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Interim report released on cause of September 2019 oil tanker explosion

06 January 2020

The UK Marine Accident Investigation Branch (MAIB) has released an interim report on an oil tanker explosion in September 2019. The blast on the Stolt Groenland, a Cayman Island-flagged tanker, injured 10 people while the ship was docked in the port of Ulsan, South Korea.

Tank rupture and ignition of released vapour - Image: MAIB
Tank rupture and ignition of released vapour - Image: MAIB

The MAIB’s report, released on December 16, says the runaway polymerisation of a styrene monomer cargo was the probable cause of the explosion, but investigators are seeking further information. On September 28, 2019, an explosion on the Stolt Groenland caused a fire which spread to a second oil tanker docked nearby, the Bow Dalian. There were 25 sailors on board the Stolt Groenland at the time of the blast and a crew of 21 on board the Bow Dalian. All sailors from both tankers were rescued.

According to the report, initial findings suggest that the exothermic heat generated by the polymerisation of a styrene monomer cargo was the likely cause. Styrene monomer is an oily liquid used to make plastics, paints and synthetic rubber. It has a flash point of 32°C (90°F).

During the incident in September, the MAIB says that vapour started to release from the pressure vacuum valve from one of Stolt Groenland’s cargo tanks which contained styrene monomer. Shortly after, a high-level alarm indicated that the level in the tank had reached 95%, soon followed by a high-high-level alarm indicating that the level had increased to 98%. The ship’s workers saw from a monitoring system that the pressure inside the tank was rapidly rising before two explosions occurred in rapid succession in way of the tanker’s cargo manifold. Both the Stolt Groenland’s and Bow Dalian’s crews immediately went to tackle the resulting fire with foam monitors before both crews evacuated the ships.

The MAIB report says that the explosions were probably caused by a rupture in the cargo tank due to over-pressurisation. The rupture released styrene monomer vapour which was then likely ignited by static electricity, sparks or elevated steel deck plate temperatures resulting from the tank rupture. Voyage data recorder (VDR) data showed that the temperature of the styrene monomer had reached 100ºC at the time of the explosion. The Stolt Groenland had been transporting 5,250 tonnes of styrene monomer from Houston, USA.

The MAIB is continuing its investigation and will publish a full report once it is complete. In the meantime, the organisation is reminding chemical tanker owners/operators to:

Fireball viewed from the Ulsan bridge - Image: MAIB
Fireball viewed from the Ulsan bridge - Image: MAIB

- 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.

The MAIB adds that crews should also be familiar with what action to take in the event of styrene monomer self-heating/polymerising.

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; further contact details are available at

Click here to read the MAIB’s interim report in full.

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