2011X12 Exposure to a Potential Killer

Exposure to a Potential Killer
MAIB Report

 

This report is taken from the UK MAIB Safety Digest number 3/2004
Exposure to a Potential Killer

Narrative

On board a passenger ship, a crossover line between the port and starboard ballast/treated black water/grey water tanks passed through an adjacent cofferdam. The pipework in the cofferdam had suffered corrosion, and this allowed sewage to build up in the tank. Ship's staff were aware of the problem, and permanent repairs were planned for the next refit which was due within a few months.

Because of the amount of liquid that had leaked into the cofferdam, it was decided to empty the contents using a portable salvage pump. The cofferdam had been opened on a number of occasions without cause for concern. The appropriate tank rescue equipment was assembled in the vicinity of the tank lid, in accordance with the company's 'Permit to Work - Entry into Confined Spaces' procedures. Two ratings removed the port aft lid to ventilate the tank, so that the senior first officer could test the atmosphere and complete the Permit to Work. Immediately on lifting the lid, the ratings noticed a strong smell of sewage. They inserted the fan extension hose into the tank and vacated the area.

A short while later, the senior first officer arrived to conduct the routine atmosphere test. While approaching the tank, his multi-gas detector registered an alarm and recorded a hydrogen sulphide (HS) reading of 98 parts per million. The compartment was immediately evacuated and the watertight access doors closed. The cofferdam lid was re-secured 15 minutes later by a rating wearing full compressed air breathing apparatus.

The ship's senior doctor examined the two ratings who had removed the cofferdam lid, and treated them for exposure to hydrogen sulphide. They remained fit for duty.

The Lessons

• Over-exposure to the potentially lethal toxic gases was prevented because the ratings vacated the area immediately after opening the cofferdam lid. The senior first officer fully recognised the dangers, and understood the meaning of the multi-gas detector alarm and reading levels. His direction to fully isolate the compartment stabilised the situation and prevented the possible contamination of other areas.
• The need to quickly replace the cofferdam lid was recognised, and this was achieved in a controlled, safe manner, making use of the compressed air breathing apparatus to provide safety to personnel.
• Strict adherence to the company's Permit to Work procedures ensured that all appropriate safety equipment was immediately available, and procedures were followed which reduced the risks associated with this potentially dangerous activity.
• It is advisable to test the atmosphere on opening tank lids, because potentially lethal levels of HS can be released if tank levels are high and the surface is disturbed by the ship's movement. In addition, it is prudent to don breathing apparatus when opening tank lids if the atmosphere in the compartment is unknown.
• Whenever corrosion or component failure compromises the integrity of sewage systems, every effort should be made to repair the defect as soon as possible to prevent exposure to toxic HS gases. If sewage systems, or compartments suspected of containing sewage, are opened, there will be a danger from the release of HS gas. Concentrations as low as 10 parts per million are toxic, as indicated in Marine Guidance Note 33 (M+F). It should also be noted that HS might be released from stagnant bilge areas that contain animal, vegetable or mineral oils which have been mixed with salt water, especially when the surface has been disturbed.