Diving, while not without risk, is relatively safe and with appropriate training and planning even many of the risks associated with tech and cave diving can be mitigated.
However, poor training and planning often leave divers exposed to increased and largely unnecessary levels of risk, and as diving accidents can be serious and potentially life threatening, thorough accident analysis is important for understanding both what happened and how to avoid similar incidents in the future.
Following is an analysis of an incident originally appearing in DAN’s Alert Diver Asia Pacific magazine. In our opinion a variety of issues tipped ‘Bob’ into the incident pit, ultimately leading to his tragic death1.
For those of you without access to Alert Diver, the incident is summarised, in good faith and without material misrepresentation, here:
- Jamie and Bob had been tech divers for about a year.
- Bob does all the planning as he enjoys it and has more experience.
- Plan: 25min @ 50m; Back gas: twin tanks, Air; Deco gas: 36% & O₂
- After descent they separated to dive independently.
- After 20min Jamie was surprised to see Bob appear and signal that he wanted to turn the dive.
- Jamie turned to signal some other divers and when he turned back Bob was gone.
- Jamie ascended to 42m, the first of their planned deco stops, to find Bob already there, wild eyed and breathing from his 36% deco gas.
- Jaime understanding that Bob was breathing 36% well bellow its MOD, attempted to share his back gas with Bob, who refused.
- During this time they sank several metres.
- Bob panicked and swam for the surface. Jamie attempted to catch him, but stopped at 36m hoping to avoid bending.
- Bob broke the surface frothing at the mouth. He was blue and not breathing when the boat crew retrieved him.
- The boat crew had no first aid training and could provide no treatment.
Given the relatively vague account provided, it’s difficult to perform any detailed or definitive analysis, but we can talk about some possibilities, make inferences about Bob’s tragedy and gain some insight into risk management and problem solving.
“Bob does all the planning as he enjoys it and has more experience.”
Dive planning: All team members should be involved in and understand dive planning. Familiarity with the dive plan and contingencies is mandatory for any dive, especially in the technical range.
“Plan: 25min @ 50m; Back gas: twin tanks, Air; Deco gas: 36% & O₂”
Inappropriate gas choice: Air at 50m has an END of 50m well beyond the 30m END where narcosis becomes a significant concern. Diving narc’d is similar to driving drunk—reaction times and mental capacity are degraded and problem solving ability is greatly impaired. Deep air is still common throughout the industry exposing divers to unnecessary risk particularly when something does go wrong. At this depth it’s likely that Bob was, on top of everything else, narc’d.
“After descent they separated to dive independently.”
Leaving the team: Diving independently exposes divers to far greater risks. Bob’s greatest asset was Jamie—a redundant gas source, a second brain for problem solving, reassurance during problems—and they decided to separate during the dive. Whatever else happened, matters were only made worse by him being alone as problems started to emerge. Jamie’s surprise at finding Bob near him at the 20min mark highlights this point even further, they were so separated that they had very little if any awareness of each other during the dive. It’s important to note that Bob made this mistake twice. Once during the planned dive, and once when he took off for the decompression stop at 42m. A better awareness of team diving and appropriate planning would have ensured that Jamie had ample gas for Bob to share while they completed their decompression.
Out of gas?
Bob appears to have been out of gas, or at least thought he was, after 20min. This could be the result of many factors including poor fitness, poor equipment maintenance, poor gas planning, no flow check or insufficient gas monitoring.
Poor fitness: Unfit divers, especially smokers, are far more susceptible to over work on a dive, especially when carrying lots of equipment as on a tech dive. Elevated breathing rates and poor gas exchange in the lungs could contribute to higher than expected gas consumption rates.
Poor equipment maintenance: Poorly maintained tanks, valves, regulators and SPGs etc could all lead to gas loss or increased consumption during a dive, either through leaks or elevated breathing rates.
Poor gas planning: Unexpected out of gas incidents can also be the result of calculation errors on the surface during the planning phase or a failure to verify tank contents before diving.
