The Office of Investigations & Casualty Analysis issued Safety Alert 05-17 to warn of a potentially dangerous situation involving modifications within gas valve unit room aboard Liquefied Natural Gas carriers.
Recently Coast Guard port state control officers performed an examination on board a Liquefied Natural Gas carrier. While onboard the inspectors discovered a significant and potentially dangerous modification within a gas valve unit (GVU) room. This room is located within the machinery space of the dual fuel, diesel-electric propelled vessel. These engines can burn petroleum oil or LNG in gaseous form. The GVU is a multi-component device which manages the liquefied natural gas 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.
During a repair on one cylinder of a main engine vessel, engineers had to remove an expansion bellows. Upon replacement of the bellows 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. Fortunately, he was able to exit the space into a safe atmosphere. After the incident the GVU room atmosphere was measured to be 22 percent methane and 17 percent 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.
As a result of this discovery, the Coast Guard strongly recommends that owners and operators in all segments of the maritime industry with a special emphasis on conventional LNG fueled vessels and Liquefied Gas Carriers:
- Emphasize to their organization’s technical shore side and vessel personnel the importance of following:
- Manufacture guidance
- Recommendations in terms of maintenance and repair procedures
- Methods to validate successful repairs involving critical systems, including leakage detection
- Procedures to evaluate corrective modifications (e.g., the use of the exhaust hose), even if intended to be temporary, to ensure they do not create additional safety hazards
- 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 proper Flag State authorities and classification societies regarding potentially hazardous conditions before making any modifications to existing approved installations, arrangements, or procedures.
For full details of the incident, read or download Safety Alert 05-17.
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