- Email: [email protected]
Pre-operational system for oil spill simulation
Pre-operational system for oil spill simulation P. Sebasti~o and C. Guedes Soares*
Instituto Superior T6cnico, Unit of Marine Technology and Engineering, Technical University of Lisbon, Portugal
Abstract A system to simulate and visualise the trajectory and fate of oil spills near the Iberian Peninsula is presented. It comprises a set of modules to calculate the weathering of an oil spill and its trajectory. A database stores geo-referenced information, such as bathymetry and mean conditions of wind, waves and currents, and, the physico-chemical properties of the most common crude oils. When used operationally the system receives the input of online met-ocean forecast data and uses it to make predictions. If forecast data is not available the predictions can also be made using the mean met-ocean conditions resident in the system. An example is provided of its application to the oil spill originated by the accident involving the tanker Prestige off the Coast of Spain, in November 2002.
Keywords: oil spills, modelling, marine pollution 1. Introduction Oil spill models are important to support decision for operations to combat oil spills at sea. Often the positions of the slicks are monitored with the help of aircraft and thus the model is expected to make forecasts of 24-48 hours. In these situations it is important that quick answers are provided but the accuracy is not as critical as ensuring that the deviations that can occur within these time periods are not very large. The oil spill forecasting system that is being developed has a database and three main modules: a weathering module, a trajectory module and a visualisation module (Guedes Soares et al., 2000). The last one is a graphical interface that allows visualisation of meteorological data as well as the model outputs. By making use of monthly mean met-ocean data stored in the database a first guess of the trajectory of an oil spill can be made, before receiving real met-ocean information. If real met-ocean data, or forecast data is available then it is introduced in the system to make the calculations. The system is being developed in order to be able to receive online met-ocean data, so that it can work operationally. A case study is presented using the system to predict the trajectory of the recent oil spill caused by the accident of the tanker Prestige off the Spanish Coast of Galice. The trajectory was calculated from three different locations where the tanker passed during the first and the second day after it started to release oil into the sea.
2. The oil spill model The trajectory module calculates the trajectory of the spilled product taking into account the local current, wind, the Stokes' drift and a deflection angle due to Coriolis effect, in a * Corresponding author, email: [email protected]
524
Pre-operational system for oil spill simulation
Lagrangian approach, as described in more detail in Guedes Soares et al. (2000). The modules to compute the weathering of the spills are described in Sebasti~o and Guedes Soares (1995, 1998). They take into account the processes of spreading, evaporation, dispersion and emulsification and compute the evolution of the viscosity and density of the spilled oil. The database of the system has geo-referenced information of the bathymetry, monthly mean values of wind (COADSmComprehensive Ocean-Atmosphere Data Set, see Dfaz et al., 1992) and currents (Richardson et al., 1984, 1986), and the properties of the most common crude oils. The data covers the Exclusive Economic Zone of Portugal (Figure 4) with a resolution of 1~ Lat x 1~ Lon (approximately 60x 46 nautical miles).
Figure 1 Exclusive Economic Zone of Portugal, which covers Continental Portugal, Aqores and Madeira Islands. The line from A to B represents the trajectory of the tanker Prestige from 13 November 2002 (A) until it sank on 19 November 2002 (B), as described below. The operational aspect of system, which is under development, consists basically of setting up the process of receiving met-ocean data via web and automatically store it in a database. Then, the oil spill model will retrieve the necessary data from the database to perform the calculations.
3. Case study: Prestige oil spill 3.1 Brief description of the accident On the 13th November 2002, the Bahamas-flagged tanker Prestige, carrying 77 000 tons of heavy fuel on board, suffered a large crack in the starboard side of the hull when it was sailing off the West of Cape Finisterre in the Galician coast of Spain, under stormy weather conditions. The tanker started taking water and leaking oil into the sea. On the same day the tanker started receiving the assistance of tugs, and initiated a journey that took it away from the coast, first to the North and then to the South, until its structure collapsed and broke into two parts that sank, with a great part of its cargo still inside, on the 19th November, some 200 km off the Spanish and the Portuguese Coast. Along that journey the tanker kept spilling oil and when it sank an additional spill occurred. The total amount of spilled oil, until that moment, was of the order of 103 tons.
