TABLE 2-3 Validity and Significance of Comparisons Between Estimates from NRC (1985) and Current Report

This Report

1985 Report

Comparability

Significance of change in estimate

Natural Seeps

Marine Seeps

Changes in methods, data, and assumptions significant, but gross comparison still valid.

Natural seeps are a significant source in both reports.

 

Sediment erosion

Differences in approach for calculated land-based loads prevent direct comparison.

Both reports point out that at worldwide or continental scales, the load from eroded source rocks is overwhelmed by natural seeps or anthropogenic loads from land-based sources. Thus, these sources may be of local significance in areas where seeps or anthropogenic loads are essentially absent.

Extraction of Petroleum

Offshore production

Changes in groupings of subcategories make direct comparison with 1985 study impossible.

 

Platforms

Platforms and pipelines

Both studies used national databases as the foundation for the resulting estimate. However, the 1985 report combined spills from pipelines with platforms. Thus, combining the estimates for these two sources in the current study should allow valid comparison at two significant figures.

Reduction in worldwide estimate from 40,000 tonnes per year to 13,000 tonnes per year is significant and is believed to reflect changes in industry practice, especially in areas where stricter regulations have been implemented.

Produced waters

Operational (produced water) discharges

Basic approach was similar but the most significant difference was in how the volume of produced water was determined. In 1985, produced water was determined as a fraction of oil production. The current study used reported data on produced water volume for North America and the North Sea, and extrapolations to worldwide production. Also, oil content was based on actual reported measurements in North America and the North Sea.

The increase from roughly 10,000 tonnes per year to 36,000 tonnes per year is significant, mostly reflecting an increase in the amount of produced water discharged as oil production fields mature, but also related to increased offshore oil production.

Atmospheric deposition

 

Not accounted for in 1985 study.

Small number calculated in the current study suggests the input is significant only in terms of its impact on local air quality.

Transportation of Petroleum

Transportation

Changes in groupings of subcategories make direct comparison with 1985 study impossible. Combining estimates for relevant categories (tanker operations, dry docking, marine terminals, bilge & fuel oil, tanker accidents, non-tanker accidents) in current study allows for valid comparison at two significant figures.

The decrease from 1.5 million tonnes to 420,000 tonnes is significant and reflects the substantial steps taken to reduce the incidence of transportation related spills and operational discharges worldwide.

Pipeline spills

 

Included with platforms in 1985 study.

 

Spills (tank vessels)

Tanker accidents

Both studies used international databases as the foundation for the resulting estimate. Thus, the results should be grossly comparable at two significant figures.

The decrease from 700,000 tonnes per year to 100,000 tonnes is significant and reflects the substantial steps taken to reduce the incidence of transportation-related spills worldwide.

Operational discharges (cargo oil)

Tanker operations

Differing quality of data and changes in methodology make comparisons of little value.

 

Coastal facility spills

Dry-docking

Both studies used international databases as the foundation for the resulting estimate. However, nature and type of facilities included makes comparison of limited value.

 

 

Marine terminals

Grouped with Dry Docking and Refineries in current study.

 

 

Refineries

Grouped with Dry Docking and Marine Terminals in current study.

 

Atmospheric deposition (tanker VOC)

 

Not accounted for in 1985 study.

Small number calculated in the current study suggests the input is significant only in terms of its impact on local air quality.



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