Never assume that third party contractors will behave safely aboard your ship. Keep an eye on them. That is one of the lessons from Brazil’s investigation into a confined space explosion aboard the Bahamas-flagged tanker Auk Arrow in Rio De Janeiro in August 2010 in which three workers died and six injured.
Cutting and welding was being carried out on scaffolding erected inside the vessel’s No 2 ballast tank in the ENAVI shipyard. Hoses carrying liquid petroleum gas, LPG and oxygen, were connected to a manifold on the main deck and passed through a vent in the ballast tank. On the day of the incident work was stopped from 1700 to 2100 for meals. The hoses were left dangling in the tank. Some 52 minutes after work resumed there was an explosion in the ballast tank. Two workers were killed, seven were hospitalised due to injuries of whom one later died.
It appears that leaking LPG pooled at the bottom of the ballast tank and eventually exceeded the lower explosive limit. Incandescent debris from the cutting and welding work fell into the LPG, ignited the now-explosive mixture.
Ventilators installed for the work were not strong enough to adequately mix air in the bottom of the tank, which might have removed the pockets of LPG and kept the atmosphere below the LEL/LFL. (MAC: where there is potential for the presence of toxic/explosive gas or fumes whch are heavier than air consideration might be given to ducting air from the bottom of the space).
Monitoring for the presence of an explosive atmosphere was not adequate.
Although the workers were experienced professionals the report says: “According to the information acquired, including in the interviews with the professionals of the area of workplace safety and workers themselves of the shipyard, remained the finding that the doctrine of prevention of accidents was not answered”.
A number incidents recorded by MAC show that it is unwise to assume that third party contractors and other visitors to the ship are safety-aware. Constant monitoring by a ship’s officer may not always be possible – in this case the officer of the watch included the work in No. 2 ballast tank in his rounds at roughly two-hourly intervals – but bear in mind that an unsafe third party remains a hazard to the ship and its crew, and when that party is working in a confined space the potential of the hazard may be significantly increased.
The Case of the Benzene Bomber (cargo surveyor uses the wrong guaging method on a chemical tank, an explosion ensues).
The Case of the Forgotten Assassin (A contractor cleaning a boiler died when he sets off a hydrogen explosion).
The Case of the Lethal Lampshade (Explosive fumes build up while coating a hold, an explosion follows)