Minimising fire risk in wind turbines

You need three things to start a fire: fuel, ignition and oxygen. And you can find all three of them in ample quantities within the nacelle of a wind turbine.

A 1.5MW machine, on the small side by today's standards, can still contain up to 900 litres of lubricating and cooling oil. The nacelle itself, probably made with flammable fibre-reinforced plastic (FRP), will house acoustic insulation materials, which are also flammable. Ignition can be provided by faulty electrical and electronic components and connections, or overheating mechanical parts. And high winds, the reason the turbine is there in the first place, can be guaranteed to feed the spark and fan the flames.

Once a fire takes hold there is practically nothing that can be done to prevent the turbine's complete destruction. The remote location of many wind projects means that fire-fighting services are often slow to arrive the scene, while the nacelle's height rules out any meaningful action to dowse the fire. The best that can be expected is that burning debris is prevented from starting ground fires.

On the plus side, catastrophic turbine fires are rare - although how rare is the subject of some dispute. Insurance underwriter GCube says that from its global portfolio of more than 30GW it expects three or four total turbine losses, usually caused by fire, in the course of a typical year. Assuming an average nameplate capacity of 1.5MW that equates to roughly one turbine in 7,000 going up in flames a year.

Daniel Kopte, expert for safety systems, renewables certification at DNV GL, estimates that worldwide 120 wind turbines suffer fire damage (not necessarily causing total loss) over the course of a year. Again assuming an average nameplate capacity of 1.5MW, that points to around one in 2,000 turbines sustaining fire damage in a typical 12-month period. The ratio is higher than GCube's because it includes damaged as well as totally destroyed turbines and, probably, a larger proportion of older machines operating in markets with less stringent operations and maintenance regimes.

There is no arguing, however, that turbine fires are expensive. "A 2MW turbine costs more than £2 million (€2.8 million) and generates an estimated income of more than £500,000 a year," says Kopte. Offshore turbines — bigger, more complex and considerably harder to repair or replace — will incur much higher costs in the event of fire.

Standards and guidelines

Various standards and guidelines for wind turbine fire protection and prevention systems currently apply. In Europe the most important is section 1.5.6 of the 2006/42/EC machinery directive, with which wind turbines must comply. It states: "Machinery must be designed and constructed in such a way as to avoid any risk of fire or overheating posed by the machinery itself or by gases, liquids, dust, vapours or other substances produced or used by the machinery."

"Taken literally, it is extremely hard to design a machine that will avoid any risk of fire," says Jamie Scurlock, head of turbine technology at global wind developer RES. "But this consideration should certainly ensure the designers think of all possibilities and eventualities."

According to Scurlock, industry standards do not state specifically how something should be designed, or lay down rules for achieving adequate protection from hazards that may impact personnel safety or asset value. "Fire safety is no exception," he says. "But there are various safety guidelines, which we have to adhere to, that set out minimum standards." A further complication for designing products that will be delivered to a number of markets is that they must also comply with differing local regulations.

Does the level of fire protection and prevention offered have any impact on a developer's choice of wind turbine for a particular project? "We have experience of many different turbine technologies, and no particular design is more susceptible to fire than others," says Scurlock. "But it is important for us to be aware of how the manufacturer has addressed the risk of fire in its design. This is commonly achieved through a design risk assessment. Our contracts are written to ensure that we have visibility of the risk assessment, and the resulting residual risks on request. Once this safety aspect has been addressed, it allows us to choose the most appropriate turbine for the project to meet the asset requirements."

Compliance testing

In March, DNV GL issued its SE0077 certification of fire protection systems for wind turbines, a replacement for a "technical note" issued in 2009 by Germanischer Lloyd (now part of DNV GL). This stresses the importance of turbine manufacturers using pre-approved components and systems for fire protection and prevention, from smoke and heat detectors to control and indicating kit.

"For a statement of compliance of a fire protection system an accepted independent testing institution performs the following steps," says Kopte. "The components have to pass different tests in laboratories of a member the European Fire and Security Group (EFSG). The system of those components also has to be tested, and the installer needs to be approved to ensure that the system works as it is supposed to do after installation. DNV GL checks for completeness before issuing the statement of compliance.

"For a fire-protection type certificate, DNV GL performs an assessment of protection class, analysing possible fire risks," says Kopte. It will then check the system's integration into the turbine, followed by inspections and function tests.

Scurlock highlights the role that wind-asset operators should play in preventing turbine fires and maximising personnel safety. "In general, the risk of fire is minimised by good design, and the incorporation of suitable protection systems such as arc detection and suppression," he says. Appropriate means of detection, such as smoke and temperature sensors, and cooling systems, and sensible sanctions such as turbine shutdown or reduced operation, plus alarm triggering and notification when temperatures exceed pre-defined limits are also vital. Remote monitoring and switching can further reduce exposure to hazards, for example using an umbilical cable to operate high voltage switchgear from a safe distance.

Swift response

"We take these residual risks very seriously, and on a few occasions have highlighted that it would be reasonably practicable to modify the protection and alarm systems to improve personnel safety and reduce risk to the asset," says Scurlock. "For example, RES identified there was a risk of fire in the tower base of one design due to the type of equipment located there. In our opinion, it would have been better protected by a smoke detector that linked to an alarm in the nacelle, warning any personnel of the hazard at entry level, enabling them to select the alternative escape route from the nacelle. The manufacturer agreed and incorporated this modification into its standard design."

"Hot work" — maintenance inside the nacelle — can lead to turbine fires, sometimes several hours after the work has been completed. A swift and coordinated response, with the safety of people uppermost in mind, is key, says Scurlock.

"In any emergency, our response combines the control-centre team, who monitor plant and personnel around the clock, highly professional site-focused managers, and an expert health, safety, quality and environment (HSQE) department," he says. "They follow a detailed and meticulously designed suite of procedures that can pinpoint personnel and coordinate evacuation, if required. Testing these responses with customers, contractors and emergency services hones the procedures and prepares staff for any eventuality."

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