Oil Slick Transport in Coastal Waters
The frequency of accidental oil spills in marine environments presents a growing concern to maritime and port authorities, marine biologists and ecologists. The environmental concern over oil spills has led to the development of mathematical models to simulate the transport and fate of oil slicks. Oil spill models are normally constructed by linking algorithms to represent oil transport and fate processes. The transport or trajectory computations determine the oil slick movement in space and time whilst the fate processes of primary interest include spreading, evaporation, entrainment or dispersion, dissolution, emulsification and shoreline deposition. Furthermore, there are the long-term processes of photo-oxidation and bio-degradation. The fate portion of the model estimates the oil transport between various environmental compartments and changes in physical characteristics such as oil concentration, density, viscosity, etc.
The Department of Civil Engineering, NUS, Maritime and Port Authority of Singapore and Prof. Poojitha Yapa of the Department of Civil and Environmental Engineering, Clarkson University, N. Y. have been involved in joint collaborative research on modelling of oil slick transport in Singapore waters. A Comprehensive Oil Spill Model (COSM2D) has been developed for Singapore waters. The following processes are considered in the model :
COSM2D utiliZes a Lagrangian numerical technique which describes the oil slick on the surface of the sea as a large number of particles, each of which represents a certain quantity of oil. The movement of each particle is governed by the above physico-chemical processes. The hydrodynamic inputs to the model, which include currents and water levels, are obtained from a well-calibrated and validated 2-D depth integrated hydrodynamic model of the Singapore Straits. With the input data comprising water levels and currents, COSM2D can simulate oil particle movement. COSM2D has been applied to Singapore coastal waters in the case of a hypothetical spill near one of the Southern Islands. The simulation was carried out over a period of 72 hours. Figure 1 shows the oil particle distribution at the end of 24 and 72 hours. The concentration distribution of the ensemble can be computed based on oil mass in a given area.
Figure 1: Oil particle distribution after 24 hours and 72 hours of accident
Besides the development of COSM2D for modelling the fate of oil slicks, a simple and elegant trajectory modelling system called the Oil Spill Tracking System (OSTS) has also been developed in both the predictor (forward tracking) and receptor (backtracking) modes. This modelling system has been applied to Singapore coastal waters for the simulation of an oil slick under the influence of tidal and wind-induced currents. The results of the simulation are depicted in Figs. 2 and 3. Figure 2 shows the circulation pattern in Singapore Waters during ebb tide. Figure 3 shows the results of forward and backtracking of a single particle from a source.
Figure 2: OSTS is Showing a current pattern.
Figure 3: The forward and back tracking of single particle. Forward tracking goes from the black point to the red during 72 hrs (left) and then backtracking from the red to the yellow (right). In this case, the error is less than 50m. Users can decrease it by taking interpolation with the hydrodynamic data.
The research team on the project comprises Prof N Jothi Shankar (PI) and Prof HF Cheong, Assoc Prof WJ Ng and Assoc Prof WT Chan, Dr X Chao and Research Scholars Y Li and Q Liu from the Department of Civil Engineering, NUS, Mr AC Toh, Deputy Director (Tech) and Dr T Song from the Maritime and Port Authority of Singapore, and Prof Poojitha Yapa from the Civil and Environmental Engineering Department, Clarkson University, Potsdam, New York.
