Accelerated emergency decompression from saturation: reading DMAC 31

What DMAC 31 sets out to cover

The Diving Medical Advisory Committee (DMAC) publishes guidance notes that function as the medical reference layer beneath the operational codes. Contractors, clients and diving medical advisers treat them as authoritative, and IMCA documents cross-reference them. DMAC 31 deals with the emergency decompression of saturation divers: the deliberate shortening of a decompression schedule to return divers to surface faster than a normal profile permits, and the clinical management that has to follow once they arrive.

The note is aimed at the people who make and carry the decision – the diving superintendent, the life-support supervisor, and the nominated diving medical adviser. It exists because the choice is not free. Compressing a schedule that was designed around inert-gas off-gassing accepts a known penalty in decompression illness. In our reading, the guidance is about making that trade deliberately, with the right authority and the right support standing behind it, rather than improvising it in the middle of an incident.

The source note is short and specific. The planning problem it opens up is neither, and that is where the rest of our analysis sits.

The physiology the limits are built on

In saturation, diver tissues are equilibrated with the inert gas in the storage atmosphere – helium in a heliox system. Normal decompression is slow because the schedule has to keep tissue gas tension below the threshold at which bubbles form and grow as ambient pressure falls. Typical saturation decompression runs on the order of a day per 30 metres of seawater at the deeper end, slowing further as the divers approach surface, and the exact rate varies between contractor tables and national requirements. The conservatism is the whole point: it holds supersaturation within limits the body can tolerate.

Accelerated decompression discards part of that margin. Bringing the schedule in faster drives tissue supersaturation higher, and the predictable result is decompression sickness – neurological (type II) hits, vestibular involvement, and in the worst case pulmonary decompression sickness. These are not theoretical risks; they are the direct, expected cost of the manoeuvre, and we read DMAC 31 as framing the decision in exactly those terms: a team is trading a certain, immediate threat against a probabilistic injury that it must then be equipped to treat.

That framing matters because it sets the acceptance criterion. In our view, accelerated decompression is justified only when the alternative – remaining at storage pressure and completing a normal profile – carries a higher and more immediate probability of death or serious harm than the DCI it is about to induce. It is not a time-saving tool and it is not a convenience.

Three responses, in the right order

The single most useful thing a management team can carry away from DMAC 31 is that accelerated decompression sits at the bottom of a hierarchy, not at the top. We see three broad responses to an emergency affecting a saturation spread, and they should be considered in this order.

Maintain pressure and control the threat. If the hazard can be contained while the divers stay at storage depth on a normal schedule, that is almost always the lowest-risk outcome. A fire that is being fought and held, a flooding event that is contained, a gas problem that is being managed – none of these automatically justify accelerating anyone.

Evacuate under pressure. Where the vessel or platform must be abandoned but the divers can be kept at storage pressure, the answer is hyperbaric evacuation through the self-propelled hyperbaric lifeboat or hyperbaric evacuation system, followed by transfer under pressure to a reception facility and a normal decompression there. This preserves life without paying the DCI penalty. IMCA D 052 covers the evacuation-system side of this, and in our experience the interface between the dive system, the evacuation asset and the reception facility is exactly the kind of multi-party arrangement that needs governing in advance – the same discipline that applies to any operation where separately contracted parties share one worksite and one emergency plan.

Accelerate the decompression in place. Only when the threat is immediate and evacuation under pressure is not viable – the system integrity is failing, the environment cannot be maintained, and there is no functional evacuation route – does accelerated decompression become the least-bad option. DMAC 31 is written for this last case.

A dynamically positioned dive support vessel that loses its position is one of the classic initiators that forces this ladder to be climbed under time pressure, which is why we argue that the DP failure modes and the way crews actually respond to them deserve attention long before a bell is in the water – the drift-off problem that persists even where ASOG is followed is not a separate topic from dive-system emergency planning. The two response chains have to be designed together.

What it demands on the spread

Reading the guidance as an operational specification, several requirements follow that a dive management team must have in place before mobilisation, not after an alarm.

A pre-established decision authority. The choice to accelerate is a medical decision executed operationally. The chain – superintendent, life-support supervisor, and a diving medical adviser reachable 24/7 – must be defined by name and role, with a communications route that survives the incident itself. DMAC 28 sets the expectation for emergency medical care provision; the accelerated-decompression decision is where we find that link tested hardest. A satellite phone to a doctor who is not on call, or a number that rings out at 03:00, is a planning failure that only shows up when it is too late to fix.

