Hazardous Methane Emissions Can Remain Undetected Offshore: Implications for Safety-Critical Operations

Abstract

Methane emissions from offshore installations are routinely monitored using gas detection systems that support safety-critical decisions during offshore operations. However, the extent to which atmospheric conditions influence the effectiveness of these systems as safety barriers remains poorly understood. This study presents a probabilistic assessment of methane detectability under representative offshore conditions, with particular focus on the implications of detection failure for operational safety. A Gaussian plume model combined with Monte Carlo simulation was used to examine the influence of atmospheric regime, emission rate, detector location, and sensor sensitivity on methane detectability. Detection probability was evaluated for representative helicopter and vessel operations under a range of offshore atmospheric conditions and emission scenarios. Results indicate that methane detectability is strongly influenced by atmospheric structure and plume geometry, in addition to emission magnitude. Under some conditions, limited vertical dispersion produces plume–sensor misalignment, resulting in false-negative detection outcomes in which substantial methane releases could remain undetected despite the presence of monitoring systems. Within the modelling framework, detectability may improve at intermediate distances as plume spreading increases plume interception, before decreasing again due to dilution. Increasing detection thresholds significantly reduces detectability, particularly under stratified marine boundary-layer conditions. The results suggest that gas detection systems should be regarded as conditional safety barriers whose effectiveness depends on atmospheric transport processes as well as sensor performance. The absence of methane detection should therefore not be interpreted as evidence that a hazardous emission is absent. The principal contribution of this study is not the derivation of operational detection thresholds, but the demonstration that physically plausible offshore atmospheric conditions can produce false-negative methane detection outcomes. The study identifies a potentially overlooked limitation of offshore methane monitoring and has implications for risk management, situational awareness, and safety-critical operational decision-making.