Question 1

You've just arrived on the offshore field and are preparing to enter DP. The chief engineer calls and says he'd like to stay on closed bus for another two hours to complete a switchboard load test — he'll open the bus before you get into the risk zone, probably.

You need to enter the field and approach the installation. What is your response?

Accepted Answer

The bus must be in open configuration before you enter DP operations near the installation. "Probably open before the risk zone" is not an acceptable operating condition — a common cause failure that takes out both power sections while you're still approaching is a scenario the whole DP2 concept is designed to prevent. Coordinate with the chief engineer to open the bus now, or delay the approach until the switchboard test is complete IMCA DPE 01/23 specifically addresses closed bus as a known risk factor. The problem with "probably open before the risk zone" is that it introduces a time-critical dependency that may not be met — and if the switchboard test is still running when you're 500m from the installation, you're now in a difficult choice between staying on closed bus or interrupting the test. The correct approach is clean separation: complete the test, open the bus, then approach. Two systems competing for the same window always creates pressure to defer the bus opening. That pressure is the mechanism behind IMCA's documented 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.