Comments from the Report
The UPS and the DGNSS source does not fall under the vessel’s planned maintenance system (PMS), as this is owned by the survey company. The equipment is maintained and tested before it is installed on a vessel. While the testing of signal dropout and UPS function is part of the vessel’s DP checklists, there is no electrical maintenance or routine testing on the specific UPS while in service onboard; the backup strategy is to keep a spare UPS onboard.
Redundancies worked as designed; no critical situation developed. Drilling operations suspended purely due to loss of redundancy.
An additional checklist to routinely test the capabilities of the UPS was developed and implemented as part of the vessel PMS.
Considerations of the IMCA Marine DP Committee from the Above Event
- The online position reference systems (PRSs), 3 x DGNSS, do not provide adequate redundancy according to industry guidance;
- Considering the time taken to select the taut wire it appears ‘stand-by’ meant that the depressor weight was still on deck and not deployed;
- Reference should be made Guidance on operational activity planning (IMCA M 220);
- The report indicates that two of the PRSs were supplied with power from a UPS that was not designated for the DP system;
- The DP FMEA should be revisited to verify the effects of failures of the DP UPSs and the power distribution supplying the UPSs. It should be verified that the arrangements, the UPS endurance and the PRS set up for the industrial mission are in accordance with the DP Class 2 requirements;
- DP equipment should not be repaired and brought online after repair without being suitably tested;
- A proper assessment should be made, and MoC procedure followed when connecting third party equipment to the DP system;
- Reference should be made to Guidelines on the shared use of sensors for survey and positioning purposes (IMCA M 235).