As steward of the Federal offshore lands known as the Outer Continental Shelf (OCS), the U.S. Department of the Interior (DOI), Minerals Management Service (MMS), is responsible for balancing the Nation's search for commercial oil and gas with protection of the human and marine environments. The MMS regulates the development of mineral resources in an environmentally safe manner by analyzing environmental consequences of the OCS program prior to lease sales or approval of industry's plans. The Oil-Spill Risk Analysis (OSRA) model was developed by the DOI for the analysis of possible oil-spill impact from offshore oil and gas operations.
The OSRA model produces statistical estimates of hypothetical oil-spill occurrence and contact from projected OCS operations. The model generates an ensemble of sea surface oil-spill trajectories by initiating thousands of oil-spill simulations at hypothetical spill locations to statistically characterize oil-spill risk in areas of prospective drilling and production and along projected pipeline routes.
The hypothetical spills are initiated every day and move at the velocity of the vector sum of the surface ocean currents plus an empirical wind-induced drift of speed equal to 3.5% of the local wind speed, with a wind-speed-dependent direction (Samuels et al., 1982). The model generates oil-spill trajectories by integrating interpolated values of the wind and ocean current fields at intervals short enough to use the full spatial resolution of the ocean current and wind fields. The OSRA model, as applied to the Gulf of Mexico, uses 3-hourly ocean current fields over 7 years (1993–1999) generated by the Princeton Regional Ocean Forecast System (PROFS) (Oey et al., 2004). The PROFS is driven by synoptic winds, heat flux, and river flows. The wind field is based on the European Center for Medium-Range Weather Forecasts surface winds enhanced by observations from meteorological buoys and Coastal-Marine Automated Network stations. The same wind field used to force the ocean model is used to move the oil in the spill trajectories.
As an example of environmental assessment, the OSRA model was used to estimate the spreading of oil spills by simultaneously modeling fractions of each spill, referred to as spillets. The spillets were used to calculate additional statistics, in particular, the length of coastline contacted by a large spill. The coastline was divided into equal length segments. Assumptions were made regarding what fraction of the spill (i.e., the number of spillets) that contacted a land segment would constitute a contact larger than the “level of concern.” Sensitivity of the analysis to key assumed parameters, such as the number of spillets and the level of concern, were tested.
We deployed ninety-seven oil-spillsimulating drifters over the continental shelf of the northeastern Gulf of Mexico during five hydrographic surveys conducted from 1997 through 1999. Earlier, sideby-side comparisons with spilled crude petroleum on the ocean surface had demonstrated that these drifters moved on the ocean surface like consolidated oil slicks under light to moderate winds. (Under high winds, a surface oil spill tends to be entrained into the mixed layer and Ekman transported, unlike the drifters, which remain on the sea surface and move mostly downwind.) The drifters were then deployed in the Gulf of Mexico as nonpolluting oil-spill proxies to compare their movements against results from an oil-spill trajectory model.The drifter trajectories were compared statistically to trajectories generated by the Oil-Spill Risk Analysis (OSRA) model. The model uses a variation of the 3.5-percent rule to compute the drift due to local wind forcing and superposes the prevailing ocean current on this wind-induced drift to obtain the total velocity of an oil spill on the ocean surface. The input fields are the European Center for Medium Range Weather Forecasting (ECMWF) winds and a data-assimilating hindcast of the ocean currents over the time the drifters were deployed.Scatter plots and linear regressions of the speeds and directions of simulated vs. modeled oil-spill drift show the extent to which they are different. Underlying these differences are the expected differences between the ocean current input field and the trajectories of satellitetracked, "water-following'' drifters deployed simultaneously with the oil-spill-simulating drifters. An earlier evaluation of the ECMWF winds showed better, but of course not perfect, agreement with meteorological buoys in the Gulf. The integrated effect of the errors in the input fields results in average discrepancies between the terminal ends of the simulated and modeled spill traiectories of 78, 229; 416, and 483 km after 3, 10,20, and 30 days of drift, respectively.These results are the consequence of integrating wind and ocean current fields, which are not perfect, and comparing the resultant trajectories against those of the oil-spill-simulating drifters, which themselves contain location errors and which are not considered to be perfect simulators of real oil spills. However, the results are useful to practical oils spill risk analysis through ongoing improvement of the model.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.