Outer Continental Shelf Drilling Blowouts, 1971-1991
Authors: Danenberger E.P. 
Published: October 2008

ABSTRACT
From 1971 through 1991, 87 blowouts occurred during drilling operations on the Outer Continental Shelf of the United States. During this 21-year period, 21,436 wells were drilled. Eleven of the blowouts resulted in casualties (61 injuries and 25 fatalities). Most of the blowouts were attributable to shallow gas influxes and were of short duration. The overall blowout rate was one per every 246 wells drilled (one per 256 wells drilled if sulfur wells are excluded). The rate for blowouts with casualties and fatalities was significantly lower.

INTRODUCTION
The control of underground pressures is the most important concern in the planning and conduct of oil and gas drilling operations. Improper well-control procedures can result in the sudden, uncontrolled escape of hydrocarbons. Such a loss of well control is commonly referred to as a blowout. Blowouts are the most spectacular, expensive, and feared operational hazard. At best, they result in costly delays in drilling programs. They also may lead to fires, explosions, casualties, serious property damage, and pollution.
The Minerals Management Service maintains recorded on all blowouts resulting from oil, gas, and sulfur operations on the Outer Continental Shelf (OCS) of the United States. (Two previous Department of the Interior reports addressed OCS blowouts [Danenberger, 1980 and Fleury, 1983].) The information in this report is derived from a review of a total of 87 blowouts that occurred during drilling operations in Federal offshore waters from January 1, 1971, through December 31, 1991. This report covers blowouts that occurred at any phase of a drilling operation but does not consider blowouts that occurred during completion, workover, or production operations. Four of the blowouts occurred during sulfur well drilling; the remainder involved oil and gas drilling operations. All of the blowouts were in the Gulf of Mexico (GOM) where 95.8 percent of all new OCS wells were drilled during this period. Casualties (61 injuries, 25 fatalities) resulted from 11 (12.6 percent) of the blowouts. Because oil was not associated with any of the natural gas influxes that triggered these drilling blowouts, pollution and environmental damage were minimal.

SHALLOW GAS
Although development wells accounted for 62.6 percent of the oil and gas drilling during this period (table 1), 46 (55.4 percent) of the 83 nonaulfur well blowouts occurred during exploratory drilling (figure 1). The greater incidence of blowouts during exploratory drilling may be attributable to the relative absence of geologic information and drilling data. Because' most exploratory wells are not commercially producible, the occurrence of drilling blowouts in the GOM may not be strongly linked to well producibility. This judgment is further supported by information on the well depths at the time the blowouts occurred (table 2 and figure 2). Fifty-eight (66.7 percent) of the blowouts occurred before the wells had been drilled to a depth of 5,000 feet and were not triggered by hydrocarbon influxes from commercially productive intervals. These wells flowed shallow gas that was over pressured, undetected, or improperly controlled.