Hydrogen Explosion Highlights IMSBC Code Deficiency
The U.K. Marine Accident Investigation Branch’s (MAIB) report on two explosions of gas released from a cargo of unprocessed incinerator bottom ash on Nortrader while at anchorage in Plymouth Sound on January 13, 2017 has been published.
The first explosion was in the forecastle store and the second in the cargo hold. The chief engineer, in the forecastle store working on the emergency fire pump at the time, suffered second degree burns requiring four months to recover. The vessel suffered extensive damage putting it out of service for over three months.
The MAIB investigation established that the explosions were caused by the ignition of hydrogen gas released from the cargo. Prior to this accident, there had been 34 similar shipments of incinerator bottom ash from Plymouth to the Netherlands and, despite it not being listed in the International Maritime Solid Bulk Cargoes (IMSBC) Code, no steps had been taken to seek approval from the competent authorities for its carriage. The investigation also found that the testing protocols in place for assessing if the waste was capable of producing flammable gases were inappropriate and inadequate.
The cargo discharged from Nortrader included items such as unburnt paper, domestic gas cylinders and wheels from motor vehicles. The predominant source of hydrogen generation is believed to be the reaction of elemental aluminum with water in alkaline conditions. Other metals such as iron, nickel and copper can also produce hydrogen, but their contribution is negligible when compared to aluminum.
The propensity of hydrogen to form an explosive mixture with air across a broad range of concentrations, and the ability of a weak ignition source to ignite this mixture, resulted in the first explosion. The ignition source for the hydrogen air mixture was most likely to have been electrical arcing between the contactors in the switch gear in the emergency fire pump starter panel when the chief engineer stopped the pump.
Several underlying conditions combined to cause the explosion:
● The presence of 2.6 percent non-ferrous metals including aluminum, the alkalinity of the cargo, and an increased moisture content due to heavy rain during loading led to the generation of hydrogen.
● The 19 hours spent at anchor allowed the generated hydrogen to accumulate in the unventilated cargo hold.
● The unsecured and unsealed cargo hold lamp access cover in the forecastle store allowed hydrogen gas to enter the forecastle store.
● The closed vents and access door of the forecastle store allowed the hydrogen to accumulate in the forecastle store.
● The broken lug of the emergency fire pump starter panel door allowed the accumulated hydrogen to enter the space in the circuit breaker.
The Maritime and Coastguard Agency has set up tripartite agreements between the U.K., the Netherlands and several other administrations for the safe carriage of incinerator bottom ash and proposed its inclusion in the IMSBC Code.
The report is available here.