ume of spills/platform/year for foreign platforms is similar to those in North American waters. However, because not all foreign platforms are subject to the same stringent anti-pollution guidelines under which U.S. oil platforms operate, it is generally agreed that their spills/platform/year exceed the estimates for North American waters. The 1985 report estimated that foreign spills/platform/year exceeded those in the United States by 300 percent, or 860 tonnes/year and these figures were used in this report as well. The maximum is estimated to be 1,400 tonnes per year or five times the calculated amount. These latter two estimates (best and maximum) are based on the lack of any reliable databases of measured discharges from platforms in the world’s oceans.

Atmospheric Deposition

During the production, transport, and refining of hydrocarbons, volatile compounds escape to the atmosphere. Some, like methane, are light and mix or degrade rapidly. Heavier compounds, like hexadecane, react more slowly and may deposit to the sea surface. These heavier hydrocarbons are labeled as volatile organic compounds (VOC) and are defined in the U.S. Clean Air Act to include all volatile hydrocarbons except methane, ethane, a wide range of chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFC), and a few others, e.g., acetone. VOC from production facilities were not estimated in the 1975 or 1985 NRC reports. With the increased number of offshore oil and gas platforms, the committee decided to evaluate the existing databases, and make estimates of this input. Appendix D describes the methodology, databases, and calculations.

The methodology used was to take estimates from regulators or producers from the four regions where these were available (North Sea, Norwegian Sea, California, and Gulf of Mexico). For the other regions, an estimate was made by multiplying the regional production volume by an average VOC rate per unit produced based on the average of the rates for the four regions mentioned above. Lower (upper) bound VOC estimates were calculated using the same technique except that the minimum (maximum) VOC rate of the four regions was used.

Equilibrium calculations as described in Appendix D indicate that less than 0.2 percent of the released VOC are deposited to surface waters. Based on the best estimate of VOC emissions of 60,000 tonnes (VOC heavier than butane) to North American coastal waters from production platforms, 120 tonnes of VOC enter the coastal oceans from production platforms. The minimum is 72 tonnes or 60 percent of the best estimate, and the maximum is 450 tonnes or 375 percent of the best estimate4. Worldwide, the best estimate is 1,300 tonnes, the minimum estimate is 380 tonnes (29 percent of the best estimate), and the maximum estimate is 2,600 tonnes (200 percent of the best estimate). The significance of these VOC inputs to the marine environment are not known and will be explored to some degree in Chapter 5. However, the size of the loads suggest that they may have an impact on local air quality. VOC is a known pollutant and has been the subject of many NRC studies (NRC 1992, 1995, 1999a,b).

Produced Waters

During oil or gas production, water from the reservoir is also pumped to the surface. Under current industry practices, this “produced water” is treated to separate free oil and either injected back into the reservoir or discharged overboard. Produced water is the largest single wastewater stream in oil and gas production. The amount of produced water from a reservoir varies widely and increases over time as the reservoir is depleted. For example, in the North Sea, a maturing oil production area, the volume of produced water has increased at a rate of 10 to 25 percent per year over the period 1993-1997 in Norway (NOIA, 1998) and the United Kingdom (UKOOA, 1999).

Produced water discharges are permitted as operational discharges. The oil and grease content is regulated by permit, and the allowable maximum concentrations vary by region and nation. For the U.S. Gulf of Mexico offshore oil and gas industry, the limit is a monthly average of 29 mg/L (USEPA, 1996a); in the North Sea and Canada, it is 40 mg/L (PARCOM, 1986, PanCanada, 1999). Conventional treatment consists of gas flotation and/or membrane filtration to separate oil and water, and there will have to be major technological advances before significant improvements in treatment efficiencies can be expected.

In the United States, operators are required to routinely monitor the volumes and oil content of produced water discharges and to submit reports to regulatory authorities to demonstrate compliance with discharge permits. For Canada, Texas Territorial Seas, California, and Alaska, compiled data were available on the volume and oil content of produced water discharges, and the best estimates were calculated using these reported data for selected time periods (usually the late 1990s). Petroleos Mexicanos published an annual report (PEMEX, 2000) reporting a total amount of oil discharged in tonnes. For discharges to the Gulf of Mexico OCS and the Louisiana Territorial Seas, only produced water volumes were available. Therefore, a default value of 29 mg/L, which is the maximum amount allowed on a monthly basis for the Gulf of Mexico discharges, was used as the maximum amount of oil and grease in produced water for these areas. Many operators are able to achieve levels below 20 mg/L (long-term average for California was 18 mg/L and for Alaska was 15 mg/L). Thus, 20 mg/L was used to calculate the best estimate, and 15 mg/L was used to calculate the minimum estimate for this region.


The 60 percent factor used to develop a best estimate and the 375 percent factor applied to develop an maximum estimate is somewhat subjective and reflects the committee’s confidence in the data available and the methods and assumptions used to complete the calculation.

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement