KiwiRail believes propeller at fault in loss
02 December 2015 3:03PM
KiwiRail has completed its investigation in to the loss two years ago of a propeller from the Interislander ferry Aratere and believes the most likely cause was an irregularity in the propeller itself.
In an extremely rare event, the starboard propeller tail-shaft fractured when Aratere was sailing from Picton to Wellington on November 5, 2013. The ship continued safely on to Wellington and the propeller was subsequently recovered from the sea floor near Tory Channel.
The Transport Accident Investigation Commission investigated the incident and is yet to report on its findings. Independently, KiwiRail launched its own investigation.
“Our investigation was led by the international engineering consultancy firm Aurecon and was lengthy and exhaustive,” KiwiRail Group General Manager Network Services Todd Moyle says.
“It involved nationally and internationally-recognised experts producing reports which considered a number of possible scenarios and causal factors.
“The thoroughness of this investigation has enabled us to be confident in the conclusion of Aurecon’s Project Director Stephen Jenkins that all potential factors, other than an irregularity in the propeller itself, are unlikely to have caused this event.
“For example, we have ruled out an external event or a power surge as causes. Importantly, the investigation also found that the fitting of a new mid-section to Aratere in 2011 did not affect or compromise the propulsion system, which is in the rear section of the ship.”
That left possible potential causes being:
Fretting at the interface between the propeller shaft and propeller hub
Cavitation arising from one or more misshapen or out-of-balance propeller blades
Vibration due to misalignment of the various individual shafts which together make up the complete propeller shaft
Stress on the shaft from the weight of the propeller cone, which was designed to be larger than the equivalent cone on the original propeller
“While our investigation points to an irregularity in the propeller as the most likely cause of the failure, we note that the manufacturer has defended the propeller and does not agree with our conclusion.
“KiwiRail uses only reputable recognised suppliers for the critical engineering products that we purchase for our ferries and this experience means we will be even more vigilant in future.
“In this case we have reached a confidential agreement with the manufacturer which means that although they accept no liability for our loss, we nevertheless have confidence about our ongoing commercial relationship with them,” Mr Moyle says.
“Since returning to service Aratere, which is Interislander’s only specialised rail freight ferry, has been a workhorse of the fleet. She has a full schedule of sailings, regularly making six journeys a day across Cross Strait and she significantly contributed to the Interislander having its best-ever season last summer.
“Together, our three ferries currently make more than 4000 crossings of Cook Strait each year, carry more than 800,000 passengers, around 220,000 passenger vehicles, and more than 2 million lane metres of freight, including rail. The reliability performance against scheduled sailings for the past year was 99 per cent.”
“Kaiarahi recently joined the Interislander fleet and forward bookings indicate another successful summer season on the way,” Mr Moyle says.
Media ph 04 498 2038
Notes to Media
Background notes on the loss of Aratere’s propeller follow this press release
Please find a link to Aurecon’s Aratere Shaft Failure Investigation final report prepared by Stephen Jenkins, Aurecon Project Director. http://www.kiwirail.co.nz/uploads/Publications/Aurecon%20Report.pdf
A number of technical examinations and reports were commissioned as part of KiwiRail’s investigation in to the loss of the propeller. These reports, which form appendices to the Aurecon report, can be found on KiwiRail’s website:
Appendices A-C http://www.kiwirail.co.nz/uploads/Publications/Appendices%20A-C.pdf
Appendices D-G http://www.kiwirail.co.nz/uploads/Publications/Appendices%20D-G.pdf
Appendices H-L http://www.kiwirail.co.nz/uploads/Publications/Appendices%20H-L.pdf
On Tuesday 5 November 2013, one of the two shafts which drive the Aratere’s propellers broke when the ship was near Tory Channel. She was able to return to Wellington where divers confirmed that the tail section of the ship’s starboard propeller shaft had broken and the propeller had been lost. Aratere is fitted with equipment which monitors all mechanical systems. These systems provided no warning of imminent failure. The crew were not aware of anything unusual prior to the failure.
At the time, Aratere sailed three return daily crossings (one more than her sister ferries) and provided approximately 45 percent of Interislander’s fleet capacity.
Initially, Aratere freight was re-allocated to the remaining two ships and what could not be carried was re-booked onto Bluebridge vessels. Despite there being a limited number of roll-on, roll-off vessels able to use the Picton and Wellington berths, Interislander found and chartered Stena Alegra in three weeks and had her in operation in eight weeks. The repair work on Aratere was carried out at Singapore’s Keppel shipyard in conjunction with a routine two-year, dry-dock that was due.
The most significant work included replacing both port and starboard propeller tail shafts and bearings and realigning the shafting system. The original smaller propellers that had been on Aratere before her lengthening were re-fitted because new propellers would have taken up to nine months to procure and fit.
Aratere re-entered service in early July last year. Initially she struggled to maintain three return sailings a day. Mechanically and electrically Aratere ran well but her smaller propellers were less effective than the larger propellers they were replacing. However, after operating improvements and hull cleaning, Aratere resumed her busy schedule of three return crossings per day.
Following the incident, KiwiRail put in place a forensic investigation, conducted by national and international maritime experts to try to establish why the failure occurred. Their findings guided the process of repairing Aratere and trying to determine cause and responsibility. The Transport Accident Investigation Commission also initiated an investigation, and is yet to report its findings.
The investigation, led by the international engineering consultancy firm Aurecon, established, recorded and analysed the physical evidence and, after consideration of extensive expert technical reports and advice, KiwiRail’s conclusion was that the most likely cause of this incident was a mis-shapen propeller blade. It should be noted that the manufacturer of the propeller, does not agree with this conclusion.