Question 1

The investigation finds that the USBL dropout was caused by increased thruster cavitation noise as the vessel corrected a slow northward position drift earlier in the watch — the noise masked the acoustic signal. The DGNSS Unit 2 failure was an unrelated UPS fault. The ASOG did not specifically address USBL vulnerability to high-thrust conditions.

What systemic gap does this investigation reveal, and what must be updated?

Accepted Answer

The ASOG for diving operations must be updated to include: thruster demand thresholds at which USBL acoustic quality must be actively monitored, contingency procedures when USBL quality degrades under high-thrust conditions, and a minimum-reference abort criterion specific to diving operations. The USBL-thruster interaction is a known vulnerability that should have been addressed in the ASOG and was not — this is a systemic SMS gap USBL acoustic dropout under high-thrust conditions is a documented and known failure mode — IMCA M103 and IMCA M140 specifically describe it. Its absence from the ASOG is a procedural gap, not bad luck. An ASOG written by people who knew about USBL-thruster interaction would have included a thruster demand ceiling for USBL-sensitive operations, or a monitoring requirement for USBL signal quality during high-thrust events. The DGNSS failure was indeed separate — but the combination of the two created the critical situation. The lesson is not to address each failure independently but to ensure the ASOG is resilient to the combination of likely degraded conditions, not just isolated failures. This is the systems-thinking requirement in DP ASOG development.

Question 2

EMERGENCY SCENARIO — DP2 DIVING SUPPORT VESSEL

Vessel: DP2 DSV, saturation diving system. Location: Subsea pipeline repair, North Sea. Diver A at 92m depth, on umbilical, working on a valve assembly. Diver B on standby in bell at 92m. Bell is deployed on umbilical from crane. References: DGNSS x2, USBL (acoustic, tracking bell transponder). Sea state: 1.2m, wind 14 knots NW. Three gyros all active and agreeing. Position very stable. DPO on watch.

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The USBL unit tracking the bell transponder suddenly drops out — no position data from the acoustic reference. Only DGNSS x2 remain. The USBL display shows 'No Fix.' Diver A is still working on the valve and the dive supervisor is monitoring via CCTV. Position is still showing as stable on the two DGNSS units.

What must the DPO do immediately?

Accepted Answer

Immediately inform the dive supervisor of the USBL dropout, place the operation on heightened alert, investigate the cause of the USBL dropout, and assess whether operations can continue on two DGNSS references alone per the ASOG — the dive supervisor must be told because they may need to decide whether to hold or recover Diver A based on the degraded reference status With a diver at 92m on umbilical, every reference system degradation is simultaneously a diver safety event. The USBL is not merely a DP reference — it is also the primary means of tracking the bell position relative to the vessel. Its dropout immediately changes the information picture for the dive supervisor, who must know. Attempting to restart USBL while Diver A continues working on the bottom defers a safety communication that the dive supervisor needs to make an informed decision. Reducing alert radius does not compensate for a lost reference — it just means smaller excursions trigger the alarm. The ASOG will specify minimum references for diving operations; dropping below that minimum requires immediate ASOG status reassessment.

Question 3

USBL has not recovered. Now, five minutes after the USBL dropout, DGNSS Unit 1 and DGNSS Unit 2 begin to diverge — Unit 1 shows the vessel's position 3.1m south of Unit 2 and the gap is slowly increasing. The dive supervisor has been informed of the USBL dropout and Diver A has been told to hold position at the valve.

With the USBL down and DGNSS units now diverging, what is the DPO's assessment?

Accepted Answer

With USBL already lost, the two DGNSS units diverging are the only remaining references — and a divergence of 3.1m that is increasing means the DPO cannot verify which unit is providing the correct position. This is now a critical reference degradation with a diver at 92m. The dive supervisor must be immediately updated and told the vessel now has no way to verify its true position. Consideration must be given to initiating Diver A's ascent while position is still believed to be acceptable The cascade is developing: first USBL (the acoustic bell-tracking reference) drops out, now the two remaining position references are disagreeing. At this point, the vessel has no verified position. This is the worst possible moment for reference degradation — 92m diver on umbilical, bell deployed, the vessel cannot confirm it is still directly over the work site. The 3.1m divergence is not "within GPS accuracy" in the context of a diver on umbilical — the combined uncertainty of two disagreeing DGNSS units, at 92m depth with an umbilical catenary, means the effective positional uncertainty for the diver is significantly larger. Beginning a controlled diver ascent now, while position is still believed stable, is categorically safer than waiting until position is confirmed to have moved.