VOC Emissions at Designated Offshore Shuttle Tanker Loader Installations in the North Sea

Shuttle Tankers are a known source of Volatile Organic Compounds (VOCs) and the International Maritime Organization (IMO) requires all member states to have in place a VOC management plan to prevent or minimises VOC emissions to air. VOCs emitted from tankers result from the displacement of inert gas in the empty tank before loading and from evaporation from the cargo being loaded. The VOC-rich gas in empty tankers on arrival at the loading point needs to be discharged, and this is often vented to the atmosphere. VOCs can also be released from a build-up of positive pressure that occurs during loading, with low molecular weight hydrocarbons occupying the space between the oil and roof of the storage tank. Further atmospheric emissions can then occur both during the voyage and during discharge at port. VOC recovery techniques should be fitted to tankers and installations but these can vary in their effectiveness depending on the composition of the gas and the technology used. In the North Sea, 70 offshore installations are currently designated as "offshore tanker loaders". We quantify using a detailed case study the emissions of VOCs from an installation regularly serviced by shuttle tankers in the North Sea. Research flights were conducted on several days with and without the presence of a shuttle tanker. Specific details outlining activity on the platform were obtained from operator logs, including the timings of name of tanker loadings. During loading, elevated concentrations of C₂-C₅ light alkanes were observed downwind of the installation on cross-wind transects of the resulting plume, most significantly for ethane and propane. Enhanced emissions calculated from loading were then compared against data for emissions from ships in transit, which indicate that the loading phase remains a substantial VOC source despite regulation for controlling emissions.