Engineers putting the housing onto a gear box: gearbox failures can result in high repair costs plus lost revenue due to unplanned downtime (image courtesy of ZF).
Engineers putting the housing onto a gear box: gearbox failures can result in high repair costs plus lost revenue due to unplanned downtime (image courtesy of ZF).

This article excerpt is taken from the September/October issue of Renewable Energy Focus magazine. To register to receive a digital copy click here.

Any operation requiring gearbox removal is bound to be expensive and a budget for a complete overhaul, whether planned or unexpected, can amount to an eighth of the cost of a new machine.

Part of that expense is incurred by having to transport a heavy and bulky gearbox to an original equipment manufacturer for the necessary attention.

The situation is worse when the OEM is overseas. Britain, for example, is one of the leaders in wind power adoption, having some of the world's best wind resources and a growing infrastructure to exploit them, so there will ultimately be many turbines.

This creates a business opportunity for organisations willing to provide a local facility for overhauling wind turbine gearboxes, particularly if that is part of a comprehensive gearbox maintenance repair and overhaul (MRO) capability.

That, at least, was the perception of German gear specialist ZF Friedrichshafen AG, three years ago, when its subsidiary ZF Services UK implemented a WT gearbox overhaul capability at its base in Nottingham, central England. UK executives felt that there was limited in-country repair and overhaul (R&O) provision at the time.

As David Morgan, business manager wind energy, told Renewable Energy Focus, “we had spent several months researching the market, discussing with operators the difficulties they face and the issues involved, so we knew the demand would be there. They weren't entirely happy with the service they were getting from the OEMs and, of course, they welcomed the idea of reducing their logistical and downtime costs. However, they did not all want the same service elements so we realised that a ‘one size fits all’ approach would not work. That's why we offer each client a service tailored to their specific needs.”

Overhaul and test facilities at Nottingham can handle gearboxes weighing up to 30 tonnes, used in wind turbines with ratings of up to 2MW but, as Morgan relates, capability for overhauling larger gearboxes may be added – as bigger onshore machines come into use and as the UK's Round 3 offshore programme takes off. Currently, though, the largest gearboxes are dealt with at sister ZF facilities in Dortmund, Germany, and Lommel, Belgium.

Although ZF is a gearbox manufacturer in its own right, its services arm is virtually autonomous and attends to gearboxes from a range of OEMs. As Morgan emphasises, “we are a multi-brand maintainer and work closely with most of the OEMs. At first there were some issues in terms of cooperation, but the majority of OEMs have since accepted the need for an independent maintainer in the market and we get a lot more support now. For our part, we use OEM spares and follow the recommendations given in their maintenance manuals.”

In addition to the gearbox, Morgan explains that the company overhauls all the ancillaries too. There is little point in going to great lengths to clean and flush the gearbox, he says, and then re-using the existing cooling matrix and pipes that will invariably be contaminated from the original failure. Keeping the gearbox oil clean has the biggest influence in prolonging times between repairs or overhauls.

Service can include gearbox removal, transport to and from the overhaul centre, and re-installation in the wind turbine affected, although ZF often works with a partner on the logistical aspects. Overhauled gearboxes are subjected to partial or full-load testing, the former in a fully-equipped test centre at Nottingham.

The tailored nature of the service provided is evident in the wide variation in contractual arrangements opted for by clients. Least sophisticated, and regrettably the most common at this stage of the industry's maturity, is the ‘fix-on-fail’ arrangement by which the maintainer is called out when a failure occurs. The client is billed on a time and materials basis for each event. This unexpected expense can be a shock.

As Morgan explains, “faults on the high speed side, such as the intermediate shaft and high-speed bearing, can often be dealt with by an up-tower working party, but if the problem is on the low-speed planetary side, you're generally looking at gearbox removal. This can involve high expense; it may cost anything from £90,000 to £160,000 to remove and subsequently re-install a gearbox for a multi-megawatt turbine.”

In the absence of a back-up gearbox that can be fitted in place of the faulty unit, the situation will be exacerbated by revenue lost while the failed gearbox is repaired or overhauled. Downtime can extend to several weeks. Alternatively, depending on the particular contract, the gearbox may be replaced by a spare unit held either by the client or by ZF Services. This can cut downtime to a few days if the necessary marine vessels (for offshore turbines) cranes and other assets, along with a favourable weather window, are available.

As Morgan adds, “we can hold a number of exchange ‘swing units’ as part of our commitment and, since these are virtually remanufactured gearboxes, they can be installed and left in service while the original units, once they have been similarly overhauled, join the spares pool ready for other clients.”

Another type of arrangement covers routine and scheduled MRO only. This can include a monitoring element so that ZF technicians can predict the best times for removing a gearbox that is known to require factory attention. They calculate the trade-off between maximising revenue time and minimising the eventual cost of overhaul.

Other contract types cover preventive maintenance including periodic on-site visits during which technicians will typically carry out visual and endoscope inspections, check vibration signatures using portable equipment and take an oil sample for analysis.

Over one aspect of this, David Morgan sounds a cautionary note: “Although many companies offer an endoscopic inspection service, understanding and interpreting the results is key. Here, depth of experience counts. It's easy to incur unnecessary costs by misdiagnosing and removing components too early. On the other hand, if tell-tale signs of wear or damage are missed, it could lead to a catastrophic failure.”

Condition monitoring

For sizeable wind turbines, monitoring the condition of key drive train elements and sending readings back to a central control room can pay dividends. Most significantly, it gives operators a chance to reduce machinery loads or take turbines out of service before a catastrophic failure can occur. Basic parameters such as wind speed, shaft speeds, bearing temperatures, power output and other electrical parameters can be measured and logged in a central database via a supervisory, control and data acquisition (SCADA) system.

However, more sophisticated techniques widely applied to rotating machinery in other sectors are gradually being adopted by the wind energy community. These include oil debris monitoring via chip detectors and vibration monitoring carried out using accelerometers mounted on bearings, gear housings and other key points.

Vibration signal analysis, in particular, holds much promise. Because most of the available diagnostic software to date has been written for machinery that rotates at constant speed and load – rather than with the variable loads and speeds that wind turbines experience – signal processing experts have had to develop statistical techniques that can overcome WT variability. Methods such as comb filtering, higher-order time and frequency domain analysis and kurtographic analysis have proved effective when applied to WT gearboxes.

One company that claims considerable success for an advanced turbine monitoring system it has developed is Danish firm Gram & Juhl A/S with its TCM solution. Tailored for wind and said to be “un-phased by variable and turbulent conditions”, TCM has proved able to detect cracks in gear teeth so early in their development that, in one instance, they could not be seen during close visual inspection and only revealed themselves when cleaning spray was applied. Engineers concluded that if it had been left, the fault might have led to a broken tooth which, in turn, could have wrecked the gearbox, replacement of which could have cost over half a million dollars.

The Danish firm has a collaboration with British company Romax Technology, in which the partners are working together to further advance the state of the vibration-based diagnostic art.

More progress in this specialist field is bound to follow, although this could be accelerated if data owners were less protective of their intellectual property and more prepared to share it for the common good.

Further reading

About: George Marsh Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.