The spill incidence record on America's OCS is in fact remarkedly low. Although 

 we cannot quantify specific reasons, the consequences are a tribute to all from 

 our society who participate in offshore oil and gas activities. 



The final stage of oil spll risk analysis combines the conditional 

 probabilities and spill likelihood to yield final (joint) probabilities — the 

 probabilities that spills will occur and will contact various resources. 



The calculation of joint probabilities involves use of hydrocarbon resources 

 expected to be produced and transported. The volumes used in the model are 

 provided by USGS and are the mean economically recoverable resources. 



Joint probabilities are calculated as follows: 



(a) Form matrix [c], where elements c^ i are the conditional probabilities 

 that a spill from site j will contact target i. 



(b) Form matrix [s], where elements Sj ^ are the expected number of spills 

 from site j due to unit volume produced at site k. 



(c) Form matrix [u] « [c] x [s], where elements u^ -s are the expected 

 number of spills occurring and contacting target i due to production 

 of a unit volume of oil at site k. 



(d) Form vector [v], where element v(k) represents the volume of oil 

 expected to be produced at site k. 



(e) Form vector [L] » [u] x [v], where element l(i) is the expected 

 number of contacts to target i. 



(f) Using the Poisson distribution discussed earlier and the vector [L] , 

 the probability of exactly n contacts to target i may be calculated 

 by: P(n,i) = [l n (i) exp(-l(i))]/n! 



The joint probability that one or more spills will occur and will contact 

 target i is obtained by summing P(n,i) over n > 0. More detailed discussion 

 of the above procedure is contained in recent lease sale specific oil spill 

 reports published by USGS, such as Samuels et al. (1982b). The steps outlined 

 above illustrate the combining of OCS production and transportation spills. 

 This is an important factor — one cannot decide to produce at a specific site 

 without concomitant consideration of oil transport. 



USE OF MODEL RESULTS 



1. Applications 



Model results are presented and analyzed in Environmental Statements (ES) 

 prepared in BLM's OCS Field Offices. The content and analyses of an ES are 

 defined by the National Environmental Policy Act and its implementing regulations 

 In compliance with these, the model addresses each proposed lease sale from 

 three perspectives. First, a portrayal of the proposed sale by itself, as 

 though there were no other activities taking place in the sale area. Second, a 



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