scope:

The capability of detecting combustible gases is directly related to the safety of offshore drilling and production operations of the oil and gas industry. Of particular concern is the presence of methane in combustible concentrations. The hazard posed by a given concentration of methane is measured in terms of the lower explosive limit (LEL), which is the lowest concentration of a gas that will, when mixed with air, propagate a flame. The LEL is not an absolute value; changes in temperature, oxygen content of the air, and purity of the gas mixture will affect the LEL. For methane, the LEL is considered to be 5.1 percent by volume. A detector for offshore platforms should produce a warning signal at 25 percent LEL and should produce a "shut-down" at 60-75 percent LEL.

Approximately 10 years ago, the United States Geological Survey (USGS) required that combustible gas detectors be installed in all enclosed areas containing gas-fueled and gas-handling equipment. As a result of the short notice given by USGS, little preliminary work was performed to collect data on the operational environment for detectors installed on offshore platforms. The work performed for this project has isolated several environmental variables that must be considered in selecting the detector best-suited for that site and in installing the sensor.

Another factor that bears on the performance of a detector has become apparent during the project work. The human-detector interaction at an installation - including maintenance of, and response to, the detector - directly affects the utility of the detector regardless of its inherent value.

The project work also has revealed an obstacle to selection of the best detector for a particular site. While there is considerable activity in the manufacture and use of combustible gas detectors, there is little freely available scientific information about commercial methods of detecting methane concentrations. In particular, manufacturers are reluctant to discuss details of their products. Much of the information that is available has resulted from efforts of the Bureau of Mines. While this information is valuable, it has not been related directly to the environment of offshore platforms.

The project work involved acquisition of information, including site visits, literature search, and survey of manufacturers. Particular attention was given to specific problems encountered in using methane detectors on offshore production platforms. Discounting human interface problems common to all instrumentation and safety systems, these problems include:

(1) Short life time of the detector head

(2) In accessibility of heads for calibration

(3) Signal returning to "safe" indication when LEL is appreciably exceeded