The Bahamas Maritime Authority issued a safety alert regarding the potential serious risk for safety on board a ship where nitrogen cylinders are used as a stored kinetic energy system for launching lifeboats. This alert was issued after the authority obtained information from an ongoing maritime incident investigation conducted by the Transport Accident Investigation Commission, New Zealand.
The vessel had hydraulically powered davits with six power packs, three on each side of the vessel. A stored energy system consisted of a piston accumulator and a bank of four high pressure (180-210 Bar) nitrogen cylinders were fitted to each lifeboat launching davit.
In February 2017, one of the nitrogen cylinders of a stored energy system onboard exploded while being topped up to maintain the correct pressure. A crew member died as a result of the explosion. The findings of the investigation indicate that significant corrosion affected the structural integrity of the cylinder.
The remains of the exploded nitrogen cylinder and other three cylinders from the same frame, along with a randomly selected representative cylinder from another bank of cylinders were taken to the Transport Accident Investigation Commission’s technical facility in Wellington, New Zealand, where an examination was made.
The examination showed that the damaged nitrogen cylinder had suffered significant corrosion at the point of failure. The corrosion had developed on the outside of the cylinder and internal wall of the surface was clear of corrosion. Corrosion had reduced the thickness of the cylinder wall by 75% at the point of failure.
The second area of corrosion was noted near the bottom of the cylinder and a third area where the protective coating had been scraped away to observe the cylinder’s identification details.
Recommendations
– The Bahamas Maritime Authority recommends to have a visual inspection of the Nitrogen bottles at pre-determined periods and expiry dates should be recorded as part of the ship’s safety management system.
– It is also recommended to include a visual inspection as part of the gas top-up procedure of the high-pressure gas cylinders. The drop in pressure may also indicate that the integrity of the pressure vessel has been damaged which should trigger a visual inspection. The visual inspection should be recorded as part of the vessel’s safety management system.
– In addition, a pressure vessel management system that ensures the fitness for service and safe operation of the pressure equipment on the vessel should be available.
– Moreover, operators must make sure that the SOLAS convention is adhered to in regard to the survey, inspection and maintenance requirements of lifeboat launching appliances.
– Finally, the Bahamas Maritime Authority recommended to have hydraulic accumulators inspected by a pressure vessel inspector to ensure any corrosion present is within acceptable limits of the design corrosion allowance.