Entrainment of Oil due to Breaking Waves

When oil is spilled in marine environments, it undergoes a series of physical chemical processes while it gets distributed further away from the source of release. The oil on the surface water tends to stay within the surface because oil has lower buoyancy than seawater. One of the main physical processes surface oil goes through is the impact from winds and breaking waves. SINTEF Ocean, MET Norway, NTNU, Imperial college in the UK, and National Oceanic and Atmospheric Administration (NOAA) in the USA will collaborate to study the impact of winds and waves on the oil distribution on the ocean surface, with the initiation of the ENTIRE project funded by the Research Council of Norway. Understanding the impact of wind and waves on the spreading and the trajectory of oil on the surface of the ocean is crucial in predicting the ultimate fate of the oil in the environment and to improve existing oil spill models. Surface oil efficiently gets mixed in rough sea with many breaking waves. This may significantly reduce the surface oil slick area compared to slick existing under calm sea conditions. Computer-based numerical models predict possible oil distribution patterns in the ocean. Which are used in planning and response activities to minimize the potential damages from accidental spills. Further, the models are essential tools to assess damages to the environment and other assets affected by spills.

Oil Experiments at SINTEF wave Flume (Photos: Tor Nordam)

The main objective of our project is to study mechanisms related to vertical mixing of oil into the near-surface water by breaking waves while drifting on the water surface caused by wind. The vertical mixing of surface oil, also known as entrainment or natural dispersion, controls the amount of oil available to spread on the surface. Natural dispersion may prevent oil from forming a wide slick impacting a large area, which affects birds, marine mammals and even reaching shorelines. But, when the oil is entrained into the water column, it may adversely impact fish, eggs, larvae, plankton and other marine life near the water surface. Hence, knowledge of the vertical distribution of oil is essential to understand the nature of the impact of an oil spill.

During our three-year study, we will carry out field and laboratory experiments to gain new knowledge and develop and improve numerical modelling of oil near the water surface. Small scale laboratory experiments will be carried out in a 14 m long wave flume at SINTEF Ocean, to measure the sizes of oil droplets, wave energy dissipation, and estimate the amount of oil that gets entrained into the water from breaking waves. The proposed field studies will be conducted during the annual oil on water exercises in Norway managed by the Norwegian Clean Seas Association (NOFO) to collect similar data for laboratory studies. Although expensive to gather, the field experimental data are invaluable. They provide information under real environmental conditions such as intermittent breaking waves, which are challenging to create in a small-scale laboratory flume. We will develop sensors to measure wave energy and deploy optical instruments (camera set up) to observe oil droplets in water. The field and laboratory-scale data collected will be analyzed to better understand the physics of entrainment of surface oil.

Oil Experiments at SINTEF wave Flume (Photos: Tor Nordam)

The newly developed modeling tools will be implemented in the OSCAR model at SINTEF Ocean and OpenDrift model at MET Norway, to improve their performance in predicting the surface slick behavior. OSCAR and OpenDrift are two modeling suites widely used in oil spill damage assessments and response planning communities. They comprise of the oil and gas industry, consultants, and the environmental regulatory agencies in Norway and internationally. Moreover, study findings will be communicated to stakeholders during the annual oil spill response forum, jointly organised by the Norwegian environmental authorities and industry. To increase oil spill awareness among the younger generations, we will have an educational program for children in a selected school in Trondheim. Thus, the project will serve several important communities by combining expertise from three different countries.

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