Entrainment of oil droplets by random waves

Abstract. A series of laboratory experiments was conducted to investigate the oil droplet size distribution generated by the entrainment of oil under random breaking waves. Experiments were performed using three different oil types and two distances between oil release and droplet sampling, to examine the evolution of droplet size distributions with prolonged wave action. The random wave field was designed to replicate open-ocean conditions through a combination of a JONSWAP spectrum and superimposed sine-wave representing swell.

The temporal evolution of the volume-based median droplet diameter, d₅₀^v, for the naphthenic Troll B oil indicated a pronounced dependence on the distance from release to sampling point, with characteristic droplet size reduced by about a factor 2 when subject to wave action for a longer distance. In contrast, the results for Oseberg Blend, with a higher wax content, showed substantially weaker dependence on release location, the characteristic droplet size differing by only a factor of 1.3 between the two release sections.

The predictive performance of several existing droplet size models was systematically evaluated against the experimental data. The relative prediction errors span a wide range, from as low as -2 % to over 8000 %, highlighting significant uncertainties in current modeling approaches. Sensitivity analysis demonstrates that model predictions are strongly influenced by the choice of characteristic length scale, and the choice of significant wave height used to derive the velocity scale. A comparison between time-resolved predictions and experimentally measured maximum droplet sizes indicates that rare, high-energy events dominate the maximum droplet diameter, highlighting fundamental limitations of deterministic breakup models under random wave conditions.