Transport Malta released the report on the explosion onboard “Seascout”, on 18 March 2021, highlighting the dangers of working with heat or spark generating tools near locations where flammable vapour or volatile material may be present.
The incident
Seascout departed from Khor Fakkan, U.A.E., in ballast and gas-free conditions, on 17 March 2021. It was bound for Al Duqm, Oman, for the scheduled dry-docking. The arrival date was 19 March 2021, with docking on arrival.
On 18 March 2021, at 0800, a toolbox meeting was carried out. The task to remove the deck ramps for inspection and maintenance was assigned to the riding team.
A cold work permit and a risk assessment for the removal of deck ramps had been prepared by the chief officer. The former document was also countersigned by the master and the technician foreman.
By 1700, the crew and technicians left for dinner. Only two more bolts on the ramp, near the accommodation block, had to be undone. After dinner, one of the assigned technicians set off to complete his task. Having already tried to use other means to undo the last two heavily corroded bolts, he decided to use an angle grinder to facilitate the job.
The pumpman and the AB, who were closeby on deck, were installing a new isolation valve on the fire line, when they heard the angle grinder running. The pumpman requested the AB to inform the technician not to use it. The AB called out to the technician, and the technician immediately ceased his work.
However, by the time the pumpman and the AB arrived at the technician’s position, they observed smoke coming out of the vent of FOT no. 2P. The pumpman directed everyone to move away and clear the area. At around 1833, while the pumpman, AB and the technician were running towards the midship store to get firefighting equipment, the pumpman and AB saw a flash and flames from the air vent, followed by a loud explosion.
Hearing the explosion, the master activated the nearest manual call point which set off the fire alarm. Soon after, all hands were accounted for and the fire squads assembled.
Fire hoses were deployed, and boundary cooling of the affected deck area was commenced, although there were no visible fire / flames.
Inspection of the deck area during the boundary cooling, revealed that the port side liferaft had fallen overboard but was still attached to the vessel. Moreover, the port lifeboat had been displaced and suspended from its davit. By 1910, whilst the vessel’s engine was stopped, the crew managed to retrieve the liferaft on board.
Cooling down continued until past midnight, when the temperature in FOT no. 2P was observed to drop. An inert gas connection was made and FOT no. 2P was flooded with inert gas. By 0400 of 19 March 2021, the crew managed to secure the suspended lifeboat and the vessel resumed her voyage.
Analysis
Residual fuel oils are known to produce light hydrocarbons inside their storage tank, even when they are kept at temperatures lower than their flashpoint. Vapour composition may also reach an explosive atmosphere, even when the fuel is unheated.
The heating of the fuel inside FOT no. 2P had been gradually increased, albeit kept at a lower temperature than the flashpoint of the fuel. It was not excluded that gas concentration may have increased inside the tank and the vent pipe during this time, some of which would have vented out and accumulated around the vent head of the tank.
The grinding wheel in use was of the conventional type, rather than, say, of the ceramic-type, which would have generated a lower cutting temperature. Moreover, given that the angle grinder was operated near the vent head of FOT no. 2P, without a protective screen, spark(s) and high temperature particles emitted from the angle grinder must have reached the position of the vent head.
The safety investigation considered that spark(s) and high temperature particles7 either passed through the mesh of the flame arrestor, or landed inside the save-all of the tank head, leading to the explosion. Oily / waxy residues on the flame arrestor, found after the accident may have contributed to the flame propagation at the tank’s vent head and into the tank.
What followed gave clear indications to the safety investigation that an explosive atmosphere was present inside and potentially around the vent head. The subsequent flash and the ejection of flames from the vent head were signs that the gases generated by the fuel within the tank were within the flammable range. However, since a fire or subsequent explosions did not occur, it was hypothesized that not enough oxygen was present within the tank to sustain combustibility.
Explosive atmosphere
Fuel sample analysis reports received for the VLSFO in FOT no. 2P indicated that it had to be kept at 10℃ above its pour point temperature. This prompted the chief engineer to increase the heating within the tank to 36℃ (i.e., 2℃ higher than required).
Two days prior to arrival at the dry-docks, heating of the FOT was gradually increased to 48℃, which was well beyond the recommended storage temperature. The rationale behind this increase was that tank heating would have to be stopped during the vessel’s stay in the dry-dock, although the fuel would still need to be kept at the recommended temperature; otherwise, the crew would risk the fuel becoming unpumpable.
Heating up of the fuel closer to its flashpoint must have resulted in an increased generation of gases, which would ultimately be vented to the atmosphere via the tank’s vent head. It so happened that for most of 18 March 2021, the vessel was heading on a course of 204° (T), at a speed of about 12 knots. Wind was blowing from the South at a speed of 11 knots. This would have created an apparent wind of around 22 knots approaching the vessel at a close angle on its port side.
Conclusions
- The VLSFO in FOT 2 P was heated up to 48℃, to maintain pumpability of the fuel during the vessel’s stay in the dry dock, which resulted in an increase in the generated volume of flammable gas.
- The safety investigation considered that spark(s) and high temperature particles either passed through the mesh of the flame arrestor, or landed inside the save-all of the tank head, leading to the explosion.
- Oily / waxy residues on the flame arrestor, found after the accident may have contributed to the flame propagation at the tank’s vent head and into the tank.
- No protective screen had been erected prior to the commencement of the task.
- The vessel was experiencing an apparent wind of 22 knots from her port bow. This wind did not assist in the dispersion of the concentration of the flammable gas at the vent head. This suggested that the vent head was somewhat protected from this wind and/or the formation of eddies affected the dispersion of flammable gas.
- The last two bolts on the deck ramp were heavily corroded and could not be removed with conventional hand tools.
- The technician’s decision to use the angle grinder was influenced by his willingness to finish the task in time, prior hours of darkness and the vessel’s arrival in dry dock.
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