Question 1

Before commencing a DP2 subsea inspection task, you review the project seabed clearance chart. In current conditions (1.8m swell from NW, 22 knots), your vessel's expected DP footprint extends approximately 12m north of the nominal station position. The seabed exclusion zone for a subsea pipeline starts 9m north.

What must happen before you can commence the task?

Accepted Answer

Operations cannot commence until either: the station position is adjusted to create adequate clearance in all weather directions, or the conditions improve to reduce the footprint, or a heading change is agreed that shifts the footprint away from the exclusion zone — and this must be confirmed with the project team. A footprint overlap is not an operational risk to accept by monitoring more closely The footprint assessment exists precisely to prevent this — a subsea pipeline or cable sitting below the expected thruster wash or hull excursion path is at genuine risk of damage if the vessel drifts or manoeuvres. A 3m predicted overlap is not a monitoring margin — it is a miss distance of zero, as the prediction itself carries uncertainty. The correct response is to change the station position, heading, or wait for better conditions to reduce the footprint below the exclusion zone boundary. This is a go/no-go decision based on clearance geometry, not an ASOG alarm threshold decision. The project team must agree the revised position.

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.