An initial report into the explosions on board the chemical Stolt Groenland during ship to ship transfer operations of styrene monomer with the Bow Dalian in Ulsan in September has been released.
The report says that the explosions on board Stolt Groenland 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. A large hole was found in way of the tank’s common bulkhead with number 9 centre (9C) cargo tank and its hatch cover had been blown off. No cargo operations or deck maintenance were in progress at the time. 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.
Stolt Groenland had loaded 5,250 tonnes of styrene monomer at the LBC terminal in Houston between 7 and 8 August. The cargo was distributed between three stainless steel cargo tanks: numbers 9S, 6P and 6C. Prior to loading, the tanks had been washed and inspected and wall wash tests had been conducted; no problems were recorded. The tanks were not purged with nitrogen before loading and none were adjacent to heated cargoes in accordance with cargo loading instructions.
The concentration of TBC inhibitor in the styrene monomer in the shore tank was 11.3ppm but this was increased by the addition of liquid TBC to each of the destination tanks before loading was commenced. The target concentration of the TBC was 17ppm. The loading was overseen by an American Cargo Assurers cargo surveyor acting on behalf of the supplier of the styrene monomer, Ineos Styrolutions, and the purchaser, Samsung C&T Corporation.
The certificate of inhibitor, issued by the cargo surveyor at the time of loading, stated that the TBC should remain effective for between 60 and 90 days. It also stated that the inhibitor was oxygen dependent and that the ideal temperature for the styrene monomer was between 60ºF and 85ºF (15.5ºC – 29.4ºC). Stolt Groenland did not carry additional inhibitor.
Styrene monomer is used to make plastics, paints and synthetic rubber. It is a colourless to yellowish oily liquid with an aromatic odour and is highly flammable. It has a flash point of 32°C. It is also toxic and reactive and is generally stable at ambient temperature. Polymerisation of the monomer is initiated by heat or contact with peroxides. The polymerisation process is exothermic and the monomer’s temperature may rise to the point where the reaction becomes very rapid and self-sustaining (runaway polymerisation). Normally, temperatures above 65°C are needed to initiate runaway polymerisations.
To reduce the possibility of polymerisation in storage and during transportation, Tert-Butylcatechol (TBC) inhibitor is added to styrene monomer, typically at a concentration of between 10-15ppm, but higher levels might be needed 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.
MAIB has said that its investigations will focus on the cause of the polymerisation, cargo operations and emergency responses. It has also issued a call for any information on problems or near misses experienced with the cargo on other vessels. At least one other more recent incident is known about.
MAIB has issued interim advice to ships carrying the cargo reminding operators to:
- 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.