On 28 November 2015, one of ten Bonus Energy 2.3MW turbines at the 14-year-old Paludan Flaks offshore wind project near the Danish island of Samso suffered a catastrophic failure; the nacelle and blades broke off and fell into the sea.
Catastrophic failure is widely defined as the complete, sudden, often unexpected breakdown in a machine or other system. This winter, there have been four such events reported across the wind industry.
With regular wind turbine service and maintenance and the possibility of sophisticated condition monitoring, how is such an embarrassing failure — appearing as it seems out of the blue — still possible?
At Paludan Flaks, the answer is still not clear.
The project owners — a group of municipalites and investors — are still negotiating over the consequences with German industry giant Siemens, which has the service contract for the Paludan Flak wind farm.
Siemens acquired Danish turbine manufacturer Bonus Energy in 2004, and the service contracts came with it.
A spokesman for Paludan Flaks told Windpower Monthly in early February: "I cannot comment on the situation because Siemens insisted on a non-disclosure agreement before it would begin negotiations with us on how the problems at the wind farm will be solved."
For its part, Siemens declined to comment on who will pay for the lost electricity output of the destroyed turbine.
Whether the machine will be replaced, and whether the tower can be re-used "is currently under clarification together with the customer", the German manufacturer said.
The remaining nine Bonus 2.3MW turbines at Paludan Flaks were inspected shortly after the incident, found to have no defects, and have been producing power since, Siemens added.
Siemens and the Secretariat for the Danish Wind Turbine Certification Scheme agreed that regular checks on the remaining turbines will be performed once a month until further notice, in line with the technical certification scheme for design, manufacture, installation, maintenance and service of wind turbines. But who pays for the extra inspections is also not clear.
Technical investigations are still under way into the root cause of the failure at Paludan Flak.
"Investigations so far have concluded that the tower top flange welding-geometry is not optimal, but it cannot be ruled out that the turbine was affected by abnormal load conditions, causing the crack in the tower to occur," Siemens said in early February.
An issue with the top flange on Bonus machines was first identified in 2003 at an undislosed site with extremely high and turbulent wind conditions.
This led, "after a thorough technical assessment", to a general update of the design of the top-flange configuration for new installations, as well as a retrofit programme for already installed turbines, depending on site-specific assessments.
In this respect, the Paludan Flaks wind farm was assessed as not being affected, Siemens said.
But after a crack in the tower top flange weld of a 1.3MW Bonus turbine was identified in January 2016, Siemens re-assessed the 2003 retrofit criteria using a more advanced calculation model. This assessment is still on going, the company added.
Under the review, Siemens launched an inspection campaign of about 750 Bonus turbines to ensure the potential risk is controlled, the company continued, without divulging the locations of these turbines.
The tower top flange is inspected visually by the local service staff operating the turbines, the company said. "It is not a major task and can be done quickly," said a spokeswoman for Siemens, declining to comment on the cost involved.
Why the company did not react earlier — considering the small outlay in time and effort for inspection, and experience with issues of the tower top flange retrofit dating back as far as 2003 — to amend other turbines with the same problem and prevent such potentially image-damaging catastrophic failure is not clear.
An incident at the Smola wind farm in Norway also gave the company some experience in the issues.
In October 2011, Statkraft technicians who carried out operations and maintenance work at the project noted a noise problem at Smola phase 1, comprising 20 2MW Bonus machines commissioned in 2002.
During additional inspections they discovered a crack in the tower top of one turbine and closed it down. Five of the other turbines were also found to be badly damaged and had to be repaired.
The repairs were carried out by Statkraft in cooperation with Siemens, Statkraft's vice president of communications for wind power and technologies, Torbjorn Steen, said in February.
Why the Smola incident did not trigger wider action by Siemens at that time is not clear.
Also in November 2015, the rotor fell off a Repower MD77-1500kW turbine in France. And in December, a Vestas V112-3MW turbine collapsed in Sweden, and a Suzlon S95-2.1MW turbine tower collapsed in Brazil.
None of the incidents resulted in injury.