Underwater explosions

We have received reports from members of four separate incidents of underwater explosions occurring when oxy-arc cutting techniques have been in use:

  1. A diver was making corner holes in the ships shell plate using ultrathermic cutting rods to mark the area that had to be cut out. The rod did not ignite and the diver was forced to tap a number of times with the rod on the cutting spot. Every time he tapped, an amount of oxygen must have been escaped from the rod and eventually the mixture in the gas pocket reached its lowest explosion level value. When the rod ignited, the heat, together with hydrogen, reached the gas pocket, which was followed by an explosion. The diver was hit by the pressure wave of the explosion and became dizzy/ unconscious for a short period. The standby diver guided the victim out of the wreck. The diver went into the decompression chamber and was then taken to the hospital for observation. He had some chest pain but was able to resume working after three days.
  2. Salvage divers were cutting an access hole into a tank for lifting slings to be installed. The hole had to be burned from the inside to the outside. Before starting with the cutting, the remaining oil and air pockets were removed by a pump. After checking that there were no visible residues of oil the diver started to cut a hole in the tank. Within approximately five seconds an explosion took place. The standby diver immediately started to retrieve the umbilical and the supervisor kept talking to the injured diver. The injured diver managed to climb the dive ladder himself. When the injured diver was on deck the paramedic examined him and he was then taken to hospital. Both of the diver’s eardrums had been perforated and he had a sore throat and nose and chest pain. The diver could not resume his work for 37 days.
  3. During cutting work in a double bottom on a salvage job while cutting the bottom plating in longitudinal direction, a diver tried to cut a small hole in the tank to avoid gas pockets building up under the tank top. When the cutting rod had initially gone through the tank top plating, the diver had reported some suction into the hole. While he drew the cutting-rod back, an explosion occurred. The diver was able to come to the surface without any problem or assistance of the stand by diver. After the decompression, the diver medic diagnosed a perforation of both eardrums.
  4. The fourth incident occurred during the cutting of tank wall plating. The intention was to flush the tank for a couple of hours using an air hose before and during the actual cutting. The diver wanted to cut a small hole in the tank wall, just beside a hole made by his predecessor, to connect a thin rope to keep the air hose in place. Before resuming cutting, the diver felt with his hand inside the hole to check for a gas-pocket and everything seemed to be okay. Within a few seconds of starting cutting, he saw a fire inside the tank and an explosion followed. The diver was able to come to the surface without any problem or assistance of the stand by diver and was transferred to a nearby rig for medical treatment. The medic diagnosed perforation of both eardrums.

Oxy-arc cutting involves the use of large quantities of oxygen and generates hydrogen during the process. When the proportion of hydrogen to oxygen reaches a certain level, an explosive mixture is formed, which will ignite when the arc, or a spark, reaches it. Reminders of the following factors known to cause an explosion during oxy-arc cutting have been identified by those involved with these incidents:

  • Gas pockets – gas pockets are formed when the shape of a structure is such that bubbles of gas are trapped on their way to the surface and allowed to accumulate in sufficient quantity;
  • Blow backs – blow backs are spontaneous explosions of varying intensity which appear to be generated at the cutting point. A research project was carried out on the oxy-arc cutting technique at depth. It showed that there is enough hydrogen produced, during the time between making the rod ‘hot’ and striking the arc, to cause an explosion. During the research an interval of four seconds was shown to be long enough to produce sufficient hydrogen to cause a serious explosion, even in half-used rod;
  • Explosive or flammable substances – depending on the substance involved, various gases or fumes can be released during cutting which can contribute to the mechanism of blow back. For instance: hydrocarbons inside a pipe, paint or bitumastic coatings, and some light alloy materials.

Actions instigated by those involved include specifying the use of hard helmets such as the super-lite for use during operations of this type and cold-cutting-drilling of holes to flush enclosed spaces prior to commencing hot work.

Particular recommendations arising from these incidents have included the following.

It is recommended that detailed risk assessments are carried out before underwater cutting operations commence, specifically when there may be a potential of gas entrapment and/or residual traces of hyrocarbons (such as in double bottom tanks/fuel tanks of vessels). Risk management measures such as diver awareness, for example divers should be familiar with IMCA D 003 (Guidelines for oxy-arc cutting), use of cold cutting techniques and flushing void spaces with inert gasses such as nitrogen. The use of Arc Air Rods could also be considered in some cases. Whenever carrying out potentially hazardous operations such as those described in the text, it is imperative that diving supervisors and diving personnel are competent in terms of the skills required by the operation and the identification and management of hazards associated with the operation.