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OIL SPILL MODELING
Oil spill modelling is an ideal tool for predicting the movement of an oil spill, both when and before a spill has occurred. Contemporary computer models are capable of plotting spill movement in various impressive ways, but these plots should be used with care as they are easily misunderstood.
There are two types of oil spill fate modeling: deterministic and stochastic. Deterministic modeling predicts spill behavior using a given wind direction, and is mainly used to predict beaching times under worst case weather conditions. Stochastic modeling is based on realistic wind conditions and is used to determine the likelihood of coastline pollution.
Deterministic modeling
Deterministic modeling usually takes several model runs whereby wind and wave conditions are adjusted to achieve the shortest beaching time to the nearest coastline. Once the shortest beaching time is determined, this can be matched with available response capabilities to ensure that equipment can be deployed to any part of the potentially threatened coastline.
A safety factor can be applied to make sure that response equipment can be mobilized in time (in the UK this is a factor of 2).
It is important to note that this deterministic modeling does not simply target the response deployment to the location of the shortest beaching time, but is used to check beaching times along a considerable stretch of coastline. This is done by altering wind direction. A graphical representation of the result of this approach is shown on the right.
When interpreting these, it should be realized that deterministic modeling concerns itself exclusively with determining shortest beaching times and gives no information about the likelihood of beaching.
In some oil spill plans, this type of deterministic modeling is presented as a single deterministic plot showing a track of oil to the nearest coastline. Whilst indeed showing the shortest beaching location, this may be based on a wind direction that is opposite to the prevailing weather conditions and does therefore not convey any useful information on which a response can be based at the time of an incident.
Deterministic oil spill modeling can also be used to determine the maximum distance that an oil spill can travel in a given time period. This can be done for any type of oil. Deterministic modeling can even demonstrate that very light oils, including diesel oil, have a maximum traveling distance, irrespective of the amount that is spilled. Even with a total loss of diesel fuel from a production platform or drilling rig, there is no likelihood that any would beach if the installation is located far enough from shore. This information can then be used to prepare a map such as shown on the left. This map shows the areas of the UK Southern North Sea from which a spill could occur while there still is enough time to mobilize the appropriate response as detailed in the oil spill contingency plan which features the map. Only in the red area immediately adjacent to the coast would additional planning be required for drilling activities, whereas additional contingency measures might also be required in the pink area if the drilling prospect would be for oil. Both coastal zones include the required safety margin of two. The map also shows the maximum distances that any diesel spill from the individual offshore installations can travel.
Stochastic modeling
The important aspect of stochastic modeling is that it takes account of prevailing weather conditions, and allows oil spill planners to target response resources to the areas most likely to be affected in a major spill event. Especially for near shore locations, where windows of response opportunities are narrow, stochastic modeling is a useful tool for determining the best location to place spill response equipment stockpiles.
Stochastic modeling is sometimes presented in oil spill plans as a contour or “spider” plot of percentage risk of surface oiling from a spill. These diagrams are the most misunderstood part of oil spill plans and on first sight present an alarming sight, showing large areas of sea covered in multicolored contours, as though vast areas of the sea could be affected by a major spill.
An alternative, and perhaps more meaningful in terms of presenting risk, is to transpose the risk contours of these plots to the coastlines where the main environmental impacts and responses are likely to be concentrated. An example of this approach is shown on the right.
The figure shows which coastlines are most likely to be affected by an oil spill from the Judy platform. It shows that, in the winter, the Norwegian and Danish coastlines are at greatest risk, rather than the UK coastline.
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