Change of heading leads to DP incident

Comments from the report:

The vessel had been on DP for the whole day and in order to lift the ROV on deck with the crane, vessel heading had to be changed 15 degrees away from the wind.  This caused a high load on two running bow thrusters and the third bow thruster was started, selected to DP and successfully shared the load.  Increased windward force on the vessel due to high wind velocity acting on it as it changed heading, caused all the three bow thrusters to trip on overspeed with no DP or thrusters’ control system alarms!  Further investigation revealed the overspeed settings were too tight and had to be increased in accordance with manufacturer’s recommendation.

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

• It is not clear why the additional bow thruster was not put online before the heading change;
• A simulated alteration of heading and or referring to the vessel’s capability plots should have been considered to ensure that the heading change could have been performed within vessel’s DP capability;
• The overspeed trip of all thrusters during DP operations shows inadequate initial commissioning and full power testing during the FMEA proving trials and annual DP trials and following any modification;
• This incident further highlights the importance of equipment protection settings with respect to DP redundancy concept, in addition to equipment safety. DP FMEAs require to consider the effects of equipment protection system on the wider DP redundancy concept.  Reference Guidance on Failure Modes & Effects Analysis (FMEA) (IMCA M 166).