The bow thruster gave an alarm that pitch was at maximum showing 114% on DP display. Investigation concluded that the full starboard maximum limit command/set point of the bow thruster proportional valve had been adjusted to 70%. During maximum pitch request the valve had to be permanently opened with constant and continuous hydraulic oil flow through the proportional valve. It was concluded that because the valve direction flow was set at 70% this resulted in the valve’s malfunction and pitch stuck at maximum. It was later discovered that the azimuth thruster tripped because of sea water contamination of the gear box oil.
Considerations from the above event:
- Consideration should always be given to making heading changes in incremental steps, a heading change of 20° in one step during critical operations is not recommended.
- To stop adverse thrust from a rogue thruster it must be shut down and not just deselected from DP control.