Question 1

The master switches to manual joystick. At the moment of switching, the position display jumps 4m northeast — the DP auto had been applying a correcting force that was masking the true position. The vessel is now manually controlled at approximately 116m from the rig. The Fanbeam is still tracking.

What does the 4m position jump reveal, and what is the immediate manual control priority?

Accepted Answer

The jump reveals that DP auto was masking a position offset — the vessel was not where the display showed it to be. The manual control priority is to use the Fanbeam and DGNSS position data to understand the vessel's true position relative to the rig and stabilise it under manual control, treating the DP auto's previous position output as potentially unreliable A position jump on switching from DP auto to manual typically means the auto system was applying forces to hold against an erroneous set point — when it releases, the vessel's true position becomes visible. This is a critical revelation: the DP auto system had been compensating for its own heading error by holding a position offset, masking the error from the position display. The manual priority is now genuine situational awareness: what does the Fanbeam show for true vessel position? Where is the rig? Is the current position safe? Do not re-engage DP auto until the gyro situation is fully resolved. The Fanbeam data (line of sight still active) and DGNSS provide the most reliable position cross-check at this moment.

Question 2

EMERGENCY SCENARIO — DP2 CONSTRUCTION VESSEL NEAR DRILLING RIG

Vessel: DP2 offshore construction vessel. Location: 120m from a mobile drilling rig, conducting pre-lay survey. Riser and anchor spread deployed from rig. Three gyrocompasses active. Two DGNSS units and one Fanbeam active (Fanbeam tracking reflector on rig). Sea state: 1.5m, wind 18 knots from the north. DPO on watch, approaching end of 10-hour watch. Master in cabin.

--- POINT 1 OF 10 ---

Gyro 2 suddenly reads 201° while Gyros 1 and 3 both read 187°. The DP system shows no alarm — its weighted average heading is 191.7°. The DPO notices the discrepancy on the heading display.

What is the correct immediate action?

Accepted Answer

Manually exclude Gyro 2 from the DP heading input immediately — two gyros agree at 187°, Gyro 2 has clearly jumped. A 4.7° heading error introduced by the weighted average will cause the DP system to apply incorrect thruster forces and may drive the vessel off station toward the rig If the DP system uses weighted averaging rather than true median voting, a 14° gyro jump on one unit produces a meaningful error in the heading output. A 4.7° heading error at 120m from a rig can produce a meaningful sideways force command. More critically, waiting for the DP alarm means accepting the erroneous input for however long the alarm takes to trigger. The DPO has already identified the problem — two gyros agree and one has clearly jumped. The correct action is to manually exclude the outlier immediately, inform the master, and arrange investigation of Gyro 2. The alarm is a safety net, not the trigger for action.

Question 3

Gyro 2 has been excluded. Gyros 1 and 3 are now the only heading inputs. The chief engineer is investigating Gyro 2 and expects a 45-minute assessment. The DP system is now running on two gyros. Position is stable.

With only two gyros now available, what additional operational consideration must the DPO apply?

Accepted Answer

With only two gyros, if Gyros 1 and 3 diverge, there is no way to determine which is correct. The DPO must increase monitoring frequency for heading discrepancy, inform the master of the degraded heading reference status, consider whether the ASOG requires a minimum of three gyros for this operation, and be aware that any future heading divergence between the two remaining gyros will require an immediate decision about whether to continue operations The third gyro is not a luxury — it is the tie-breaker. With three gyros, a divergent reading can be identified and rejected. With two, if they disagree, the DPO has a 50/50 problem with no way to resolve it from the heading data alone. The master must know the heading reference has degraded. The ASOG may specify minimum gyro requirements for different operation types — near a rig with a deployed anchor spread is exactly the type of sensitive operation where three-gyro minimums are commonly set. Running on two gyros 120m from a rig is not automatically acceptable: it requires a formal ASOG status reassessment.