Worn components create unwanted thrust
- DP Event
- Published on 25 August 2021
- Generated on 26 October 2026
- DPE 03/21
- 2 minute read
Incident
A worn key within the coupling of the step motor caused unwanted travel of the thruster pitch. The vessel was forced to return to port.
Comments
It was found that a worn key within the coupling of the step motor (providing feedback) caused unwanted travel of the thruster pitch. The vessel was forced to return to port to source a spare part, a ‘used but good’ part was located and fitted.
Considerations
- In most direct driven CPP systems there are some failures that can lead to uncontrolled thrust and the
emergency stop needs to be employed by the DPO, before the excursion becomes too significant. DPOs should not hesitate to use the emergency stop on thrusters if thruster alarms indicate a thruster control issue. - Use of a “second hand” part might be acceptable as a temporary stop gap measure, provided that the part is still within specification.
- This event highlights the need to annually check and test controllable pitch propellers.
IMCA M190 Chapter 4.0 Paragraph 4.10:
“There are many ways in which a variable speed thruster can fail, but the failure effects are generally safe. For vessels with variable speed thrusters of proven reliability, it may be acceptable to carry out the control loop wire break tests on a rolling programme where all tests are carried out over a five-year period. Controllable pitch propellers should be tested annually.“ - Although not related to the event, the number of references is inadequate. IMO MSC Circ. 645, IMO MSC.1 Circ. 1580, and IMCA M252, Guidance on position reference systems and sensors for DP operations guidance documents state that for DP 2 or 3 operations:
“There is a requirement for three independent position references based on two different principles to be simultaneously available during operation.”
Latest DP incidents
-
The domino effect
The event demonstrates the importance of maintaining the approved DP power configuration, addressing known defects before equipment is relied upon during DP operations, and ensuring DP personnel understand how changes to the generating arrangement can affect redundancy and failure propagation
DPE 03/26
19 October 2026
Incident
-
Lost in calibration
During activity associated with calibration of a vessel Vertical Reference Unit (VRU), the DP control system experienced degradation of its attitude reference arrangement. Shortly afterwards, the vessel generated an Off Position Warning followed by an Off Position Alarm
DPE 03/26
19 October 2026
Incident
-
Watch what you are pressing
While transiting away from a wind farm, the Master of a DP equipment class 2 vessel observed that the vessel was operating in Backup Mode and queried this with the bridge team and attending electrician
DPE 03/26
19 October 2026
Incident
-
DP drill scenario – Consequence Analysis Class DP2 & DP3
DP drill scenario – Consequence Analysis Class DP2 & DP3
DPE 03/26
19 October 2026
Drill Scenario
-
The UPS and downs
While a vessel was transiting in DP mode, the DPO observed the loss of Gyros 2 and 3 together with alarms from a 230V backup power supply. The telephone system remained powered but was inoperable.
DPE 03/26
19 October 2026
Incident
The case studies and observations above have been compiled from information received by IMCA. All vessel, client, and operational data has been removed from the narrative to ensure anonymity. Case studies are not intended as guidance on the safe conduct of operations, but rather to assist vessel managers, DP operators, and technical crew.
IMCA makes every effort to ensure both the accuracy and reliability of the information, but it is not liable for any guidance and/or recommendation and/or statement herein contained.
Any queries should be directed to DP team at IMCA. Share your DP incidents with IMCA online. Sign-up to receive DP event bulletins straight to your email.