UK’s Marine Accident Investigation Branch (MAIB) report said the temperature of the styrene monomer cargo on board the chemical tanker was not monitored and heat transfer from other cargoes was "not fully appreciated"
The MAIB report into an explosive blaze on board a chemical tanker that spread to a second tanker in the South Korean port of Ulsan in September 2019 has pointed to heated cargo tanks and a lack of temperature monitoring as underlying causes in the fire.
Residual heat from Stolt Groenland’s heated tanks acted on a cargo tank containing the chemical compound styrene monomer that makes up polystyrene, causing the styrene monomer to combine, creating a heat-producing chain reaction known as runaway polymerisation. The styrene monomer cargo’s temperature was not being monitored.
"Cayman Islands-registered chemical tanker Stolt Groenland ruptured due to runaway polymerisation. The catastrophic rupture released a large quantity of vapour to the atmosphere, and it subsequently ignited," the MAIB report said.
The MAIB report made a series of recommendations with regard to the incident. Investigators told Stolt Groenland owner Stolt Tankers to work to ensure "the wider marine chemical sector benefits from the lessons learned" from the incident and the research that resulted from the accident.
Stolt Tankers issued a statement in response saying it has "used the learnings from this regrettable accident to improve its procedures for handling inhibited cargoes".
"During 2020, we made several improvements in our stowage planning processes afloat and ashore, and in our 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’s statement 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 MAIB report recommended that Cayman Island Shipping Registry, along with the Chemical Distribution Institute and Plastics Europe’s Styrene Producers Association, work to ensure "the guidance provided in certificates of inhibitor and styrene monomer handling guides is consistent and achievable given the limitations of equipment and testing facilities on board ships".
MAIB also asked industry associations the Internantional Chamber of Shipping and Intertanko to distribute the incident report to their members.
The fire from Stolt Groenland spread to another vessel, Odfjell’s Bow Dalian, which was docked nearby, and South Korea’s state-owned news agency Yonhap reported 17 people were injured in the blaze including 11 sailors and six rescue workers "mostly from burns and smoke inhalation".
Fire-fighting efforts by the emergency services on site in the port of Ulsan lasted for more than six hours and involved some 700 personnel and dozens of fire trucks, water pumping units and fire-fighting tugs.
MAIB released interim reports on the incident that chronicled the vessel’s movements and gave further detail on happenings related to the fire.
Stolt Groenland loaded 5,250 tonnes of styrene monomer at the LBC terminal in Houston, USA, between 7 and 8 August 2019. The styrene monomer was distributed across three stainless steel cargo tanks: numbers 9S, 6P and 6C. The tanks were washed, inspected and wall-wash tests conducted before loading. Styrene monomer is a regular cargo carried by Stolt and other chemical carrier operators and its properties are well known. It is a colourless oily liquid and is highly flammable. Styrene monomer has a low flash point of 32°C and exothermic runaway polymerisation can occur from temperatures above 65°C.
To counteract the possibility of polymerisation, TertButylcatechol (TBC) inhibitor was added to the tanks before cargo loading. TBC is added in concentrations of between 10 and 15 ppm. The concentration of TBC inhibitor in the styrene monomer in the shore tank was 11.3 ppm, but this was increased by the addition of 11 litres of liquid TBC to each of the destination tanks before loading commenced. The target concentration of the TBC was 17 ppm. The MAIB notes that TBC effectiveness can decline over time, and it is affected by temperature, oxygen levels and water.
The interim report related the following sequence of events leading up to the fire on board the tanker: At 10:43 on the morning of 28 September 2019, vapour started to release from the pressure vacuum valve for 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 9S cargo tank had reached 95%, soon followed by a high-high-level alarm indicating that the level had increased to 98%. By now, Stolt Groenland’s on-watch deck officer and chief officer had made their way to the cargo control room and they saw from the cargo monitoring system that the pressure inside 9S cargo tank was rapidly rising. Suddenly, at 10:50, two explosions were seen and heard in rapid succession in the tanker’s cargo manifold.
Initial examinations show the two explosions occurred almost simultaneously. The first is believed to have been the rupturing of the deck immediately above tank 9S, due to the excess pressure in the tank from the runaway polymerisation of the styrene monomer cargo. The second explosion is believed to have been caused by the ignition, possibly by static, of the styrene monomer cargo. Recovered instrument data shows the cargo reached 100°C immediately before the explosion. It is important to note that no cargo operations were underway at the time.
Stolt said that Stolt Groenland remains afloat in Tongyeong, South Korea, with all styrene residues removed from the ship. The company said its plans for repairs are continuing as it awaits approval from local authorities to tow the ship to China.