Assessment of Alternative Devices for Trace Hydrocarbon Monitoring in the Environment Around Offshore Oil and Gas Facilities

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Abstract The offshore O&G industry is committed to protecting the environment surrounding operated facilities, thus requiring simple and robust environmental monitoring to be performed periodically. Several conventional methods have been developed to monitor hydrocarbon into the environment when effluents are discharged. Nevertheless, trace substances in seawater remain challenging due to high dilution factors and required sample representativeness. Passive samplers (PS) for in-situ monitoring are considered by regulators and stakeholders as an alternative to spot sampling. PS for specific trace substances (PAHs, PCBs˙.) are deployed to assess the time-weighted average concentration of organic substances in aquatic systems at very low quantification levels, thus assimilating contamination variations over time. However, offshore implementation remains challenging and PS have never been used for total dissolved hydrocarbons (TDH), for mandatory OiW measurements. The feedback from preliminary trials and offshore PS deployment resulted in optimized laboratory experiments, carried out using pilots with controlled flow rates, specifically designed to: (1) assess the capacity of two sampler types to assimilate time-variable TDH concentrations, and (2) measure the uptake rates of TDH in seawater. Both Empore™ SPE C18 disks and silicone rubbers were deployed in the pilots for three 14-day experiment periods corresponding to three scenarios of hydrocarbon (HC) releases in seawater. They simulated respectively: continuous HC release, discontinuous HC release and a short, abrupt release of HC. Low concentrations of HC were injected in seawater for each scenario; the concentration of TDH measured in seawater was compared to the mean concentration estimated by the samplers. The results show that both C18 disks and silicone rubber reliably integrate changes in TDH concentration over time. In most of the exposure scenarios tested, TDH concentrations estimated using both C18 disks and silicone rubber were comparable to the concentration directly measured in seawater, especially at the end of the experiment periods. To optimize sample preservation for analysis and ensure simple field applicability, a set of the PS exposed to HC were stored for 10 days under different temperature conditions (freezer, fridge and room temperature) before extraction. The analysis on PS after storage at different temperature conditions showed that the loss of HC cannot be avoided. Alternatively, if the PS are rapidly extracted, the extract could be stored for days without HC loss at any storage temperature. The results of this work show that passive samplers can be a reliable alternative to spot samples' measurement for the monitoring of TDH in seawater, requiring simpler logistics that will allow for more representative periodic monitoring in aquatic environments surrounding O&G facilities. Additional experiments with varying parameters, especially with different types of HC mixtures, are still required for optimization and validation of the protocol before implementation around offshore platforms.