The runaway polymerisation of a styrene monomer cargo may have been the cause of the explosion onboard Stolt Groenland, according to an interim report by the UK Marine Accident Investigation Branch (MAIB), which is seeking more information about the incident
The initial findings of the interim report on the investigation of the explosion and fire on board the chemical tanker Stolt Groenland on 28 September 2019, Ulsan, Republic of Korea, suggests that the exothermic heat generated by the polymerisation of a styrene monomer cargo may have been the cause.
Stolt Groenland loaded 5,250 tonnes of styrene monomer at the LBC terminal in Houston, USA, between 7 and 8 August 2019. The styrene monomer was distributed across three stainless steel cargo tanks: numbers 9S, 6P and 6C. The tanks were washed, inspected and wall wash tests conducted before loading. Styrene monomer has a flash point of 32oC and exothermic runaway polymerisation can occur from 65oC. TertButylcatechol (TBC) inhibitor was added to the tanks before the cargo loading.
The interim report related the following sequence of events leading up to the fire onboard the tanker:
At 1043, vapour started to release from the pressure vacuum valve for Stolt Groenland’s number 9 starboard (9S) cargo tank, which contained styrene monomer. About 2 minutes later, a high-level alarm indicated that the level in 9S cargo tank had reached 95%, soon followed by a high-high-level alarm indicating that the level had increased to 98%. By now, Stolt Groenland’s on-watch deck officer and chief officer had made their way to the cargo control room and they saw from the cargo monitoring system that the pressure inside 9S cargo tank was rapidly rising. Suddenly, at 1050, two explosions were seen and heard in rapid succession in way of the tanker’s cargo manifold (see image above labelled Figure 1).
The ignition may have been a static spark caused by the tank and deck rupturing from the pressure. Recovered instrument data shows the cargo reached 100oC immediately before the explosion.
The MAIB interim report is available here. In the meantime, the MAIB said that chemical tanker owners/operators are reminded to:
The MAIB is seeking any information from ship owners, ship and terminal operators, or individuals regarding any accidents or ‘near-misses’ involving the carriage of styrene monomer on board ships, including any actions subsequently taken. Those with information are encouraged to contact the MAIB via email at email@example.com; further contact details are available at www.gov.uk/maib.