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Monday, December 16, 2019 

The UK's Marine Accident Investigation Branch (MAIB) has published an interim report - requested by the Cayman Islands Maritime Authority - into an explosion which occurred onboard the ‘Stolt Groenland’, a Cayman Islands registered chemical/products tanker operated by Stolt Tankers, while moored in harbour in Ulsan, Korea.

The tanker was carrying 20 different chemical cargoes in 37 of its 39 cargo tanks. Before mooring, the vessel had discharged 10 of its cargo tanks, and a further two had been unloaded via a ship to ship transfer with Stolt Voyager. On completion, Stolt Voyager moored ahead of Stolt Groenland.

The Singapore registered chemical/products tanker Bow Dalian was then secured alongside Stolt Groenland’s port side. The purging of Bow Dalian’s cargo tanks with nitrogen supplied from shoreside vehicles was then commenced in readiness for a ship-to-ship cargo transfer with Stolt Groenland.

Vapour started to release from the pressure vacuum valve for Stolt Groenland’s number 9 starboard (9S) cargo tank, which contained styrene monomer. A high level alarm then indicated that the level in 9S cargo tank had reached 95%, soon followed by another alarm indicating 98%. Stolt Groenland’s deck officer and chief officer noted that the pressure inside 9S cargo tank was rapidly rising. Suddenly two explosions were seen and heard in rapid succession in way of the tanker’s cargo manifold.

Stolt Groenland’s and Bow Dalian’s crews immediately directed foam monitors towards the respective cargo manifolds. Bow Dalian’s cargo manifold drench system was also activated. The two vessels' crews were evacuated, and the fire fought from the shore and from the water

Stolt Groenland suffered extensive damage to the cargo manifold and accommodation block, and several persons were injured.

MAIB's interim report notes that styrene monomer, which is used to make plastics, paints and synthetic rubber, is highly flammable, with a flash point of 32°C (90°F). It is also toxic and reactive and is generally stable at ambient temperature. Polymerization of the monomer is initiated by heat or contact with peroxides. To reduce the possibility of polymerisation in storage and during transportation, TertButylcatechol (TBC) inhibitor is added. TBC depletes over time and its effectiveness as an inhibitor is affected by temperature, oxygen levels and water.

The interim report says that the explosions were probably caused by the rupture of the deck above 9S cargo tank, followed immediately by the ignition of the styrene monomer vapour that was then released. The rupture was due to over-pressurisation and the likely sources of the ignition were static electricity, sparks or elevated steel deck plate temperatures resulting from the tank rupture. VDR data showed that the temperature of the styrene monomer had reached 100ºC at the time of the explosion. Such an elevated temperature indicates that the cargo was polymerising. The certificate for the TBC inhibitor, issued by the cargo surveyor at the time of loading, stated that the TBC should remain effective for between 60 and 90 days.

MAIB will continue to progress the investigation and ensure appropriate action can be taken to reduce the likelihood of similar accidents. Meanwhile, chemical tanker owners/operators are reminded to:

● Adhere to the carriage and storage instructions details on the safety data sheet, the certifcate 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/polymerising.

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