Comments from the report:
The vessel was operating in shallow water and a strong current. There were several heading changes prior to the event. It was noted that heading changes, particularly in strong current should be completed at very low rates of turn. In such a situation the report commented that the vessel position should be independently monitored, such as by a survey screen, to ensure the required position setpoint remains as required.
Other actions taken:
- CAMO updated, maximum Rate of Turn (ROT) & maximum allowance for heading change amended;
- Heading change procedure generated; inc position of personnel, drillers to be evacuated from Rooster box, checks to be performed and communication protocols;
- 6 hourly checklist revised: offsets of DGNSS added & tick box added for survey position;
- Revised DP standing orders: Offset of DGNSS to be checked after rebooting OS PC. Compare survey position with DP all times during operations.
Considerations of the IMCA Marine DP Committee from the above event:
- There is a lack of information available within the report however, it states that the drill string snapped 50 minutes after an “incorrect button” was pressed. It also appears that the ROT used for heading changes was too high for the operation and the offsets for the DGNSS were incorrect. Therefore, it is assumed that there was a position excursion.
- Following the installation or relocation of position reference sensors, offsets should be permanently changed in the controllers. On at least one DP control system, position reference system offsets can be changed at the operator stations but revert to the original setting when the controllers are reset. Offsets should therefore be verified as part of operational planning.
- Rapid heading changes in such environmental conditions may destabilise the DP model resulting in unpredictable behaviour with significant consequences. Heading and position changes should be carried out in small increments and with adequate waiting time to enable the model to stabilise between increments.
- The use of three DGNSS is an over reliance on one PRS operating principle (DGNSS) at the expense of the other single PRS.
The following case studies and observations 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 DP technical crew in appropriately determining how to safely conduct their own operations. Any queries should be directed to IMCA at [email protected]. Members and non-members alike are welcome to contact IMCA if they have experienced DP events which can be shared anonymously with the DP industry.
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