Comment on “A Tale of Two Spills: Novel Science and Policy Implications of an Emerging New Oil Spill Model”

The American Petroleum Institute believes that subsea dispersants played a critical role during the Deepwater Horizon (DWH) response by reducing the surface oil near the well site. Peterson and his colleagues (2012), however, published an article on post-DWH research and policy priorities questioning the use of subsea dispersants. This letter provides evidence that subsea dispersants worked.

A primary concern is Peterson and his colleagues’ statement that subsea dispersants “may have only marginally augmented the high degree of natural oil dispersion” (p. 463). They believe that “the turbulent mixing induced by the pressurized discharge of hot oil and gas into entrained cold seawater was sufficient by itself to induce massive dispersion of oil into fine droplets” (p. 463).

Figure 1.

Aerial photos taken over the Macondo well site before the subsea injection (top left, 9 May 2010), 11 hours after start of the dispersant injection (top right, 10 May 2010), and 5 hours after the injection was stopped (bottom, 11 May 2010). Abbreviations: Avg, average; hrs, hours; CST, Central Standard Time.

Aerial photos, such as those in figure 1, taken during a 24-hour test of subsea dispersants suggests other-wise. Eleven hours after the subsea injection had begun, the surface near the well had 90% less oil, according to estimates made using these images. The slicks reappeared 5 hours after the injection was stopped.

There are other factors that explain the change on 10 May. Winds and currents could have increased. Wind speed, however, actually decreased. In an evaluation of current data collected near the Deepwater Discovery III (NOAA National Data Buoy Center station no. 42916;  www.ndbc.noaa.gov/station_page.php?station=42916), combined with droplet rise velocities, it was found that the surfacing locations of large oil droplets showed little variability between 9 and 10 May (figure 2).

To support their statement, Peterson and his colleagues referenced Johansen and colleagues (2003), who described a field release of hydrocarbons in approximately 800 meters of water. The data in the report ( www.boemre.gov/tarprojects/377.htm) underlying Johansen and colleagues (2003) indicate, however, that a significant amount of the crude oil released (untreated with dispersant) may have reached the surface. During the crude oil discharge, spotter planes observed a 9 × 1 kilometer sheen at the surface. The sheen was estimated to have a thickness of between 0.3 and 5 microns, which represents 2.7–45 cubic meters of oil. Considering that 50 cubic meters of crude oil was released, a significant amount reached the surface at the low estimate. At the high estimate, 90% of the oil reached the surface!

Figure 2.

Horizontal surface location (in meters) of large oil droplets relative to the well head (the center circle). The numbers next to the plus sign (+) show the number of hours after midnight on 9 May. To calculate a surface location at a given time, the measured current profile from the Deepwater Discovery III was applied to a 10-millimeter oil droplet (the diameter of a large oil droplet that might form when dispersants are not used) that was assumed to rise at 20 centimeters per second. Although this model is quite simple, it should be adequate to give a relative measure of the importance of ocean currents on the rise of the oil.

Furthermore, the role of dispersants is not just to facilitate the formation of small, slowly rising oil droplets but also to hinder recoalescence into larger droplets (Young 1945, Ivanov et al. 1979, Vincent 1983) that rise more rapidly to the surface.

Evidence indicates that subsea dispersants reduced the volatile oil surfacing near the DWH well, which helped protect responders attempting to control the well. Clearly, some oil surfaced, but it was mostly away from the well and in smaller amounts, which helped reduce the amount of oil reaching sensitive shorelines.