P. Sebasti,~o and C. Guedes Soares*
525
3.2 Trajectory simulations Figure 2 shows the trajectory of the tanker Prestige from the location where it first suffered a crack on the hull to the site where it would sink 6 days later.
Figure 2 Trajectory of the tanker Prestige while it was being towed.
Figure 3 Trajectory simulation using mean wind and current for November.
Figure 3 shows the simulation of the Prestige oil spill from the initial position using the mean wind and current resident in the database of the system. The simulation indicates that the spill will reach the coast. Three other simulations were done, considering measured wind and current data supplied by Puertos del Estado (2002), from their deep-sea buoy network. A Seawatch buoy station moored at 323 m water depth performed the measures. The measurements are transmitted every hour via satellite to Puertos del Estado and directly posted on the web. Three instantaneous spills were simulated starting at points 1, 2 and 3 on Figure 4 - - t h e first one starting on 13 November 2002 19h (point 1) and the other two on 14 November 2002 at 10h (point 2) and 18:05h (point 3). The simulation period was 8 days.
Figure 4 Three oil spill trajectories simulated starting from three locations (points 1,2 and 3) of the trajectory followed by the tanker Prestige. The tanker symbol highlights the trajectory of Prestige while the other trajectories correspond to simulated spill trajectories.
526
Pre-operational system for oil spill simulation
In this way the three simulations intend to represent three different trajectories followed by the spill at different points and different instants of time, along the trajectory followed by the tanker, as shown in Figure 4. Although there was in fact a continuous spill, the simulations that were done showed a general good agreement both in space and time with the observations. Simulating more instantaneous spills would represent better the continuous spill situation and this would lead to intermediate trajectories and landing sites. It can be observed that at the different locations the oil released was submitted to the action of the local met-ocean conditions, which induced different trajectories. The three simulations indicate that part of the spill would clearly reach the Galician Coast and that another part of it would travel along the Northern Coast of Spain towards the coast of France. These results were in fact observed: the spilled oil reached the Galician Coast (both the Western and Northern parts), the Cantabrian Coast (Northern Coast of Spain) and reached the French Coast by the end of December 2002.
4. Conclusions The system that was presented proves to be useful to predict the trajectories of oil spills at sea. It was able to simulate approximate trajectories of the Prestige Oil Spill using measured currents and wind, which can be very important when planning response measures. The mean conditions of current and wind stored in the database allow a first estimate of the trajectory while met-ocean forecast data allow more accurate estimates.
Acknowledgements This work has been performed within the project "Simulation of the Fate of Oil Spills in the Portuguese Coast" which is conducted in cooperation with the Service to Combat Pollution at Sea from the Directorate General of the Maritime Authority and is funded jointly by the Foundation of Portuguese Universities and the Ministry of Defence.
References Dfaz, H.F., K. Wolter, and S.D. Woodruff (Eds.), 1992, Proceedings of the International COADS Workshop, Boulder, Colorado, 13-15 January 1992, NOAA Environmental Research Laboratories, Climate Research Division, Boulder, Colo., 390 pp. Guedes Soares, C., P. Sebastiao, and F. Silva, 2000, System for Oil Spill Prediction, Hydraulic Engineering Software VIII (Hydrosoft 2000), W.R. Blain and C.A. Brebbia (Eds.), WITPress, Southampton, pp.217-226. Puertos del Estado, 2002, Oceanography and Meteorology--deep-sea network, Minist6rio do Fomento, Spain, www.puertos.es. Richardson, P.L. and T.K. McKee, 1984, Average seasonal variation of the Atlantic equatorial currents from historical ship drifts, J. Phys. Oceanogr., 14, 1226-1238. Richardson, P.L. and D. Walsh 1986, Mapping climatological seasonal variations of surface currents in the tropical Atlantic using ship drifts, J. Geophys. Res., 91, 1053710550. Sebastiao, P., and C. Guedes Soares, 1995, Modelling the Fate of Oil Spills at Sea. Spill Science & Technology Bulletin. 2, 2/3, 121-131.
