Modelling and measuring the weathering and bioavailability of fuel spills in polar marine environments, and assessment of the applicability of fuel dispersants
thesisposted on 28.03.2022, 02:34 by Konstantinos Kotzakoulakis
Shipping activity is increasing continuously in Antarctica in the last couple of decades. During the 2014/2015 summer season 191 tourist expeditions visited Antarctica carrying 36,702 passengers in addition to the illegal fishing activity and shipping related to research station support operations. Recent incidents such as the stranding of Akademik Shokalskiy on 25 December 2013, the grounding of the MS Nordkapp at Deception Island on 30 January 2007 and the fire aboard the Nisshin Maru in February 2007, which was carrying approximately 1000 tonnes of heavy fuel oil, have highlighted the risk of a major fuel spill in the Antarctic waters. Currently, much needed data on the behaviour of these fuels in the Antarctic marine environment in order to plan response measures is missing. The three fuels that are used in the Australian Antarctic Territory are the Special Antarctic Blend (SAB), the Marine Gas Oil (MGO) and the Intermediate Fuel Oil 180 (IFO-180). During this study we examined the rate of weathering and the path to bioavailability of these fuels to the Antarctic marine biota. The main mechanism of weathering for SAB and MGO in the Antarctic marine environment is evaporation with 80% of SAB and 33% of MGO evaporated in 6 days and 30 days respectively. Both SAB and MGO are pure distillates consisting of hydrocarbons in the range of C10-C15 and C7-C26 respectively which explains the fast evaporation rate. IFO-180 is a heavy fuel consisting of around 90% residual distillation fuel (Bunker C) and less than 10% light distillate. It was found that the loss from evaporation during the first 30 days is 7% which corresponds to the majority of the added distillate and then evaporation almost stops. These results show that the majority of both MGO and IFO can persist in the Antarctic marine environment for long periods of time and response measures such as mechanical recovery or treatment methods need to be considered. The main path to bioavailability of these fuels is through dissolution in the seawater column. For SAB and MGO, the main groups of components that are becoming bioavailable are benzenes, nathphalenes, phenols, indanes, tetralins and biphenyls whereas for IFO-180 except for the aforementioned groups that are present we also identified some smaller amounts of heavier bioavailable groups such as the fluorenes, phenanthrenes, anthracenes and dibenzothiophenes that are entering the water column. Due to the slower rate of evaporation under Antarctic conditions the dissolution time window is extended and the potentially dissolved amount of hydrocarbons larger in comparison to more temperate regions. This is an additional reason for response measures to be deemed necessary. We examined the efficiency of a suite of six chemical dispersants under Antarctic conditions in combination with the MGO and IFO-180 as well the Kuwait crude oil since it is used by the Australian Maritime Safety Authorities as a reference for dispersant certification purposes. The order of efficiency was determined for the Antarctic and sub-Antarctic temperatures of 0°C and 5°C. Additionally, we examined the Kuwait crude oil and the IFO-180 on an extended temperature range from 0°C to 22°C in order to study the effects of temperature on the efficiency of the dispersants. An interesting observation during this study was that heavy fuels seem to exhibit a logarithmic relationship between the viscosity and the dispersant efficiency due to the temperature change. A second observation was the change in the dispersant efficiency order at different temperatures, as it appears that each dispersant is affected to a different degree by the temperature change. During the study of the evaporation mechanism an effort has been made to construct a mathematical model that can predict the rates of evaporation of the different fuels at the Antarctic temperatures. The result of this effort was the successful development of a general model that takes into account the diffusion forces and the concentration gradient in the body of the fuel and can predict the evaporation rate of the fuels but also of other complex mixtures and crude oils in a wide range of temperatures and can be applied in different regions. Additionally, and for the needs of this model a new correlation had to be developed that can predict the viscosity of the weathering fuel or crude oil based on the average boiling point, the density and the prevailing temperature. Further work is underway in order to incorporate the prediction of the dissolution rate into the model.