Lessons learned from an explosion on a chemical tanker caused by unnoticed heating of styrene monomer cargo, leading to a thermal runaway and tank rupture.
A chemical tanker was moored alongside waiting for another vessel on its offshore side to finish purging its cargo tanks with nitrogen. Without warning one of the cargo tanks exploded, sending a large fireball skywards towards a major road bridge. Remarkably, just one crew member on each ship was injured.
The tanker had loaded a multiple parcel cargo some weeks before.
Some of the cargoes required heating, inert atmospheres and/or inhibiting chemicals for stabilisation. One of the cargoes, styrene monomer (an aromatic hydrocarbon used to manufacture plastic, rubber and polystyrene products), was inhibited for the voyage. However, the inhibitor was only effective for a time-limited period within a specific temperature range.
Under normal circumstances, styrene monomer was a benign cargo that did not create any issues when stowed in ambient temperatures away from heated cargoes. Consequently, the chemical was not monitored by the crew, and alarm systems were not enabled.
The styrene monomer was in three of the chemical tanker’s 39 cargo tanks. The other tanks contained various chemicals, some of which were heated (Figure 2).
Unfortunately, the heated cargoes in this case were adjacent to cargoes that readily absorbed heat and transferred it to the styrene monomer cargo. The raised temperatures affected the polymerisation inhibitor, reducing its time‑limited efficacy. The ineffective inhibitor had triggered a thermal runaway incident that caused high pressure to breach the tank and ignite, likely due to static electricity.
Lessons learned
Check → There are strict rules and regulations for the safe carriage of chemicals. However, while the cargo might be correctly stowed to meet both these and the requirements of the charterer there is a risk that less apparent issues could be missed. Check loaded cargoes frequently to determine whether they are behaving as expected and, if they are not, contact the ship’s operating company or the chemical’s supplier for advice.
Equipment → Liquid cargo monitoring equipment usually has a temperature alarm function. Check the safety data sheet and carriage instructions for crucial temperatures and set the alarms in line with these.
Plan → A phone call ashore could offer solutions, but might prove ineffective mid-voyage when the ship is several days from port. The preparation of a robust shipboard response procedure will ensure that you are well-equipped to deal with unexpected events during the carriage of sensitive cargoes.
Risk → Inhibited cargoes are stable for a limited amount of time. The inhibitor could lose its effectiveness if the voyage is extended, causing the cargo to become unstable and change to a hazardous state. Make sure risk assessments account for this possibility and that appropriate mitigations are in place.