Independent Expert Report on Windflow 500 Turbines
27 November 2009
Independent Expert Report on Windflow 500 Turbines at Te Rere Hau
As part of the process of seeking to obtain International Electrotechnical Commission (IEC) Class 1A Type Certification Windflow Technology Limited (WTL) has made various design changes to the Windflow 500 wind turbine generator (WTG). Seventeen (17) of these changes affect matters relevant to certification. Some of these changes, but not all, have been incorporated progressively into turbines supplied to New Zealand Windfarms Limited (NWF) for the Te Rere Hau windfarm (TRH).
An Independent Expert has provided WTL with a report of the assessed significance through calculation of the 17 design changes and in particular whether by not incorporating them there is a materially increased risk the WTG’s will not operate properly at TRH.
The Expert has concluded from information provided by WTL (including calculations and design modelling) that:-
• One (1) proposed modification is no longer found to be necessary.
• In the case of seven (7) modifications, on recalculation the unmodified design is considered to be suitable for IEC Class 1A applications.
• In the case of a further three (3) modifications, the unmodified design is considered to be suitable for use at TRH without special measures.
• In respect to the remaining modifications, specific comments follow:-
Forty-four (44) WTGs at TRH have blades that are of a butt jointed construction. Two blades of this construction failed during IEC fatigue tests undertaken in 2007. A redesigned blade has passed the IEC fatigue testing and this redesign has been incorporated in all subsequent WTGs supplied. It has now been established that the original test blades which failed were loaded to a considerably higher level than was intended or calculated at the time. The Expert has determined that at the test load actually applied, the cycles required to pass the type test were actually exceeded in both tests.
This outcome greatly reduces the degree of concern regarding these blades although there is still a margin of uncertainty about the design validation for the blades due to the somewhat limited information about the material properties of the butt-jointed laminate. On the basis of the new calculations the Expert considers that these blades will not now be expected to fail but recommends that the blades of this type be given a visual inspection at six-monthly intervals in the region where the test blades failed. The estimated cost to replace a single blade is in the order of NZ$40,000 so the latest analysis of the original test results is a welcome outcome for the project.
The Expert has determined that failures of taper roller bearings and plain spherical bearings in the pitch mechanism are likely at some stage during the turbine life if the pitch bearing friction is as assumed in the calculations and in accordance with the theoretical model widely used by bearing manufacturers. In service measurements from the Gebbies Pass Windflow 500 prototype suggest actual friction is lower than assumed. If this is normally the case with the WTGs at TRH the bearings could all survive the entire life. As this cannot be guaranteed a monitoring program has been recommended for the 49 affected WTGs and the estimated cost of having to replace one bearing assembly with an upgraded assembly is in the order of $16,200 per WTG providing suitable tooling and methods are developed and practised.
35 WTGs have unmodified gearbox casing feet that could be at risk of distortion under rare extreme load events. These 35 WTGs also have a bolted flange joint within the gearbox that is at risk under cyclic loading. Some risk of failure of the brake adaptor on these WTGs has also been identified, but any such failure would have safe consequences. Remedial modifications have been identified and the Expert considers these could be incorporated at an estimated cost of $3,100 per WTG.
The Expert has identified that the mid-height welds around the 5 Batch 1 WTG towers are at some risk of fatigue damage and has recommended that a welding specialist should be sought to develop a suitable monitoring programme. Repairs have been estimated at an approximate $2,000 per WTG.
The Expert has estimated the total liability that would arise if all the identified issues resulted in a requirement to repair (or a decision was made to implement a pro-active retrofit, where applicable) to be approximately $966,200 (excluding blades or any consequential failure).
Inspection costs to monitor these potential risks are estimated at around $12,000 per annum when such work is performed in conjunction with scheduled maintenance.
To summarise, the Expert considers that the overall risks to the project are addressed through calculation or may be mitigated through monitoring, but acknowledges that there may be financial implications associated with some of the above issues and the wind farm owner may wish to consider how such potential costs might be covered.
A copy of the Expert’s report has been made available to NWF. NWF has advised WTL that it is largely satisfied with the report. WTL looks forward to working with NWF to resolve the issues identified above.