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What happened

On 9 February 2017, the passenger cruise ship Norwegian Star departed Melbourne, Australia, on a scheduled cruise to Dunedin, New Zealand. There were 2,113 passengers and 1,017 crew on board. On departure, the starboard propulsion unit was operational following its recent repair, and the port propulsion unit was under repair following its failure on 24 January. This meant that the ship was operating only with the starboard propulsor.

At about 0134 on 10 February, Norwegian Star was about 22 NM south-west of Cape Liptrap, Victoria, Australia, when the starboard propulsion unit failed. Propulsion could not be restored and tugs were deployed from Melbourne to tow the ship back to Melbourne. The ship arrived back without further incident at about midnight on 11 February.

What the ATSB found

The ATSB found that Norwegian Star lost function of its starboard propulsion as a result of the failure of the recently repaired starboard exciter. The configuration of the exciter unit had been modified as part of its repair, and the unit failed as a result of an error in the design of the modification. Insufficient clearance had been provided to allow for movement and thermal expansion of components during the unit’s operation. This error was not detected during the design, installation and trialling of the modified exciter unit.

The need for the repair of the starboard exciter unit had followed its failure on 11 December 2016. The failure of the unit in its original (as-built) configuration, and the subsequent failure of the original port exciter unit on 24 January, were both probably due to a breakdown in electrical insulation. Processes for the monitoring of exciter unit condition were ineffective in detecting deterioration prior to unit failure.

The ship operator’s decision to sail from Melbourne with only the starboard propulsion unit did not breach any regulatory requirement and was based on confidence in the reliability of the repaired starboard unit. While the repaired exciter was of proven design concept, the detailed design specific to this propulsion system application had not previously been used in-service.

What's been done as a result

The ATSB has issued recommendations to the ship operator Norwegian Cruise Line Holdings and the equipment manufacturer ABB Industry Oy to review the processes for monitoring the condition of brushless exciter units in Azipod installations, considering the mechanism of failure of the port and starboard brushless exciter units on Norwegian Star.

Safety message

Operation of newly designed equipment without redundancy increases operational risks. Equipment manufacturers and ship operators must apply extra diligence when designing, installing and operating modified equipment, especially safety critical equipment.

The passenger cruise ship Norwegian Star

The passenger cruise ship Norwegian Star

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The occurrence

Context

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Preliminary report published: 27 April 2017

The information contained in this Preliminary report is released in accordance with section 25 of the Transport Safety Investigation Act 2003 and is derived from the initial investigation of the occurrence. Readers are cautioned that new evidence will become available as the investigation progresses that will enhance the ATSB's understanding of the occurrence as outlined in this Preliminary report. As such, no analysis or findings are included in this report.

What happened

On 9 February 2017, the passenger cruise ship Norwegian Star departed Melbourne, Australia, on a scheduled cruise to Dunedin, New Zealand. There were 2113 passengers and 1017 crew on board. On departure, the starboard propulsion unit (Azipod[1]) was operational and the port Azipod was under repair.

At about 0134 on 10 February, the vessel was about 18 nautical miles south-west of Cape Liptrap, Victoria, when the starboard Azipod failed. Propulsion power could not be restored and two tugs were deployed from Melbourne to tow Norwegian Star back to Melbourne. The vessel arrived back without further incident at about midnight on 11 February 2017.

What the ATSB has found so far

Based on the preliminary information, the ATSB found that the Norwegian Star experienced three separate propulsion unit failures over a period of about nine weeks. In each case, the field exciter unit for the main propulsion motor failed. The first two failures (the starboard unit in December and the port unit in January) involved a breakdown of electrical insulation and the third failure (on 10 February 2017) related to a modification made to the starboard Azipod exciter unit during its earlier repair.

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  1. Azipod is the registered trademark of ABB Oy (Finland).
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The occurrence

Context

Investigation direction

Sources and submissions

 

Safety issue

MO-2017-003-SI-01 -  

Processes for the monitoring of the exciter unit insulation resistance

The processes for monitoring the condition of the brushless exciter units’ electrical insulation were ineffective in detecting deterioration prior to unit failure.

Safety issue details
Issue number:MO-2017-003-SI-01
Who it affects:Those responsible for maintaining Azipods
Status:Not addressed

 
General details
Date: 10 February 2017 Investigation status: Completed 
Time: 0134 EDT (UTC +11 hours) Investigation phase: Final report: Dissemination 
Location   (show map):Bass Strait, 22 nautical miles south-west of Cape Liptrap Investigation type: Occurrence Investigation 
State: Victoria  
Release date: 24 May 2017  
Report status: Final Occurrence category: Incident 
 Highest injury level: None 
 
Vessel details
Operator: Norwegian Cruise Line, Bahamas 
Vessel: Norwegian Star 
Flag: Bahamas 
IMO: 9195157 
Sector: Passenger 
Type of Operation: Passenger/Cruise 
Damage to Vessel: Substantial 
Departure point:Melbourne, Victoria
Destination:Dunedin, New Zealand
 
 
 
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Last update 24 May 2018