The Australian Transport Safety Bureau (ATSB) has published its investigation report on the liquified hydrogen (LH2) carrier Suiso Frontier. According to the report, the failure of an incorrectly-fitted electrical solenoid valve led to the brief propagation of flame.
While berthed on the evening of 25 January 2022, the chief mate and cargo engineer made plans to start the ship’s gas combustion unit or GCU (see the section titled Gas combustion unit and Figure 1) to burn excess boil-off gas (BOG) from the LH2 cargo tank. 2 They notified the wharf operator and followed standard procedures for preparing the GCU, including conducting required safety checks. The ship was equipped with 2 radial fans to supply combustion, cooling and dilution air to the GCU, and fan number 1 was pre-selected for use.
At 1947, the chief mate and cargo engineer initiated the starting sequence of the GCU from the unit’s compartment in the ship’s forecastle space. The unit’s control system then performed a series of automated function tests to ensure the correct parameters were present for stable hydrogen combustion. By 2007, the GCU was in operation.
Over the next 8 minutes, the cargo engineer gradually increased the hydrogen flow until the maximum combustion rate of 40 kg of hydrogen per hour was reached. All GCU parameters and temperatures were observed to be within their normal range. Meanwhile, the third mate had started their watch and was assigned to monitor the GCU from the cargo control room (CCR).
At about 2147, an able seaman (AB) was conducting routine safety rounds on deck when they saw a 1 m high yellow flame propagate for about 5 seconds from the GCU vent stack on the port side of the ship’s foredeck (Figure 2). The AB immediately notified the third mate in the CCR via handheld radio. Seconds later, the GCU flue gas temperature reached 450°C, triggering the high flue gas temperature and common alarms in the CCR. The third mate quickly shut down the GCU and closed its main hydrogen supply valve. After confirming with the AB that there was no flame from the vent stack, the third mate telephoned the chief mate, master and cargo engineer.
1 Failure of damper actuator solenoid
The solenoid valves installed on the gas combustion unit’s air fan discharge damper actuators were of the incorrect specification. Due to the mismatched specifications between the 24 V DC solenoid valves and the control system’s 230 V AC power supply, the solenoid valves had been subjected to damaging vibrations and high temperatures during operation. After a relatively short 400-hour service life, one of the solenoids eventually failed at the time of the incident, most probably due to overheating and material fatigue, leading to a short circuit or functional fault.
2 Closing of vent damper
Following the failure of the damper actuator solenoid valve during GCU operation, the damper closed, significantly restricting air supply to the GCU for combustion, cooling and dilution.
Due to the wide flammability range of hydrogen gas, enough air remained within the GCU to support combustion of the gas for several minutes after the damper closed. However, as the airflow for cooling and dilution was significantly restricted by the closed damper, temperatures in the GCU began to rise. Following the restriction of air to the GCU, the hydrogen flame gravitated towards the ambient air outside of the vent. As the flame rose up from the vent, it probably reacted with sodium chloride (salt from the environment at sea) ingrained on the inner flue surfaces resulting in the yellow flame that the AB reported.
3 Gas combustion unit design
The GCU’s safety systems did not detect and respond to the malfunction in time to prevent the hydrogen flame propagating from the vent stack.
The manufacturer’s failure mode and effect analysis (FMEA) predicted that, in the event of the damper closing during operation, the GCU’s 2 flame scanners would detect the resulting instability of the hydrogen flame and shut down the GCU. However, when the damper closed, the scanners did not detect any abnormality despite the hydrogen flame transitioning out of the vent stack.
The GCU was not equipped to detect the failure of the vent damper solenoid valve or the subsequent closing of the damper. A low air pressure transmitter was fitted to the GCU but was located between the fan and the damper. As a result, it did not activate when the damper closed since the air pressure on the fan side of the damper did not drop.
Probable cause
– An incorrect type of solenoid valve had been installed on the pneumatic damper actuators for the gas combustion unit’s (GCU) 2 air fans. The 24 V direct current solenoid valves installed were incompatible with the system’s 230 V alternating current power supply.
– During operation of the GCU, the fan discharge damper providing combustion, dilution and cooling air closed when the damper’s actuator solenoid valve failed. Consequently, the temperature in the GCU increased, making the hydrogen flame unstable, which then propagated from the unit’s vent stack on the ship’s deck.
– The GCU was not equipped to detect an air damper closing during operation, and its flame scanners were ineffective in detecting the abnormal condition as per the manufacturer’s risk assessment. As a result, the GCU alarm and shut-down mechanisms did not activate in time to prevent the flame propagating from the vent on deck.
