Shell has declared victory over the major oil spill from their Bonga FPSO off Nigeria, claiming the slick was halted 12 miles offshore and has mostly dissipated, thanks to evaporation plus the use of chemical dispersants. Our observations of satellite images over the past few days don’t indicate anything to the contrary.

How big was this spill?  We think the amount spilled is near the high end of Shell’s estimate of “up to” 1.68 million gallons, based on the size of the oil slick observed on December 21 and the photos provided by Shell showing a rainbow sheen.  The thickness of “rainbow sheen” is in the 5 to 10 micron range according to the CONCAWE guidelines, and 0.3 to 5 micron range according to the BONN convention. The overlap — 5 microns — would mean a spill of at least 1.2 million gallons (28,571 barrels).

On their website Shell reported the slick was “less than a hundredth of a millimeter” thick in most areas. 1/100th of a millimeter is 10 microns, which would be a spill of 2.4 million gallons — 58,000 barrels.

Assuming Shell, like most successful companies, is fanatical about inventory control they should be able to provide an accurate measurement by comparing the amount pumped out of the FPSO with the amount that actually ended up in the shuttle tanker. Flow meters on the pumps and transfer lines, and gauges in the tanks, should allow them to calculate the spill with precision.  Let’s ask them for those numbers and settle the question.

Regardless of the specific amount spilled, we’re left with some troubling questions, most notably: how could up to 1.7 million gallons of oil steadily leak into the ocean before anybody noticed and took action? The crack in one of the transfer lines that Shell blames for this leak looks like it could only divert about 5-10% of the flow through that line. How long would that take to amount to 1.7 million gallons?  This is just the latest example of the many mundane, low-tech ways that modern offshore oil production still poses risks — even when it’s being done by one of the biggest, technically accomplished, retail-brand-sensitive multinational oil companies (hmm, that sounds familiar…).