Oil spill occurrence estimates were generated for several estimated future oil and gas development scenarios (including exploration, production, and abandonment) in the Chukchi Sea Outer Continental Shelf (OCS) lease sale region. Because sufficie nt historical data on offshore oil spills for this region do not exist, an oil spill occurrence model based on fault tree methodology was developed and applied. Using the fault trees, base data from the Gulf of Mexico including the variability of the data, were modified and augmented to represent expected Arctic offshore oil spillage frequencies.