# Oil Spill Risk Analysis Model (OSRAM)

**Information to Be Used In Oil Spill Response Plans**

A Notice to Lessees, Operators, and Pipeline Right-Of-Way Holders (NTL) was effective on August 10, 2012 (BSEE NTL 2012-N06) that provides additional guidance on the review and update of Oil Spill Response Plans (OSRP).

This web site contains tables providing the conditional probability results of the most up-to-date trajectory simulations and instructions for the operator. These tables are available in the following document:

Ji, Zhen-Gang, Walter R. Johnson, Charles F. Marshall, and Eileen M. Lear. 2004. Oil-Spill Risk Analysis: Contingency Planning Statistics for Gulf of Mexico OCS Activities. OCS Report 2004-026, Herndon, VA: U.S. Dept. of the Interior, Bureau of Ocean Energy Management, Environmental Division.

In the future, please check this website for any updated OSRA model information and instructions when preparing future updates of OSRP's.

**What are Oil-Spill Trajectory Simulations? **

Oil spill trajectory simulations are generated by BOEM to be used to estimate spill risk. The BOEM uses a numerical computer model that simulates the likely trajectory of a surface slick, represented as a point launched from locations projected onto a gridded area. For more information on the trajectory model, called the OSRA (oil spill risk analysis) model, see Ji et al. (2002; Smith et al, 1982; and LaBelle and Anderson, 1985). The point’s trajectory simulates a spill’s movement on the surface of water by using modeled ocean current and wind fields. The model uses temporally and spatially varying, numerically computed ocean currents and winds.

The model tabulates the number of times that each trajectory moves across or touches a location (contact) occupied by polygons mapped on the gridded surface. These polygons represent locations of various environmental features or shoreline segments. The OSRA model compiles the number of contacts to each feature that result from the modeled trajectory simulations from all of the launch points for a specific launch area, discussed below. Each trajectory was allowed to continue for as long as 30 days and contacts within 3, 10 and 30 days were compiled. However, if the hypothetical spill contacted shoreline sooner than 30 days after the start of the spill, the spill trajectory was terminated, and the contact was recorded.

**What are Conditional Probabilities of Contact? **

The probability of contact to an environmental feature is calculated by dividing the number of contacts by the number of trajectories started within each launch area. The output from these trajectories provides information on the likely trajectory of a spill by wind and current transport, should one occur and persist for the time modeled in the simulations. The probability that an oil spill will contact a specific location within a given time of travel from a certain location or spill point is provided in the attached tables. These probabilities are termed conditional probabilities, the condition being that a spill is assumed to have occurred and persist for the time period modeled.

**TABLES OF PROBABILITIES**

- Probability Tables for Gulf of Mexico Launch Areas 70-145

- Probability Tables for Gulf of Mexico Launch Areas 30-69

- Probability Tables for Gulf of Mexico Launch Areas 1-29

**What are the onshore areas I must identify?**

The onshore areas that must be identified are counties and parishes. All Gulf of Mexico coastal counties and parishes must be listed that can be potentially contacted (i.e., have a probability greater than 0.5% that they will be contacted) by an oil spill starting at the determined worst case discharge locations. The probability tables (see above) show the parishes/counties having a probability of contact.

How can I find the launch area where my lease or facility is located?

If you do not know the launch area(s) where your lease or facility is located, we have included an Excel file listing Lease Block Areas, Blocks within each Lease Block Area, and the launch area for each block.

If the facility in your nearshore worst case response scenario is located in state offshore waters, choose launch area for the nearest block in the OCS.

OSRA Launch Areas by Block* (Excel spreadsheet - August 2008)*

**What time frame should I use? **

Three time frames (within 3, 10, or 30 days), are provided in the probability tables. The time frame that you choose should best represent the likely time period that the hypothetical oil slick would persist on the surface of the water based on what you know about how quickly it will weather.

Climatological wind and ocean estimates are available for your use in oil weathering calculations.