the main structural joint of its unique GEV HP wind turbines, where the two halves of the turbine nacelle are joined. Vergnet’s design allows the main turbine to be quickly


disassembled into two halves so that the upwind nacelle section, complete with rotors, can be lowered to the ground and easily secured. This can be either as a protective measure in extreme weather conditions, or for much quicker service and maintenance. Each turbine uses thirty M90 RotaBolts on the main nacelle structural joint.


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