MAIB publishes findings from STOLT GROENLAND investigation

The incident occurred on 28 September 2019 when a cargo tank containing styrene monomer ruptured as a result of runaway polymerisation. Runaway polymerisation is a chemical reaction that can occur if the cargo polymerises and heat which is produced by the reaction is not removed from the cargo. When runaway polymerisation occurs, the cargo will expand rapidly leading to an increase in tank atmosphere pressure. If venting systems cannot cope with the increase in tank pressure, the expansion can result in a cargo tank rupture.

The catastrophic rupture released a large quantity of styrene monomer vapour into the atmosphere which then ignited.

The MAIB have identified the following safety issues which led to the incident:

• the styrene monomer was affected by other heated cargo tanks
• heat transfer from other cargoes was not fully appreciated
• the styrene monomer temperature was not monitored

The full report can be read here. 

Previous Update: 17 Dec 2019

Maib issues alert on Styrene Monomer Cargoes

The UK Marine Accident Investigation Board (MAIB) have published an interim report on the investigation of the explosion on board the chemical tanker STOLT GROENLAND.

Owner, operators and crews of chemical tankers are urged to read the MAIB’s interim report, which concerns the over-pressurisation of a cargo tank of styrene monomer and its subsequent explosion.

The investigation of the explosion at Ulsan, Rep. of Korea is ongoing, however, the MAIB has seen it appropriate to issue recommendations based on initial findings.

Styrene Monomer Cargo

Styrene monomer is used to make plastics, paints and synthetic rubber. It is highly flammable with a flash point of 32°C (90°F). Although generally stable at ambient temperature, polymerization of the monomer can occur, initiated by heat or contact with peroxides. The polymerization process generates heat and can become very rapid and self-sustaining.

VDR data from the vessel showed that the temperature of the styrene monomer had reached 100°C at the time of the explosion. The MAIB reports states that this is an indication that the cargo was polymerizing.

Inhibitors

TertButylcatechol (TBC) inhibitor is added to reduce the possibility of polymerization. Typical concentrations are between 10-15ppm but may be higher depending on storage conditions and expected voyage duration. The depletion of TBC over time and its effectiveness as an inhibitor are affected by temperature, oxygen levels and water.

In this instance, the concentration of TBC inhibitor in the shore tank was 11.3ppm compared to a target of 17ppm. As such, three US gallons of TBC was added to each designated tank before loading.

MAIB reminder to industry

The report includes the following reminders to chemical tanker owners and operators:

• Adhere to the carriage and storage instructions details on the safety data sheet, the certificate of inhibitor, and those provided by the charterer.
• Witness the addition of inhibitor into each cargo tank.
• Closely monitor cargo temperature for unexplained increases. They should also ensure that their crews are familiar with the action to take in the event of the styrene monomer self-heating/polymerizing.

Read the interim report here.