Droplet formation, drift, weathering, and natural dispersion in two North Sea experimental oil spills

Accurate oil spill modelling is crucial for environmental risk assessment and operational preparedness for marine oil spills. Predicting drift, weathering, and droplet sizes in natural dispersion is essential. Existing droplet size models are fitted to laboratory results, and a lack of field data makes it uncertain how well these models apply in real conditions. Here we show for the first time in situ measurements of droplet size distributions during natural dispersion in the field. Observations have been made from two controlled experimental spills of 10 m³, each followed for 24 h. Droplet sizes were measured from a novel spar buoy with integrated silhouette camera. Supporting measurements included oil properties, wind, waves, currents, and hydrography, as well as remote sensing from multiple platforms. The data was collected during a field experiment conducted in the North Sea by NOFO in collaboration with the NCA. Two oil types were studied at two weather conditions. MGO released under calm wind (2-7 m/s) resulted in few droplets, and the slick remained thicker than expected, up to 8 mm after 24 h. Oseberg crude drifted in up to 12 m/s wind, with observed d₅₀^v in the range 90-1667 μm. Previously published droplet size models were evaluated. While no model was clearly better in terms of absolute errors, the model by Johansen closely followed an observed trend of reduced droplet size at later times. These results provide the first direct field evidence to evaluate and constrain droplet size parameterizations for oil spill modelling.