Mix of absolute and relative position reference systems – DP incident

Comments from the Report:

It was considered that the movement towards the drill ship could have been caused by a combination of position reference system degradation and interaction from the Drill ship’s thrusters.  There was an advisory regarding service disruption from the DGNSS supplier in force at the time.  It was stated that the DPO initially selected ‘present position’ to remove any doubt that he/she had entered an incorrect step movement for a change of position.

It was further noted that selecting ‘present position’ is not following DP watch-keeping procedures and is a fundamental error.  Selecting ‘present position’ resulted in the thruster demand being reset with the momentum of the vessel still being in the same direction

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

• Operating on DP alongside a mobile platform which also provides the reference(s) for the PRS needs to be planned carefully. It is recommended that relative PRSs are used.
• Surface movement of the drill ship, position moves, and thruster interaction should be carefully considered during operational planning and in the decision support tool (ASOG).
• IMCA have received a number of reports this year where it has been noted that the DP Operator selected the ‘present position’ button as the best option for halting an unplanned move. It should be noted that this action resets the wanted position and therefore does not assist the DP System to maintain the original wanted position.