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Bell contamination

A member has reported that during a recent diving project, a subsea fitting failure caused divers to become contaminated with a hazardous substance released from a hose under pressure. Diver 1 subsequently returned to the bell and entered the bell and was allowed to enter without taking the necessary precautions to ensure that he was free from contamination. On removal of his diving helmet, he reported to the bellman that he felt faint. The diver was put on to the built-in breathing system (BIBS) and the bell was flushed through. Decontamination measures were then taken and diver 2 was subsequently recovered to the bell. During this incident a Hyper-Gas II analyser situated in the bell did not alarm and the bellman said he was unaffected.

A review of the incident was carried out by the member company, noting the following points:

  • The analyser in use was an infra-red analyser that looks at the specific band of light related to crude oil and condensate and cannot necessarily be relied upon when vapour from only one or two specific hydrocarbons are present, as these may fall outside the band of infra red light being analysed;
  • Where the analyser is to be used in an environment where vapour from other hydrocarbons could be present, it may be advisable to reset the analyser specifically to respond to these hydrocarbons;
  • While the Hyper-Gas II analyser is capable of seeing specific fractions of hydrocarbons as part of crude or condensate mix, such fractions need to be at a relatively high concentration before the unit will alarm, e.g. 10% of the anaesthetic requirement. Concentrations of other chemical substances may also be at levels well above the occupational exposure standard (OES) and be well on the way to a level that will incapacitate the bell occupants, with potentially fatal consequences, before an alarm is given;
  • The levels of individual hydrocarbons which cause anaesthesia are several orders of magnitude higher than the allowed occupational exposure standard for each compound. This being so the Hyper-Gas II cannot be, and has never been presented as, a first line warning of hydrocarbon contamination in the bell;
  • Human sensory awareness can, in some circumstances, detect (but not quantify) substances before electronic devices. However, this must never be relied upon as a means of detection;
  • Although the bell is small, contamination may occur in one area of the bell but not in others. This will depend on gas flows within the bell, composition/molecular weight of the contaminant chemicals and also how the contaminant is introduced into the bell, e.g. by the umbilical, contaminated clothing or gaseous ingress into the bell trunk. In two- and three-man bells mixing would naturally occur due to the restricted volume.

Members are reminded of the following:

  • Analysers can usually only warn of an event once it is occurring and they should not be relied upon as the primary warning of bell contamination. Prevention of contamination and the use of enhanced diving techniques should be the primary defence against bell contamination;
  • Where contamination is possible, enhanced diving procedures should be used. These include: correct bell positioning, the use of BIBS by the bellman prior to diver recovery and prior to assisting the diver when contamination is suspected, decontamination of umbilical, removal of protective coveralls and thorough diver decontamination prior to bell entry, testing of the bell atmosphere using the appropriate Draeger tubes to confirm bell conditions;
  • At the planning stage of an operation, the project engineer should ensure any chemicals/hazardous substance used during the operation, or contained within the system to be worked on, are identified and a material safety data sheet (MSDS) is obtained;
  • The MSDS for each substance should be presented to the project safety adviser, who will arrange for a safety assessment to be completed, including a diving assessment;
  • A review of the chemical/hazardous substance will consider the need for any additional specific Draeger tubes to identify contamination/quantify bell atmospheric conditions.

A video showing the effects of an anaesthetic chemical within a contaminated bell is available from Analox.

Safety Event

Published: 20 December 2007
Download: IMCA SF 10/07

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