Mix of absolute and relative position reference systems
- DP Event
- Published on 3 September 2019
- Generated on 16 June 2025
- DPE 03/19
- 2 minute read
Incident
Movement towards the drill ship may have been caused by a combination of PRS degradation and interaction from the drill ship’s thrusters.
Comments
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
- 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.
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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.
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