Article: Investigating fires in box ship casualties
11 July 2019
Partner – Resident Director,
Dr JH Burgoyne & Partners DMCC
T +971 501 455 121
The investigation of the cause of a cargo fire or explosion on a mega box ship can be a complex affair involving a number of different scientific disciplines. This article outlines some key considerations.
The larger modern-day container ships typically carry several hundred individual containers in each hold, with many more on deck, and the outbreak of a fire can have catastrophic consequences, particularly during
sea passage when firefighting resources are limited to those on board. There may be flammable liquids or oxidising solids stowed on board, which whilst they may not have caused the fire, can greatly enhance its ferocity and rate of spread. With large numbers of diverse cargoes, many of which will be combustible, the extent and severity of the damage can therefore be significant and, in the worst cases, extend to adjacent holds, deck cargo, or accommodation and machinery spaces.
Many such incidents are the result of cargo self-heating or self-reaction.
Some cargoes naturally undergo chemical reactions, such as oxidation in air (for example, charcoal) or a natural but slow decomposition (for example, calcium hypochlorite), and these processes are often exothermic (heat producing). If the heat that is produced cannot be safely dissipated, because of the insulating effect of immediately adjacent material, dunnage, packaging or even other containers, the increase in temperature can lead to an increase in the rate of reaction, which then further increases the amount of heat produced. As a rule of thumb, the rate of chemical reactions will double for every 10°C in temperature rise. Under these circumstances, the material can begin to self-heat and conditions for thermal runaway, where the rate of reaction and heat production rise uncontrollably and beyond ‘the point of no return’, might then be achieved.
Chemicals susceptible to decomposition in this manner can be characterised by a self-accelerating decomposition temperature (SADT), defined as the owest ambient temperature at which self-accelerating decomposition may occur in a substance in the packaging as used in transport. The SADT is influenced by many factors, such as the type and size of individual packages, the method of stowage and the presence of moisture or impurities. In combination, those factors can lead to the depression of the SADT, below the ambient temperatures experienced in a typical ship’s hold for example, and the onset of a violent reaction.
Other chemicals may be susceptible to hazardous polymerisation, in which the individual molecules react together to form long chains, or polymers, with the evolution of heat. Similar to the SADT, materials subject to polymerisation may be characterised by a self-accelerating polymerisation temperature (SAPT), defined as the lowest temperature at which the reaction may occur in a substance in the packaging as offered for transport. There are standard UN Test procedures for both SADT and SAPT.
In some cases, and particularly with chemical decompositions, huge quantities of gaseous products can be liberated, generating an overpressure within the container that results in its violent and explosive rupture and it is not unusual to find debris scattered at considerable distances from the source. Many of these gaseous products are also highly toxic which greatly limits or precludes firefighting. The decomposition of calcium hypochlorite and other oxidising solids is particularly problematic as those processes are not only highly exothermic but generate their own supply of oxygen or other oxidising gas. This combination of heat and oxygen enrichment can readily cause ignition of nearby combustible materials. Moreover, the generation of an oxidising environment means that traditional hold fire suppression systems employing carbon dioxide gas, which work by diluting atmospheric oxygen, may be rendered ineffective.
Despite the best endeavours of carriers to ensure that dangerous goods are stowed correctly and segregated from other incompatible materials, in line with the guidance set out in the International Maritime Dangerous Goods (IMDG) Code, the risks can be impossible to manage if the cargoes have been incorrectly declared. All dangerous goods have special stowage provisions, which set out measures such as whether that cargo can be stowed under deck or whether that cargo should be kept away from sources of heat. Thus, incorrect declarations frequently lead to situations where dangerous goods are stowed inappropriately, such as having heat-sensitive cargoes placed in direct sunlight or adjacent to heated heavy fuel oil tanks. This has been recognised as a primary factor in a number of containership casualties.
The investigation of fires and explosions on board container ships centres, of course, on a visual inspection of the stow, which is usually best achieved during the discharge of containers and debris. In some cases, the location of origin may be visually apparent, involving perhaps the outward bulging of the shell of a violently ruptured incident container coupled with the inward creasing of adjacent boxes. In cases where a fire has burned for a prolonged period, the overall damage might be so extensive that definitive physical evidence to demonstrate the container of origin is very difficult to obtain. In all cases involving fires in containerised cargoes, knowledge of the chemistry of the substances in question is crucial in order to determine their propensity to react and the factors that may have promoted that reaction to a state of self-acceleration. Crew accounts and interrogation of electronic data may also provide useful evidence.
In order to establish liability, it is essential to determine all factors that may have contributed to the onset of self-heating or other uncontrolled cargo reactions. As mentioned above, misdeclaration can lead to heat sensitive cargoes being inappropriately stowed adjacent to heated fuel tanks for example.
However, the development of selfheating to the point of thermal
runaway may be solely due to the inherent properties of the cargo itself,
irrespective of stowage position, conditions on board and segregation from other cargoes. Factors such as excess moisture in the formulation, impurities or contamination with incompatible materials can promote exothermic reactions and lead ultimately to uncontrolled heating.
Whilst chemical analysis can assist in this regard, the cargoes in question
may have been totally consumed during the reaction and ensuing fire, in which case the investigation must consider analysis of the residues from the decomposition reaction, which may provide useful indicators of the precursor materials from which they might have derived.
In summary, the investigation of the cause of containerised cargo fires and
explosions is typically a complex affair that involves many different aspects
across a range of scientific disciplines, from the traditional forensic fire
investigation techniques to the interpretation of complex analytical results. A detailed knowledge of the chemistry of unstable materials is required, and in addition to keeping on top of the various enquiries, the investigator must also be prepared to take into account many other issues, such as the serious safety hazards arising from the generation of toxic gases, the practical difficulties of discharging distorted container shells, and the disposal of contaminated firefighting water and other debris.