Evaluating Hg speciation in an offshore gas export pipeline and the utility of pXRF for screening Hg contamination

Mercury (Hg) has been established as a prominent contaminant in oil and gas (O&G) production and has been suggested to be present in high concentrations along the internal walls of subsea pipelines. Contaminants, such as Hg, are critical to upcoming decommissioning processes which may opt to leave infrastructure, such as pipelines, in-place, along the seabed. For decisions on decommissioning to be made while protecting marine environments, relevant data must be generated identifying the concentrations of Hg and its species present in pipelines. The aim of this study was to investigate Hg speciation throughout a decommissioned carbon steel dry gas export pipeline (GEP) to better understand the risks associated with leave in-place decommissioning strategies. The evaluation of Hg concentrations in pipeline samples by cold vapour atomic fluorescence spectrometry (CV-AFS), X-ray absorption spectroscopy (XAS), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) found that total Hg concentrations ranged from 0.003 g Hg∙m -2 at an onshore site, to 1.1 g Hg∙m -2 at the furthest point taken offshore, showing heterogeneity across the length of a single pipeline. The evidence available suggested that, where species characterisation was possible, metacinnabar (β-HgS) was found to be the dominant compound, which is a critical finding for subsequent evaluations of potential routes for bioaccumulation. The efficacy of portable X-ray fluorescence (pXRF) as a screening tool for Hg contamination on steel pipelines was investigated, showing that pXRF is a useful but limited technique which may be used primarily for hotspot detection. Any application which intends to claim that a pipeline is free of Hg runs the risk of underestimating the Hg contamination on the steel.