Question 1

You're DPO on a DP2 construction vessel. Suddenly, Gyro 1 shows a heading of 187° while Gyros 2 and 3 both show 179°. The DP system has not yet flagged an alarm — the three-gyro average is 181.7° and the system appears to be taking a weighted average.

What is your immediate action?

Accepted Answer

Immediately exclude Gyro 1 from the DP input manually. The two remaining gyros agree at 179° — Gyro 1 has clearly jumped. Waiting for the DP system to raise an alarm risks the system applying a weighted average that introduces a heading error into the DP control loop, causing an unintended position command. Inform the master and the chief engineer to investigate Gyro 1 Some DP systems use median voting (which would correctly reject the outlier), but others use weighted averaging — and the 8° error from Gyro 1 can produce a meaningful heading error in the DP output even with averaging. More critically, "waiting for the alarm" is exactly the passive approach that allows the erroneous input to affect DP performance before corrective action is taken. An 8° heading discrepancy with two gyros agreeing at 179° is unambiguous — the DPO knows which reading is wrong. Manual exclusion of the faulty gyro is the correct proactive action. The system alarm is a backstop, not the primary response mechanism.

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.