USCG issues gas carrier safety alert
Repairs to the main engine of a dual-fuel gas carrier that had been completed wrongly could have caused a major incident was discovered during a USCG PSC inspection of the ship’s Gas Valve Unit (GVU) room. The GVU is a multi-component device which manages the LNG pressure supplied to the propulsion engines. The GVU room has an air intake and exhaust system designed to continuously ventilate and exchange the air within the space to reduce fire, explosion, and hazardous atmosphere risks from developing if gas leaks should occur from the equipment. The atmosphere of the room is monitored by a catalytic methane sensor located near the inlet to the room’s ventilation exhaust trunk. The GVU room’s ventilation creates a vacuum within the room when the two access doors are shut. External to the GVU room the gas supply piping to the main engines is double walled including through manifolds and expansion bellows all the way to each cylinder. The purpose of this double walled construction is to contain any leakage of gas and return it back to the GVU room via a small gas evacuation fan assembly. During a repair on one cylinder of a main engine vessel, engineers had removed an expansion bellows but when reassembling an O-ring, separating its inner and outer sections, was damaged. This error went unnoticed until a crew member was making a round in the enclosed GVU room while the engine had been operating on gas. After entering the GVU room he was overcome by methane gas and nearly lost consciousness. After the incident the GVU room atmosphere was measured to be 22% methane and 17% oxygen by volume. Methane is an asphyxiant which displaces oxygen and is extremely flammable. The installed methane sensor failed to detect the accumulation of gas despite not having malfunctioned. Engineers traced the methane leakage to the recently removed bellows and replaced the damaged O-ring. To prevent future accumulation of gases they rigged a hose from the outlet of the gas evacuation fan, across the GVU room to the sensor at the entrance exhaust duct. This unauthorised arrangement, which was identified during the USCG inspection, could have likely disabled the sensor’s ability to detect methane leakages from other components within the GVU room. The inspectors also noted that a second exhaust hose was similarly installed in the vessel’s second GVU room. These hoses were in place for about eight years. It is also important to note the engine manufacturer’s manual highlights the need for caution when replacing the bellows and to use care when installing the O-rings. The manufacturer also emphasises the need to check for gas leaks after repairs or maintenance are performed. As a result of this discovery, the USCG recommends that owners and operators in all segments of the maritime industry with a special emphasis on conventional LNG-fueled vessels and LNG carriers
- Urge their vessel personnel to request additional expertise such as classification societies and manufacturer representatives when technical issues arise (e.g., the failure of the GVU room sensor to detect gas leakage or potential inadequate diffusion and circulation of air exchanges within the GVU room) to ensure the most effective corrective actions and system alterations take place as needed.
- Make notifications to the flag state authorities and classification societies regarding potentially hazardous conditions before making any modifications to existing approved installations, arrangements, or procedures.