Case Studies - Modelling of Offshore Gas Detector Location

Accidental releases of gases on offshore installations can lead to the accumulation of explosive mixtures with potentially catastrophic consequences. Gas detection equipment is installed on offshore installations to help warn of such accumulations of gas, but recent statistics show that only 62% of releases were initially detected by gas detection systems. Platform personnel detected the remaining releases using hearing and smell. Thus, there is room for improvement in the effectiveness of offshore gas detectors.

At the request of HSE, HSL examined how flammable gas detector networks respond to natural gas releases. A network of gas detectors was modelled using measurements of the dispersion of gas releases in a naturally ventilated, simulated offshore process module. This model incorporated the number, location and types of detectors, alarm levels and response times based on current industry practice. The response of this network to the gas releases was evaluated and the network optimised by altering these factors to increase the detection rates and reduce the response times.

The findings showed that a number of long beam detectors (open path) spaced at five metre intervals and at least two different heights provided an effective gas detection system. Infrared long beam detectors are recommended because they provide good coverage of a module, reliably detect gas leaks and are usually quick to respond. These should be supplemented with infrared point detectors located close to the most likely release points or in areas highly congested with equipment.

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