Accidental formation of flammable gas from chemical waste on a ship

An explosion occurred on a ship loaded with a cargo of 2333 tonnes of unprocessed incinerator bottom ash. Flammable gas generated by the ash produced two explosions: the first one was in the forecastle store and the second in the cargo hold.
They caused extensive damage to ship structures and injuring a crew member.

Laboratory tests carried out during the investigation have established that hydrogen gas was the only flammable gas released by the load on board. The presence of 2.6% non-ferrous metals including aluminium, the alkalinity of the cargo, and an increased moisture content due to heavy rain during loading led to the generation of hydrogen.
Enough hydrogen could have been released to form an explosive atmosphere.

The INITIATING CAUSE was the accidental production of hydrogen from the reaction between ash and moisture, assisted by heavy rain.

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 free pump starter panel when the chief engineer stopped the pump.

The ROOT CAUSE was related to “inappropriate and inadequate” testing protocols in place for assessing if the waste could produce flammable gases were. Moreover, the no autorisation for ashes transportation had been asked to the authorities.

Cargo ship, transportation of incinerator bottom ash

Prior to the accident, there had been 34 similar shipments of incinerator bottom ash. In all these cases no step had been taken to seek approval from the competent authorities for its carriage and its potential risks.

The chief engineer, in the forecastle store at the time, suffered second degree burns requiring four months to recover.
The vessel suffered extensive damage and was out of service for two months.
According to the MAIB report, "The explosions dislodged and distorted all nine of the vessel’s steel hatch covers, breaking all but one of the 66 hatch cleats in the process. Some of the hatch covers dropped into the cargo hold and were found resting on top of the cargo. The cargo hold coaming bar was also damaged in several locations and the cargo ventilation trunking tower flaps were deformed. " (for picture see mentioned source)

The investigation found that the testing protocols in place for assessing if the waste was capable of producing flammable gases were inappropriate and inadequate.
It also found that no step had been taken to seek approval from the competent authorities for its carriage and its potential risks, despite it not being listed in the International Maritime Solid Bulk Cargoes (IMSBC) Code.

Following the accident, the MAIB (UK’s Marine Accident Investigation Branch) has published a safety flyer to disseminate the lessons from this accident and improve awareness of the IMSBC (International Maritime Solid Bulk Cargoes) Code.
The ship owner has been recommended to review its safety management system to reflect the requirements of the code.
The Maritime and Coastguard Agency has set up tripartite agreements between the UK, the Netherlands and several other administrations for the safe carriage of incinerator bottom ash and proposed its inclusion in the code.
The agency has been recommended to update the Merchant Shipping (Carriage of Cargoes) Regulations to refer to the IMSBC Code and to work with the Environment Agency to ensure that test protocols for the classification of cargoes are fit for purpose.