On April 20, an explosion on Deepwater Horizon, a British Petroleum (BP)-operated oil rig in the Gulf of Mexico, led to the most publicized oil spill in decades. Another blowout in the same waters 31 years ago, farther south on the Mexican side of the Gulf, turned into the largest peacetime oil spill ever.
The platform where that accident happened, called Ixtoc 1, was operated by Pemex, the state-owned Mexican oil company. The two accidents and the spills they caused have a number of similar features, although marine oil spills in general have profoundly changed in character in the three decades between the two events.
Oil tankers used to be responsible for the bulk of oil that was spilled. Tank-washing gave rise to a huge number of small spills and tanker accidents such as those involving the Torrey Canyon, Exxon Valdez, Metula, and St. Peter resulted in huge, concentrated spills. Blowouts were not infrequent, but most occurred on land or in shallow water, and most could be stopped relatively easily.
The ban on tank-washing, technical features such as double hulls and sectioning of tanks, introduction of one-way sea lanes and, most important, the use of Global Positioning Systems, have greatly reduced the amount of oil entering the sea from tankers.
Meanwhile, the technology used at drilling and production platforms has developed tremendously, but the challenges regarding depth, weather and sediment structure have grown even more, as easy-to-exploit oil fields have been exhausted. Blowouts have become the most troublesome type of oil spills, and in deep water they tend to continue for a considerable time because of the difficulties faced in containing them.
After the explosions at Deepwater Horizon and Ixtoc, oil and gas under high pressure entered the water at the seabed. This resulted in a three-phase emulsion of oil, gas and water that also contained sand and dirt particles.
The properties of this oil emulsion differ from normal crude oil. Some part of it will float on the surface, but parts may also form plumes at different depths in the water mass. Standard aerial or satellite imaging techniques to measure the quantity of oil spilled do not work well.
In the Ixtoc case, using such methods, Pemex (wanting to keep the loss figure low) estimated the spill to amount to a little under a 454,000 tonnes. A UN expert group that I led put the figure significantly higher. Similarly, whereas BP has been using a release figure of 725 tonnes per day From Deepwater Horizon, estimates by independent experts are many times higher.
Pemex tried to put a device called a “sombrero” over the Ixtoc leak to collect the oil at the ocean floor, but the plan failed because the structure could not be kept in place. Formation of methane hydrate was noted, but was not decisive. In the case of Deepwater Horizon, a huge dome could be put in place, despite the greater depth, thanks to deployment of modern unmanned underwater vehicles, robots and positioning tools, but the formation of methane hydrates clogged the device and rendered it useless.
In both cases, efforts were made to burn off the oil from the ocean’s surface. But, as the 1967 Torrey Canyon experience showed — the stranded tanker was bombed with napalm — oil on water doesn’t burn well and the emulsified oil called “chocolate mousse” hardly burned at all.