Instituto Superior T6cnico, Unit of Marine Technology and Engineering, Technical University of Lisbon, Portugal
Abstract A system to simulate and visualise the trajectory and fate of oil spills near the Iberian Peninsula is presented. It comprises a set of modules to calculate the weathering of an oil spill and its trajectory. A database stores geo-referenced information, such as bathymetry and mean conditions of wind, waves and currents, and, the physico-chemical properties of the most common crude oils. When used operationally the system receives the input of online met-ocean forecast data and uses it to make predictions. If forecast data is not available the predictions can also be made using the mean met-ocean conditions resident in the system. An example is provided of its application to the oil spill originated by the accident involving the tanker Prestige off the Coast of Spain, in November 2002.
Keywords: oil spills, modelling, marine pollution 1. Introduction Oil spill models are important to support decision for operations to combat oil spills at sea. Often the positions of the slicks are monitored with the help of aircraft and thus the model is expected to make forecasts of 24-48 hours. In these situations it is important that quick answers are provided but the accuracy is not as critical as ensuring that the deviations that can occur within these time periods are not very large. The oil spill forecasting system that is being developed has a database and three main modules: a weathering module, a trajectory module and a visualisation module (Guedes Soares et al., 2000). The last one is a graphical interface that allows visualisation of meteorological data as well as the model outputs. By making use of monthly mean met-ocean data stored in the database a first guess of the trajectory of an oil spill can be made, before receiving real met-ocean information. If real met-ocean data, or forecast data is available then it is introduced in the system to make the calculations. The system is being developed in order to be able to receive online met-ocean data, so that it can work operationally. A case study is presented using the system to predict the trajectory of the recent oil spill caused by the accident of the tanker Prestige off the Spanish Coast of Galice. The trajectory was calculated from three different locations where the tanker passed during the first and the second day after it started to release oil into the sea.
2. The oil spill model The trajectory module calculates the trajectory of the spilled product taking into account the local current, wind, the Stokes' drift and a deflection angle due to Coriolis effect, in a * Corresponding author, email: [email protected]
524
Pre-operational system for oil spill simulation
Lagrangian approach, as described in more detail in Guedes Soares et al. (2000). The modules to compute the weathering of the spills are described in Sebasti~o and Guedes Soares (1995, 1998). They take into account the processes of spreading, evaporation, dispersion and emulsification and compute the evolution of the viscosity and density of the spilled oil. The database of the system has geo-referenced information of the bathymetry, monthly mean values of wind (COADSmComprehensive Ocean-Atmosphere Data Set, see Dfaz et al., 1992) and currents (Richardson et al., 1984, 1986), and the properties of the most common crude oils. The data covers the Exclusive Economic Zone of Portugal (Figure 4) with a resolution of 1~ Lat x 1~ Lon (approximately 60x 46 nautical miles).
Figure 1 Exclusive Economic Zone of Portugal, which covers Continental Portugal, Aqores and Madeira Islands. The line from A to B represents the trajectory of the tanker Prestige from 13 November 2002 (A) until it sank on 19 November 2002 (B), as described below. The operational aspect of system, which is under development, consists basically of setting up the process of receiving met-ocean data via web and automatically store it in a database. Then, the oil spill model will retrieve the necessary data from the database to perform the calculations.
3. Case study: Prestige oil spill 3.1 Brief description of the accident On the 13th November 2002, the Bahamas-flagged tanker Prestige, carrying 77 000 tons of heavy fuel on board, suffered a large crack in the starboard side of the hull when it was sailing off the West of Cape Finisterre in the Galician coast of Spain, under stormy weather conditions. The tanker started taking water and leaking oil into the sea. On the same day the tanker started receiving the assistance of tugs, and initiated a journey that took it away from the coast, first to the North and then to the South, until its structure collapsed and broke into two parts that sank, with a great part of its cargo still inside, on the 19th November, some 200 km off the Spanish and the Portuguese Coast. Along that journey the tanker kept spilling oil and when it sank an additional spill occurred. The total amount of spilled oil, until that moment, was of the order of 103 tons.
