2 gas cylinder ruptures delay vessel operation over 8 months
Ships and buildings are outfitted with inert gas fire suppression systems. In the event of a fire the cylinder valves activate, filling a room or compartment with an inert gas, displacing the oxygen. Since fire requires oxygen to continue combustion, removing the oxygen from the room or compartment stops the fire. This is accomplished without the use of water or other liquid or powdered contaminates which may affect the components in the room or compartment.
In this situation a ship was outfitted with 448 cylinders of an inert gas. Enough cylinders to contain a fire if it broke out in sensitive areas of the vessel. Most cylinders, after being put into service, require a hydrostatic requalification every 10 years to ensure safe continued use. In this circumstance the vessel was required to test 10% of its 448 cylinders. This vessel opted for an Ultrasonic testing of the cylinders instead of the hydrostatic requalification.
In January of 2018 one of the high-pressure cylinders ruptured. It is unknown the amount of damage it caused to the vessel or system and no injuries were reported. Upon inspection of the ruptured cylinder it was determined that Stress Corrosion Cracking was the cause. To make matters worse, a second cylinder, on the same vessel, ruptured 3 months later. From that time until November the vessel was unable to perform basic maintenance of the fire suppression system. The vessel sat idle for the protection of the workers.
Since two cylinders had ruptured, the concern was for any worker going into the area to perform further maintenance. The fear was that further maintenance may cause another rupture. The vessel had 446 remaining pressurized cylinders, only 10% of which had been tested, and their integrity was in question.
10 year requalification’s are an important part of safety. However, to solely rely on this one test may not be enough. Cylinders, when placed into service, are exposed to damaging environmental elements. These elements may be external and/or internal. More frequent visual inspections may help identify problems before they become catastrophic. And documenting those inspections may provide the safety officers with more data and allow them to pull an unsafe cylinder from service.
In this specific case the vessel operator chose an ultrasonic examination procedure. These tests are accurate and valid, but they have some limitations. The sensors detect wall thickness and damaging sidewall corrosion. The sensors don’t analyze the crown (neck) or base region. It is unknown if the cylinders on this vessel had the base and crown regions analyzed for cracks or corrosion.
Having a visual inspection protocol in place, with proper documentation, may have provided some assurance of safety. To be clear, an external visual inspection would not have identified stress corrosion cracking. However, having a visual inspection procedure in place would provide an opportunity to monitor for exterior corrosion. And, if the visual inspection procedure would allow for periodic draining of the cylinder (in a safe environment), and subsequent examination of the interior and neck of the cylinder, forming cracks may have been identified. Those added precautions may have given the decision makers the confidence to carefully remove all or any questionable cylinders.
As in most situations of a rupturing cylinder, there is no documentation of a visual inspection process. The cylinders were delivered in good condition, placed into service, and then likely forgotten. Cylinders are manufactured to very high standards, but they are made of metal or composites. These two elements can be damaged over time. Because of their composition, all high-pressure cylinders should be visually inspected and documented on a frequent basis. It is hoped that a frequent inspection, by trained professionals will decrease the number of ruptures. This may help prevent operations from being shut down for months at a time and keep facilities and vessels safe.