MAIB Calls for Information on Styrene Monomer Management

The U.K. Marine Accident Investigation Branch (MAIB) has released an interim report into the explosion and fire on board the chemical tanker Stolt Groenland, but before concluding its investigation, MAIB is calling for information regarding any accidents or near-misses involving the carriage of styrene monomer, including any actions subsequently taken.

The fire on Stolt Groenland occurred in Ulsan, South Korea, on September 28, 2019. The Cayman Islands registered vessel had sailed from Houston on August 17 carrying 20 different chemical cargoes in 37 of her 39 cargo tanks. At about 0600 on September 28, the Singapore registered chemical/product tanker Bow Dalian 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.

At 1043, vapor started to release from the pressure vacuum valve on 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 tank had reached 95 percent, soon followed by a high-high-level alarm indicating that the level had increased to 98 percent. At 1050, two explosions occurred in rapid succession, and the resulting fireball passed very close to a road bridge above the quay.

The MAIB report states that the explosions were probably caused by the rupture of the deck above the cargo tank, followed immediately by the ignition of the styrene monomer vapor that was then released. The rupture was due to over-pressurization, 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 explosions. Such an elevated temperature indicates that the cargo was polymerizing.

The MAIB notes a subsequent incident which occurred on November 20, 2019. The Cayman Islands registered chemical/products tanker Stolt Focus had a situation where the temperature of a styrene monomer cargo steadily increased. Efforts to stabilize the temperature, including the injection of additional inhibitor, were unsuccessful. Following consultation with chemists ashore, the crew distributed the cargo between four cargo tanks and mixed it with sea water, which stabilized the styrene monomer.

The MAIB’s investigation continues and will focus on the factors contributing to the initiation of polymerization, cargo monitoring and emergency response.