Deepwater Horizon – Learning from Past Experience and Defending Against Future Oil Spills
of it will also quickly turn into suspended oil droplets with varying amounts of gas bubbles and sand grains in them. These droplets can have a wide range of densities. Some will rise to the surface, others fan out over the bottom and still others ride on the density gradients between different layers of water.
Furthermore, while the quantity oil discharged at the surface can be reasonably well estimated from airplanes or satellites from the size of the surface area covered and with the darkness of the oil slick as an indicator of thickness, this technique will lead to a severe underestimate
The reason for this ‘learn; forget; relearn; forget again’ cycle so typical for oil spills is the special funding schemes for research and study.
if applied to oil from a sea floor blowout. Not only will much of the oil fail to reach the surface, but the part that does will be emulsified and brownish or reddish in colour rather than black, and it will not all spread out over the surface but will be in, as well as on, the water.
These peculiarities of blowout oil were known to the US Coast Guard and National Oceanic and Atmospheric Administration (NOAA) at the time of the Ixtoc 1 blowout in 1979, but apparently forgotten in 2010.
With regard to effects on biota and ecosystems, oil harms organisms in two very different ways. One is by smearing them. Warm-blooded animals, insulated by feathers or fur, lose their insulation on the oil- covered patches and may succumb to hypothermia. If they get into thick enough layers of oil, such as if they have to surface to get air when under a new surface spill, they might also simply get stuck in it and be unable to free themselves. This is, on another scale, what happens to plankton that get caught by suspended oil droplets: they sit there fighting in vain to get free, much like flies on fly-paper.
The other mechanism is chemical toxicity. Many of the chemical compounds that make up crude oil are toxic and some have anaesthetic properties. Organisms with gills can pick up these compounds either when they have dissolved in water or when oil droplets in the water get stuck on gill membranes when they breathe. Others, like sea turtles or dolphins, might inhale anaesthetic compounds, get confused and lose orientation, or simply eat the oil, e.g. in the form of ‘chocolate mousse’, mistaking it for food.
Seabirds could be especially exposed if they land in an oil slick, as happened on a stormy winter night in 1976 in the Baltic. An oil tanker passing the island of Öland washed its tanks with seawater and released a mere five tons of oil that calmed the waters and made the spot an inviting place to land for some 60,000 long-tailed ducks, who all succumbed.
Similarly, it was found in the studies of the effects of the Ixtoc oil spill that organisms with light-induced vertical migration that rise towards the
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surface at night, thereby both passing through underwater oil clouds and exposing themselves to surface oil, were hit the hardest. Shrimp, squid and a large number of zooplankton belong to these organisms.
Another important early lesson was that oil spills in tropical and sub-tropical waters differ a lot from those in temperate and boreal waters. In warm waters everything occurs faster than in cold ones. The old rule of thumb called q-10 is a fair approximation also here. It says: “A 10°C increase in temperature doubles the speed of a chemical or biochemical reaction.” As a consequence the acute effects tend to be more severe in warm waters but the recovery is also much faster. After the Ixtoc spill, one had to look quite hard to see any ecological effects five years later. The hardest-hit fisheries, for shrimp and squid, even reported higher than pre-spill catches, helped, no doubt, by some years of reduced fishing pressure.
A type of lasting consequence repeatedly reported after oil spills is what is called cascading populations. Oil might kill grazing snails on beaches or zooplankton at sea. When the toxicity is gone, attached or plankton algae can grow wild and cause blooms, much like the consequences of eutrophication. Then the grazers return and with ample food supply, they in turn surpass their normal numbers, pushing down the numbers of the organisms they feed on. Simultaneously, they themselves form a richer than usual food base for their predators, and from predation and food shortage they again become less abundant than usual. And so it goes for a while, until it stabilises within normal bounds of variation.
The question about the use of dispersants is, as said above, also an old one. They work the same way as the detergents in dishwashers do when they clean grease from plates, by breaking it up into small droplets that disperse in water. Dispersants speed up the normal process of oil weathering and ageing as described above, but in doing so they change the balance between the processes. When chemically dispersed, less of the volatile oil components evaporate and photo-oxidation becomes far less important as a breakdown and detoxification process. As a result, gill-breathing organisms get much more exposed to oil toxicity, while those that risk getting smeared by oil on the surface fare better. An old summary is: “If you want to protect birds and beaches as your priority, use dispersants; if you want to protect fisheries and under-surface marine life – don’t.” Today, another consideration often seems to be decisive: oil-smeared birds and contaminated beaches on pictures in media make bad PR for companies and governments alike, while shrimp and fish larvae in the open ocean die unseen by cameras.
Oil spills from blowouts don’t show a clear temporal trend as those from tankers and pipelines do. That does not mean that they are very rare. There are a number of small such spills every so often, medium size and large ones now and then and huge ones decades apart. The Norwegian Foundation for Industrial and Technical Research (SINTEF) marine blowout database has catalogued 573 blowout spills since 1955, mostly from European and North American waters, and that is far from all. Less than a year before the BP disaster, there was a blowout spill in the Timor Sea in Australian water, and while the Mexican Gulf had the media’s full attention, there was another in the Egyptian Red Sea. The huge blowout spills, though, luckily, are more rare. With the exception of the oil that the Iraqis pumped into the Persian Gulf during the Kuwait war in 1991, the two Mexican Gulf
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2
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