Stolt Tankers says it has improved its chemical cargo-handling processes after it came in for criticism in a UK accident report into a “catastrophic” explosion on a tanker in South Korea.

The 43,000-dwt Cayman Islands-flag Stolt Groenland (built 2009) was torn apart by a blast in Ulsan on 28 September, 2019, that injured two seafarers and 15 shore workers.

The UK Marine Accident Investigation Branch (MAIB) said a cargo tank containing styrene monomer ruptured due to runaway polymerisation, a chain reaction where molecules bond together.

This breach released a large quantity of vapour to the atmosphere, which subsequently ignited.

The gangway watchman was blown over the guardrails on the main deck while a colleague initially held on to one of the rails, but was soon forced to let go due to the heat from the fire.

He fell into the water between the quay and the vessel, but he was able to climb over a fender and on to the quay.

Firefighting efforts took more than six hours and involved more than 700 personnel and 117 units of fire trucks, pumps and fire tugs.

The MAIB found that the styrene monomer was affected by other heated cargo tanks.

The temperature was not monitored, the report said.

The elevated temperature due to other cargoes caused the inhibitor — added to prevent the chemical’s polymerisation during the voyage — to deplete more rapidly than expected.

“Although the styrene monomer had not been stowed directly adjacent to heated cargo, the potential for heat transfer through intermediate tanks was not fully appreciated or assessed,” MAIB said.

Critical temperature limits had been reached before the vessel berthed under the road bridge in Ulsan.

The crew was not aware of the increasingly dangerous situation, MAIB said.

Similar incident avoided

A similar dangerous styrene monomer polymerisation incident had occurred a couple of weeks earlier on board another Stolt Tankers vessel, but this was spotted before disaster could strike.

After the Groenland accident, Stolt Tankers took immediate action to ensure that the temperatures of all cargoes carried on board its ships were monitored and reported to its shore management.

The owner also took steps to enhance crew awareness on the hazards of inhibited and heat sensitive cargoes.

The company is developing technological and administrative initiatives to assist with the safe stowage and monitoring of heat sensitive cargoes, MAIB added.

The Stolt-Nielsen group company said in a statement that safety for people and the environment remains its number one priority.

“Stolt Tankers has used the learnings from this regrettable accident to improve its procedures for handling inhibited cargoes,” the owner said.

Improvements made

During 2020, the company made several improvements in stowage planning processes afloat and ashore, and in practices relating to inhibitors, managing cargo alarms and reporting of cargo temperatures to shore staff.

“Our seafarers also receive additional training to increase their understanding and awareness of the importance of these changes. The MAIB report acknowledges that these actions have already been taken,” Stolt Tankers said.

“We are working with industry organisations to improve the inhibitor management process and we continue to improve and develop our cargo stowage software to better predict the movement of heat between tanks and around a ship,” the company added.

Stolt Groenland remains afloat in Tongyeong, South Korea, with all styrene residues removed from the ship.

Planning for repairs continues as the owner awaits approval from local authorities to tow the ship to China.