The runaway polymerisation of a styrene monomer cargo may have been the cause of the explosion onboard Stolt Groenland
The initial findings of the interim report by the UK Marine Accident Investigation Branch (MAIB) on the explosion and fire on board the chemical tanker Stolt Groenland on 28 September 2019, 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 is a regular cargo carried by Stolt and other chemical carrier operators and its properties are well known. It is a colourless oily liquid and is highly flammable. Styrene monomer has a low flash point of 32oC and exothermic runaway polymerisation can occur from temperatures above 65oC.
To counteract the possibility of polymerisation, TertButylcatechol (TBC) inhibitor was added to the tanks before cargo loading. TBC is added in concentrations of between 10 and 15 ppm. The concentration of TBC inhibitor in the styrene monomer in the shore tank was 11.3 ppm, but this was increased by the addition of three US gallons of liquid TBC to each of the destination tanks before loading commenced. The target concentration of the TBC was 17 ppm. The MAIB notes that TBC effectiveness can decline over time, and that it is affected by temperature, oxygen levels and water.
The interim report related the following sequence of events leading up to the fire onboard the tanker: At 10:43 on the morning of 28 September 2019, vapour started to release from the pressure vacuum valve for Stolt Groenland’s number 9 starboard (9S) cargo tank, which contained styrene monomer. About two 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 10:50, two explosions were seen and heard in rapid succession in way of the tanker’s cargo manifold.
Initial examinations show that two explosions occurred almost simultaneously. The first is believed to have been the rupturing of the deck immediately above tank 9S, due to the excess pressure in the tank from the runaway polymerisation of the styrene monomer cargo. The second explosion is believed to have been caused by the ignition, possibly by static, of the styrene monomer cargo. Recovered instrument data shows the cargo reached 100oC immediately before the explosion. It is important to note that no cargo operations were underway at the time.
The MAIB has requested that chemical tanker owners/operators are reminded to: