Watch Handover Reveals Suppressed PRS Alarm
At 06:00, a DP2 vessel in the 500 m zone of a semi-submersible rig conducts a DP watch handover. The vessel is holding about 80 m from the rig. The incoming DPO notices that only one DGPS and one laser position reference are active; the second DGPS is deselected. The ASOG display is green. The outgoing DPO says the second DGPS was 'noisy' and was deselected two hours ago, and that 'there was no need to change the ASOG because the laser is good'. The DP alarm history shows a 'position reference disparity' alarm that was suppressed. Wind is 15 kt, current 0.3 kt, and the vessel has two generators online with no thruster isolated. The rig is expecting a crane lift in 30 minutes.
1. What immediate actions should the incoming DPO take before accepting the watch?
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The incoming DPO should not accept the watch as a simple continuity handover. First, take positive control of the DP console and confirm the actual configuration: which PRS are selected, their quality, the ASOG status, thruster and generator status, and the alarm history. Second, inform the OIM and the rig that the DP configuration is under review and that the crane lift should be held until the review is complete. Third, assess whether the deselection was justified: check the second DGPS quality, the reason for the disparity alarm, and whether the laser alone is providing reliable position. If the second DGPS is healthy, consider reselecting it under controlled conditions; if it is not healthy, the configuration may be acceptable only with a revised ASOG and a risk assessment.
The suppressed alarm is a red flag. The DPO should not accept 'no need to change the ASOG' without verification. The ASOG must reflect the actual configuration and the worst-case failure. Until that is confirmed, the vessel should be treated as degraded.
2. How should the ASOG be assessed and changed, and who should be involved?
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The ASOG should be assessed against the vessel's DP configuration and the operation in progress. A green ASOG normally requires the approved redundancy, including the specified number and diversity of position references. Deselecting one DGPS reduces diversity and may change the worst-case failure case. The DPO should check the ASOG decision support matrix: if the current PRS configuration does not meet the green criteria, the ASOG must be changed to amber or red, with the corresponding operational limits. The change should be agreed with the master, the OIM, and the DP technical authority as required by the vessel's procedures.
The assessment should consider whether the laser and the remaining DGPS are truly independent, whether the laser has line-of-sight and good quality, and what happens if the laser fails. If the vessel cannot survive the loss of the laser, it is not in a green configuration. The ASOG change should be recorded, displayed, and communicated to the rig and deck crew. The crane lift should not proceed until the ASOG and configuration are aligned.
3. What follow-up is needed after the immediate situation is stabilised, including human factors and reporting?
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After stabilisation, the vessel should complete a DP near-miss report covering the unrecorded PRS deselection, the suppressed alarm, the incorrect ASOG status, and the inadequate handover. The report should identify why the outgoing DPO believed no ASOG change was needed and why the alarm was suppressed. The handover checklist should be reviewed to ensure that PRS changes, alarm suppressions, and ASOG changes are always recorded and communicated.
Human factors are central here: a two-hour-old configuration change was passed on verbally, without a written record, and the ASOG displayed a status that did not match reality. The vessel should reinforce the rule that any PRS deselection requires a risk assessment, an ASOG review, and a log entry. The DP alarm suppression policy should be checked; suppressing a disparity alarm without investigation is not acceptable. Finally, the OIM and company should be informed, and a DP drill or training session on configuration control and handover should be conducted.