Question 1

Your vessel has been operating for two days with Thruster No.4 restricted to 60% output due to a mechanical governor fault. The capability plot displayed on the DP console and used in the ASOG was generated pre-voyage at 100% thruster availability. The master says 'we checked the capability mentally and we think we\'re fine.'

What is the correct position on this?

Accepted Answer

The capability plot must be recalculated to reflect the actual current thruster configuration — a plot generated at 100% availability is incorrect and potentially misleading when used to make ASOG go/no-go decisions with a 40% reduction on one thruster. The updated plot must be used to verify that DP2 capability is still maintained and to confirm any revised weather limits The capability plot is the formal quantitative basis for ASOG weather limits. Operating against a plot that reflects 100% availability when actual availability is materially reduced (40% on one thruster) means the ASOG limits may now be set above the vessel's true capability. "We think we're fine" is not a recalculation. Modern DP systems and capability plot tools (e.g., DP Capability Plot software) can incorporate degraded thruster scenarios — this is exactly the scenario they are designed for. The updated plot must be run, the revised limits confirmed, and the ASOG amended before operations continue. IMCA DPE 02-24 specifically cited outdated capability plots as a contributing factor in incidents.

Question 2

You're on DP2 watch during a cable survey operation. The ROV pilot requests you shut down the port aft azimuth thruster because its cavitation noise is interfering with the multibeam sonar. The thruster is currently not contributing significant thrust in the prevailing conditions. You shut it down.

What critical DP risk has this action introduced?

Accepted Answer

Shutting down a thruster changes the Worst Case Failure and may remove the vessel from DP2 compliance — the remaining thruster configuration must now be consequence-tested to confirm DP2 capability is maintained, and the ASOG status must be reassessed before operations continue Even a thruster contributing little current thrust is part of the vessel's redundancy configuration. Shutting it down may change which thruster now becomes the WCF, and the consequence analysis result for the new configuration has not been verified. IMCA 2024 incident data specifically recorded cases where DPOs deactivated thrusters in response to operational requests (noise, vibration, ROV interference) without reassessing the WCF impact — with some subsequently operating outside DP2 class capability without knowing it. The ROV team's operational preference cannot override the DP2 redundancy requirement without a formal reassessment and ASOG review.

Question 3

During a DP2 anchored-spread mooring operation, your two DGNSS systems begin showing positions that diverge by 4.2 metres and are moving further apart. The Fanbeam is not available at this worksite. Both DGNSS units show normal signal strength and differential correction status.

What is your immediate decision process?

Accepted Answer

With two diverging DGNSS units and no third reference, you cannot determine which is correct. Immediately inform the master, assess whether operations can safely continue with degraded reference confidence, and consider whether an additional reference source (taut wire, radar range/bearing, another DGNSS unit) can be deployed or activated before the divergence worsens Two reference systems that disagree cannot be arbitrated without a third — you simply cannot know which is correct. Averaging two potentially erroneous positions may drive the vessel toward the error. This situation directly mirrors the gyrocompass argument for having three units: with only two, a disagreement produces a binary uncertainty with no way to resolve it. The correct response is to treat this as a reference degradation event, inform the master, and seek a third independent reference before the situation develops. Operating near a mooring spread with an unresolved 4.2m reference discrepancy that is growing is a significant risk.