P. Sebasti,~o and C. Guedes Soares*
525
3.2 Trajectory simulations Figure 2 shows the trajectory of the tanker Prestige from the location where it first suffered a crack on the hull to the site where it would sink 6 days later.
Figure 2 Trajectory of the tanker Prestige while it was being towed.
Figure 3 Trajectory simulation using mean wind and current for November.
Figure 3 shows the simulation of the Prestige oil spill from the initial position using the mean wind and current resident in the database of the system. The simulation indicates that the spill will reach the coast. Three other simulations were done, considering measured wind and current data supplied by Puertos del Estado (2002), from their deep-sea buoy network. A Seawatch buoy station moored at 323 m water depth performed the measures. The measurements are transmitted every hour via satellite to Puertos del Estado and directly posted on the web. Three instantaneous spills were simulated starting at points 1, 2 and 3 on Figure 4 - - t h e first one starting on 13 November 2002 19h (point 1) and the other two on 14 November 2002 at 10h (point 2) and 18:05h (point 3). The simulation period was 8 days.
Figure 4 Three oil spill trajectories simulated starting from three locations (points 1,2 and 3) of the trajectory followed by the tanker Prestige. The tanker symbol highlights the trajectory of Prestige while the other trajectories correspond to simulated spill trajectories.
526
Pre-operational system for oil spill simulation
In this way the three simulations intend to represent three different trajectories followed by the spill at different points and different instants of time, along the trajectory followed by the tanker, as shown in Figure 4. Although there was in fact a continuous spill, the simulations that were done showed a general good agreement both in space and time with the observations. Simulating more instantaneous spills would represent better the continuous spill situation and this would lead to intermediate trajectories and landing sites. It can be observed that at the different locations the oil released was submitted to the action of the local met-ocean conditions, which induced different trajectories. The three simulations indicate that part of the spill would clearly reach the Galician Coast and that another part of it would travel along the Northern Coast of Spain towards the coast of France. These results were in fact observed: the spilled oil reached the Galician Coast (both the Western and Northern parts), the Cantabrian Coast (Northern Coast of Spain) and reached the French Coast by the end of December 2002.
4. Conclusions The system that was presented proves to be useful to predict the trajectories of oil spills at sea. It was able to simulate approximate trajectories of the Prestige Oil Spill using measured currents and wind, which can be very important when planning response measures. The mean conditions of current and wind stored in the database allow a first estimate of the trajectory while met-ocean forecast data allow more accurate estimates.
Acknowledgements This work has been performed within the project "Simulation of the Fate of Oil Spills in the Portuguese Coast" which is conducted in cooperation with the Service to Combat Pollution at Sea from the Directorate General of the Maritime Authority and is funded jointly by the Foundation of Portuguese Universities and the Ministry of Defence.
References Dfaz, H.F., K. Wolter, and S.D. Woodruff (Eds.), 1992, Proceedings of the International COADS Workshop, Boulder, Colorado, 13-15 January 1992, NOAA Environmental Research Laboratories, Climate Research Division, Boulder, Colo., 390 pp. Guedes Soares, C., P. Sebastiao, and F. Silva, 2000, System for Oil Spill Prediction, Hydraulic Engineering Software VIII (Hydrosoft 2000), W.R. Blain and C.A. Brebbia (Eds.), WITPress, Southampton, pp.217-226. Puertos del Estado, 2002, Oceanography and Meteorology--deep-sea network, Minist6rio do Fomento, Spain, www.puertos.es. Richardson, P.L. and T.K. McKee, 1984, Average seasonal variation of the Atlantic equatorial currents from historical ship drifts, J. Phys. Oceanogr., 14, 1226-1238. Richardson, P.L. and D. Walsh 1986, Mapping climatological seasonal variations of surface currents in the tropical Atlantic using ship drifts, J. Geophys. Res., 91, 1053710550. Sebastiao, P., and C. Guedes Soares, 1995, Modelling the Fate of Oil Spills at Sea. Spill Science & Technology Bulletin. 2, 2/3, 121-131.