Routine maintenance leads to loss of thruster – DP undesired event

Comments from the report:

During a routine weekly maintenance operation, a ships engineer took an oil sample from the starboard azimuth thruster sample point, which is a small valve located on the line to the pressure switch.  The location of the oil sample point on the line to the pressure switch creates a risk of inadvertently activating the switch if the sample valve is opened too much.  When obtaining the sample, the valve was opened fully, reducing the pressure to the pressure switch.  The pressure switch then tripped causing the thruster to stop; the oil pressure in the thruster was maintained and not at risk.  Once the sample valve was closed the pressure on the line to the pressure switch returned to normal.  The starboard azimuth was started back up in DP and, following a short investigation, pre-dive checks were carried out and the bell deployed to continue diving operations.

Considerations of the IMCA Marine DP Committee from the above event:

• The loss of redundancy should have initiated a yellow alert but this was not in the report.
• Planned maintenance of critical equipment should be controlled by a permit to work (PTW) system.
• It appears that during the event, co-operation between the bridge, engine room & diving departments was good.
• The event highlights a poor design, for example the sample point could have been down stream of the oil pressure sensor and in this situation, it would seem reasonable for an alarm to be triggered rather than the thruster shut down.