No flow check: An often overlooked pre dive step, the flow check is a simple procedure for verifying that all valves are open before diving. On twin tanks this ensures that both posts are open and breathable and that the isolation valve is open. If Bob’s manifold was closed he may have gone ‘out of gas’ with an entire tank still full. In the worst case, a diver could enter the water with no valves open, leaving them with not only no breathing gas but no inflation. This could be a potentially fatal error, particularly for divers with limited experience in valve manipulation.
Insufficient gas monitoring: Divers should have a good knowledge of their SCR and regularly monitor gas consumption during a dive. Regular checks would have alerted Bob to any problem long before it became urgent, allowing him (and his team!) to either deal with the issue or make a controlled ascent while gas sharing.
“… wild eyed and breathing from his 36% deco gas.”
Panic response: CO₂ is a nasty gas, often overlooked in diving. It is extremely narcotic (many, many times more so than nitrogen), causes drowsiness, unconsciousness and ultimately death, is implicated in an increased risk of CNS oxygen toxicity, it drives our breathing cycle, causing increased breathing rate and gas use, and it also drives our panic response, its build up creating a feedback loop that leads to extreme panic and irrationality. All of these factors make it arguably the most dangerous gas we have to deal with as divers and most of the factors mentioned so far are likely to have increased Bob’s exposure to CO₂ leaving him ‘wild eyed’ and panicked.
Incorrect gas switching procedures: Not only did Bob leave his team and race off to the decompression stop, he switched to a decompression gas without verification from his team. At 42m he was well bellow the MOD of his 36% decompression gas, exposing himself to a PPO₂ of almost 1.9 bar, well over what is considered safe, and a real risk of CNS oxygen toxicity. Given he was carrying two decompressions gases, was probably narc’d and was already in a panicked state, Bob also risked mistakenly switching to his 100% gas. If that had happened Jamie would most likely have discovered him already dead.
“During this time they sank several metres.”
Poor buoyancy: Poor buoyancy control simply makes everything more difficult underwater. Divers expend more energy and expose themselves to greater risk in problem situations. Bob was so panicked at this point that he refused to share gas and while they tried to resolve this, Bob and Jamie descended several metres, exposing Bob to an even higher PPO₂ and increasing their decompression obligation.
“Jamie ascended to 42m, the first of their planned deco stops.”
Poor understanding of deep stops: 42m is an incredibly deep first stop for a 50m dive. Decompression planning is a balance between getting shallow enough to start offgassing, slowing the ascent enough to control bubble growth and limiting time at depth to control ongassing. VPM conservatism 2 indicates that offgassing does not start until 36m, which is where their first deep stop should have been. Staying too deep during decompression is not conservative, it increases your decompression obligation.
“Bob panicked and swam for the surface.”
Blowing all stops: Bob’s final and ultimately fatal mistake was to blow his decompression and shoot to the surface. After so long at depth, it is unlikely Bob would have recovered, even had the boat crew been appropriately trained.
“The boat crew had no first aid training”
Poor planning: This is inexcusable and negligent on the part of the operator and the divers. They simply should not have entered the water without appropriately trained and equipped surface support.
These sort of incidents are tragic and made worse when the actual problems are not identified. In our opinion Bob’s tragedy was the result of poor training, procedures and planning, lack of understanding and poor situational and team awareness.
No single issue or event caused his death (other than in the direct sense that blowing that sort of decompression obligation is likely to cause significant injury or be fatal). Rather, Bob’s death was the result of a series of mistakes and poorly managed problems. At every juncture there were opportunities to resolve the situation before it spiralled out of control.
This is what we call the incident pit. Accidents are rarely the result of a single issue. They snow ball. Missed procedures, mistakes and unresolved problems build and build, increasing diver stress and eventually leading to panic and unresolvable problems that are ultimately unsurvivable.
1. There are no doubt more details than those appearing in the article and this post represents our opinions and observations based on the incident as reported. It is really about risk management rather than the specifics of Bob’s tragedy. ↩