Three contractor employees died at a natural gas processing plant as a result of inhalation of H2S released during the unloading of molecular sieves from a NGL drier. Two of the victims were trying to rescue the first worker. The incident description and the learning points derived from the analysis of this tragedy are presented below.
Three contractor employees died at a natural gas processing plant as a result
of inhalation of H2S released during the unloading of molecular sieves from
a NGL drier. Two of the victims were trying to rescue the first worker. The
incident description and the learning points derived from the analysis of this
tragedy are presented below.
Identified main areas for remedial actions are:
- Lack of detailed knowledge of the properties of molecular sieves.
- Inadequate awareness of hazards and management of risks.
- Insufficient and inadequate controls (system of work)
- Ineffective emergency response.
Description of the process
The Processing Plant concerned produces lean gas and natural gas liquids from
associated gas from oil wells. This associated gas contains water vapour and
H2S. The process involves compression of gas, refrigeration followed by separation
of condensed liquids, dehydration of vapours and liquids and final separation
into lean gas and NGL by a cryogenic process.
The natural gas liquids are passed through a bed of molecular sieves to remove
water prior to the cryogenic process step. When the bed of sieves becomes saturated
with water, it is regenerated by passing a stream of hot gas (250 deg. C) through
the bed, followed by cooling of the bed with cold gas.
After some 3-4 years the beds have to be replaced. Normal procedure at this
location was to dump the sieves, after regeneration, cooling and purging with
Nitrogen into a truck for subsequent disposal.
Description of the incident
The drier was prepared for dumping the sieves in a similar way that had been
done many times over the previous 20 years. Appropriate safety precautions and
equipment were provided for the entry of personnel into the drier in order to
remove the top guard and mesh. Removal of the sieves was done by raking them
from the drier onto a chute ending above a high-sided tipper truck. The truck
had been wet with water and the dumped sieves had been wetted using a fire hose
in order to reduce the risk from any pyrophoric material and to restrict dust
in the windy conditions. After a while a mound of molecular sieves had formed
at the back of the truck. A contract labourer decided to enter the truck to
level the mound by shovelling the sieves to the front of the truck. Entry to
the body of the truck was by a ladder behind the cab. After some 10 minutes
a second contractor also entered the body of the truck to help. Shortly afterwards
he collapsed. The first contractor went to his assistance and was joined by
a third contractor who jumped into the truck from the elevated platform on the
drier. All three became unconscious and died. A fourth man who climbed the vehicle
ladder to see what was happening also became unconscious but was pulled from
the area by rescuers. Emergency response was delayed by unclear radio communications.
Three main events were identified:
- H2S was present in the truck (semi-enclosed space) at sufficient concentration
to overcome workers within minutes and ultimately cause death.
- Workers in the truck were unprotected (no SCBA/escape masks/personal
- There was a lack of awareness of the H2S hazard associated with the
dumping of the molecular sieves
and thus inadequate management of the risks.
Initial emergency response was not effective. A major contributing condition was also identified:
The H2S was evolved (de-sorbed) from the molecular sieves in the
truck. The gas used for regeneration of the sieves prior to dumping is a residue
gas containing approximately 830 ppm of H2S. The molecular sieves
will start to adsorb H2S from the regeneration gas during the cooling
of the bed. This H2S will not be removed from the molecular sieves
during the Nitrogen purging stage. However, since the affinity of the molecular
sieves for water far exceeds the affinity for H2S, the H2S
will be released when the sieves are contacted with water (dumping in a layer
of water in the truck and spraying with water). Further, during the levelling
of the molecular sieves any trapped H2S will be released.
Failings identified were:
- Staff and contractors did not know that H2S could be released from the
- The information provided by the sieve manufacturer did not give explicit
adequate warning of the risk of desorption of H2S after contact with water.
- Contractors could not recall the content of the site safety induction
that they had received some years before.
- The effectiveness of this induction was limited (no test, no records,
- Over several years the H2S content of the gas had increased but adequate
action had not been taken to enhance awareness of staff and contractors of the hazard. The need to carry
escape masks was not recognised, there was no requirement to carry personal H2S monitors.
- Staff did not react to the unpleasant smell which was apparent for some
time before the event.
- There were no warning signs in or around the driers indicating the presence
of H2S in hazardous concentrations.
- Dumping the molecular sieves, which was supervised by the Civil group,
was not included in the planning of the overall molecular sieve replacement by the Mechanical group.
- Multiple jobs requiring different precautions were on a single Work
Permit. The requirements of the Company Permit to Work System were not met.
- No job safety analysis (task risk assessment) was conducted for the
task of dumping the molecular sieves, and no tool box talk was given.
- There was no Company supervision present at the job location, during
unloading of the molecular sieves.
- There was no immediate availability of rescue staff with breathing apparatus
and resuscitation equipment
- The immediate first aid response was inadequate.
Lessons to be learned from this incident
- Understanding of the Hazards and Effects Management Process (HEMP) needs
to be improved, in particular the relationship between HEMP and the planning of activities through
identification of incident scenarios and job safety analysis (task risk assessment).
- Incident scenarios and appropriate job safety analysis (task risk assessment)
should be performed with the involvement of first line supervision. Method statements should be prepared
which clearly define roles, responsibilities and the controls to be applied. Communication through
tool box talks should be carried out.
- Manufacturer’s recommended practices for safe handling of molecular
sieves should be understood, communicated and applied.
- Safety induction should be tailor made for the target audience, be multilingual
if necessary and preferably visual. Effectiveness needs to be checked and recorded and refresher
training requirements defined.
- The awareness of the hazard of H2Sshould be enhanced for all staff and contractors. The effectiveness of such awareness training should be checked and refresher training
- The use of adequate PPE should be enforced, including the provision of warning notices.
- Emergency drills should address a range of scenarios and involve all staff who may have a role to play.