Cyclone winds exceeded survival margins

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Wind farm owners in India are still counting the cost of the cyclone which swept through Gujarat in early June reducing swathes of wind turbines to piles of twisted wreckage. Over a third of the more than 300 turbines in the Lamba, Navadra and Bhogat wind farms along the Saurashtra coast were destroyed. Before the cyclone, their combined generating capacity was 85 MW.

"The area presented a staggering picture of wind turbines totally uprooted from the base, blades scattered around, generators on the ground," says wind consultant Dr A Jagadeesh, who travelled to view the scene of devastation from Nellore in Andhra Pradesh. Towers in many cases had crumpled, bringing nacelles crashing to the ground, he reports. Power lines linking the wind farm to the grid were still down last month.

Nonetheless, of 335 wind turbines, at least 206 survived the raging winds, which lashed the area for seven hours, knocking out railways, roads, 88,000 electricity poles and the entire port of Kandla. An estimate of damage caused to the property of the Gujarat Electricity Board (GEB) runs at INR 5 billion. Gujarat's prime minister, Keshubhai Patel, has described it as the "worst ever natural calamity in the history of the state." The cyclone hit the wind farms in the early hours of June 9.

There is heated discussion in India about the intensity of the storm, with accusations that the authorities contributed to the deaths of more than 3000 people because they did not issue a sufficient warning. The authorities say average winds during the storm were only 160 kilometres an hour, 45 m/s, according to the Business India news magazine. But independent assessments of the damage caused make it more likely that mean winds were in the 55-56 m/s range, with gusts up to 70 m/s.

Jagadeesh points out that a head count of the failed turbines reveals that machines made under licence from Danish manufacturers -- NEG Micon, REPL Bonus, AMTL Wind World and RRB Vestas -- were generally more prone to collapse, while those of German design -- Enercon, Tacke, TTG Husumer and BHEL Nordex -- weathered the storm well, with the notable exception of Südwind, (tables).

Tacke's Ulrich Schumakers says the four TW 250 units in the area -- designed to survive wind speeds of 65 m/s -- were unscathed other than damage to the nacelle covers and a blade hit by parts blown off other machines. Winds dropped stones into the nacelles and blew a container into the sea, he says. German over-engineering coupled with a robust design were the main contributors to the TW 250's survival, believes Schumakers. The Tacke units were not certified, unlike several models which failed in the storm.

Enercon lists a series of reasons for the survival of all 13 of its turbines in the region, including a tower shaped to gradually transfer extreme loads to the foundation, the ability of the model to turn out of the wind even after grid failure, the speed at which the turbines shut down, and the reduced loads of using lightweight blades and pitch control.

Unlike its countrymen, Südwind suffered badly with 24 of its 38 turbines knocked out of action. The S33 is certified by both German and Indian authorities. Once the grid was down, the yaw systems were unable to turn the machines out of the wind, says the company's Martin Kühl, though this might not have been an important factor. "During the cyclone the wind changed direction by as much 180 degrees in fractions of a second," he says. Loads on the turbines were over seven times greater than design loads, he says. Südwind has asked Germanisher Lloyd to report on the failures.

A major lesson from the catastrophe is that new standards have to be set for weather conditions in India, Kühl believes. "Standards for Europe shouldn't simply be copied by countries in other parts of the world."

Commenting on the total failure of all seven NEG Micon turbines, company director Jens Erik Kristensen says winds were so strong at the centre of the cyclone that nothing could have withstood them -- and the Micon units were directly in its path. All the machines were under one year old, but they were subject to loads way beyond their design criteria. "We are talking here of wind strengths which only happen once in the course of hundreds of years," says Kristensen.

Vestas' Johannes Poulsen echoes Kristensen's words, calling the cyclone "a terrible and atypical situation where so far we have found that winds were over the survival speed of 54-56 m/s the turbines were built to withstand." Vestas has subsequently raised its wind speed safety margin, he adds.

All 34 Bonus turbines in the area were destroyed, confirms the company's Henrik Stiesdal. He says that investigations carried out so far by Bonus support claims by the Jamnagar airport authorities of winds upwards of 64 m/s -- when the anemometer mast at the airport collapsed. These are winds way above anything expected in India, says Stiesdal. "Our investigations reveal that our turbines stopped as they should with their blade tips out when the cyclone hit. We have an idea what happened afterwards but the final conclusion must wait for our damage report."

It is not clear if the turbines which survived the storm did so because they were not subject to its full ferocity, or if powerful vortices, so called "twisters," hit some groups of turbines and not others.

Call for help

The Indian Wind Energy Promoters Association (WEPA) is calling for public money to help get the wind farms back up and running, says Jagadeesh. It has also issued a plea to insurance companies to settle claims without delay. At a meeting on July 27 in Baroda, Gujarat, WEPA called for all money invested in repairing the wind farms to be 100% tax deductible; for the pool of cash set aside by GSEB for a new power line from the wind farm to be used for restoring the destroyed 11 kV cabling; for GEB to compensate wind farm owners for their loss of income by crediting them with a sum equal to that earned in the same period last year; for GSEB to waive its bills for use of power lines; and for the Gujarat Energy Development Authority waive its land lease charges.

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