The Club would like to draw attention to a recent
notice (safety alert 02-20) issued by the United States Coast Guard (USCG) on
gas detection vulnerabilities on gas carriers.
One critical safety measure on liquefied gas
carriers is the fixed gas detection system. If gas vapours are detected in a
monitored space, an alarm will activate and alert the crew of the dangerous
condition. The International Code for the Construction and Equipment of Ships
Carrying Liquefied Gases in Bulk (IGC Code) requires the activation of alarms
at specified vapour concentrations.
However, during exams on three separate Liquefied
Natural Gas (LNG) carriers in Boston, Port State Control Officers (PSCOs)
discovered issues with the fixed flammable gas detection system that resulted
in the issuance of deficiencies and delay of cargo operations. While witnessing
tests, multiple sensors measured outside the tolerances established by the
manufacturer and subsequent calibration checks failed. In each case,
crewmembers were not following established procedures as specified in their
Safety Management System (SMS).
Testing procedures and allowable tolerances should
be understood by the crewmembers responsible for maintaining and testing the
fixed gas detection system. Although the examined gas carriers’ SMSs and
manufacturers’ instructions specified procedures for testing the fixed gas
detection systems and stated the acceptable parameters for the sensors, many of
the sensor tests observed by the PSCOs had readings outside of the established
tolerances. In these types of situations, a “drifting sensor” often causes
detectors to exceed tolerances specified by the manufacturer. Sensors operating
outside of established tolerances pose a significant safety threat and could be
grounds for vessel control actions, such as delayed departure from port,
delayed cargo operations, or detention. Enforcement details are available in
the issued order.
Another common discrepancy observed was the use of
improper span gas on catalytic sensors. To prove proper operation of the fixed
gas detection system, the span gas used to test the sensors or complete
calibration checks must be appropriate for the type of sensor. There are three
main types of sensors: catalytic, infrared, and electrochemical. Catalytic
sensors rely on the presence of oxygen to function correctly. Absent the means
to introduce oxygen into the sample, if the composition of the span gas used on
a catalytic sensor does not include oxygen and is balanced with an inert gas,
the sensor will not operate correctly. Tests or calibrations conducted with the
incorrect span gas can cause the sensor to operate outside established
tolerances.
The
Coast Guard strongly
recommends
that liquefied gas carrier owners and operators ensure the following:
- ·
Testing
and calibration of fixed gas detection systems are completed per the vessel’s
SMS and manufacturer’s instructions.
- ·
Sensors
are operating within established parameters as required by the vessel’s
procedures and manufacturer’s instructions.
- ·
The
appropriate span gas for the type of sensor is correctly applied to the sensor,
as established by the manufacturer and vessel’s SMS.
- ·
The
crewmember responsible for maintaining the gas detection system has adequate
training, is fully knowledgeable on the system, and is proficient in conducting
system tests.
Investigating
officers, inspection personnel, servicing technicians, and shipboard personnel are encouraged to maintain an acute awareness of
these issues and initiate corrective actions as needed.
This safety alert is provided for informational
purposes only and does not relieve any domestic or international safety,
operational, or material requirements.
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