A reception capability that can still recompress. This is the requirement we see teams most often get wrong. If a spread accelerates to surface and then vents the system, and a diver develops a neurological hit twenty minutes later, that diver needs to be recompressed immediately – on a therapeutic table, with treatment gas and oxygen available. The plan must keep a chamber functional and gassed, and must not strand the divers at a location or in a state where recompression is impossible. Accelerated decompression that leaves no route to treat the injury it predictably causes is not a plan; it is a gamble with a known bad outcome.

A consumables budget that covers the whole manoeuvre. Getting to surface fast does not remove the environmental-control demand. The chamber still needs CO2 scrubbing, and helium’s thermal conductivity means the divers cool quickly, so heating and thermal balance have to hold through the descent in pressure. On top of that, the treatment reserve – therapeutic oxygen and treatment gas for a full recompression table – has to be ring-fenced. IMCA D 014 sets the baseline for gas reserves; the point we stress for emergency planning is that the treatment reserve is additional to, not drawn from, the reserve already committed to the accelerated profile.

Medical monitoring through and after surfacing. Symptoms may present during the ascent in pressure or hours later. The life-support team needs a monitoring and documentation regime that continues well past the point the divers reach surface, and a clear threshold for initiating recompression rather than waiting to see whether a symptom resolves.

Where teams get this wrong

1. Treating accelerated decompression as the default emergency response

The most damaging misreading we encounter is planning the emergency around accelerating everyone to surface, when maintaining pressure or evacuating under pressure would preserve life without inducing DCI. Accelerated decompression is a last resort. If it appears near the top of an emergency response plan as a general-purpose reaction, the hierarchy has been inverted and the plan will cause harm it did not need to.

2. Surfacing the divers and losing the ability to treat them

A team can execute the accelerated profile competently and still fail, if the sequence ends with the divers out of the system and no functional, gassed chamber to recompress them. We treat the decision to accelerate and the decision about what recompression capability remains available afterwards as a single decision. They must be planned as one.

3. A decision chain that has never been exercised

Emergency drills are a standard expectation for saturation diving operations, but in our experience most drill programmes rehearse hyperbaric evacuation far more than they rehearse accelerated decompression and the medical decision behind it. The result is a team that can launch an SPHL under pressure but has never worked through the call to accelerate – who authorises it, on what information, and how the medical adviser is brought into a decision measured in minutes. A response that is written but never rehearsed behaves, under stress, as if it were never written.

4. Confusing the two emergencies

Hyperbaric evacuation and accelerated decompression are different responses to different situations, and we see conflating them produce bad decisions in both directions: accelerating when evacuation under pressure was available, or attempting evacuation when the system was already failing and speed to surface was the only option left. The distinguishing question is simple and must be answered explicitly in the plan – can the divers be kept at pressure, or not.

What to put in place now

• Write the response hierarchy into the dive-project emergency plan explicitly, in the order maintain-pressure, evacuate-under-pressure, accelerate. State the specific conditions that move a team from one rung to the next, so the decision is bounded rather than invented on the night.
• Name the decision authority and the 24/7 diving medical adviser by role, with a verified out-of-hours contact route, and test that route as part of mobilisation checks. Log the test.
• Ring-fence the treatment reserve – therapeutic oxygen and treatment gas sufficient for a full recompression table – as a separate line in the gas budget, distinct from the accelerated-profile reserve, and confirm it against IMCA D 014 before the first bell run.
• Confirm, in writing, what recompression capability remains available at every stage of an accelerated profile, including after the divers reach surface. If any stage leaves no route to recompress a symptomatic diver, redesign that stage.
• Drill the accelerated-decompression decision, not just the evacuation launch. Include the medical-adviser call, the environmental-control demand during the manoeuvre, and the post-surfacing monitoring and treatment-initiation thresholds. Record who authorised, on what information, and how long the chain took.
• Design the DP and dive-system emergency chains together. The initiators that force accelerated decompression are often position or power events, so the ASOG, the DP FMEA response, and the dive emergency plan must reference each other and be exercised in combined scenarios rather than in isolation.

DMAC 31 is short because the medical principle is simple: accelerating decompression trades an immediate threat for a treatable injury, and is worth doing only when the threat is the greater risk. Turning that principle into something a spread can execute safely is the harder work, and in our view it is entirely front-loaded. The plan, the authority, the medical link and the treatment reserve are cheap to establish before mobilisation and impossible to assemble once the alarm has sounded.

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Based on: DMAC 31 – Accelerated emergency decompression from saturation