Geology And Geophysics

How to contain an Offshore Oil Leak



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To contain an offshore oil leak, its source must be known. If the leak is directly from the wellhead, it must be capped or otherwise closed off as soon as possible. If it is from the riser, as in the 2010 Deepwater Horizon drilling rig disaster, the riser must be repaired or sealed off below the leak. A new, slant-drilled well which taps into the same reservoir can be used to pump in seawater and concrete to plug the well. Only after the submarine leak is capped can the surface oil slick be properly dealt with. If, however, the blowout preventer cannot seal the leak, further technology for stopping offshore deepsea oil leaks at the source remains extremely experimental.


Emergency oil diversion

The blowout preventer

A fully functional blowout preventer should automatically seal off the wellhead. In case of sudden pressure change, blowout preventers are supposed to seal off or even shear right through the annulus, preventing methane bubbles and other formation fluid from rising any further. The 2010 Deepwater Horizon drilling rig disaster was caused in part by a blowout preventer failure. Had it functioned properly, the explosion should not have happened in the first place, since either the crew or a dead-man’s switch should have severed the drill string at the first sign of a methane bubble.

Where the leak is due solely to a malfunctioning blowout preventer, it could potentially be stopped by sending remotely controlled submarine robots to the sea floor to fix it. This approach is handicapped, however, by uncertainty over whether any readings are accurate to physical reality. Even if the robots do succeed in deploying BOP valves, subsequent damage from the explosion and rig collapse may render those valves useless.

Subsea oil recovery system

If a pollution dome can be successfully placed around the source of the leak, leaking oil can be restricted to a limited area. The oil-water mixture can then be pumped up to the surface through a new pipeline without contaminating the rest of the water. At the surface, the oil can be separated from the water and any natural gas; and then sent off to be refined more or less normally. The Deepwater Enterprise, the processing ship brought in to deal with the Deepwater disaster, is capable of processing up to 15,000 barrels of oil per day. This system could potentially divert as much as 85% of all leaking oil - but it has only previously been used in shallow water less than 100 metres deep. It has never been tested in a deepsea environment.

Placing the container precisely over the leak makes the difference between success and failure. Any subsea wreckage further complicates placement which is already difficult due to normal ocean currents and turbulence.

Another complication was observed on May 8, 2010, when the natural gas leaking from the Deepwater riser combined with seawater to form buoyant methane hydrate crystals which clogged the top part of the container where the new riser was to be connected. A possible, as yet untested solution for the crystal problem is to surround the riser with a second pipe containing a mixture of seawater and antifreeze. A much smaller container with proportionately more vertical height, the so-called ‘top hat’, has successfully been lowered into the vicinity of the leak, but will only be deployed if other approaches fail.

Another subsea oil recovery approach is a direct pipeline diversion. By placing a tube inside the leaking riser, oil gushing from that riser can theoretically be diverted directly to the surface for processing. During the Deepwater disaster, British Petroleum succeeded in placing a 15-cm wide tube into a leaking pipe within three days. Again, all this work is done by remotely controlled submarine robots.

Relief drilling

Previous approaches dealt only with emergency recovery of oil. To actually stop the flow, a relief well is required. This new well must be drilled into the same reservoir as the damaged well but at a slant, avoiding the high oil pressure at the original well. Once the connection is established, first seawater and then mineral mud is pumped into the reservoir to displace the oil. If these have successfully displaced the oil and gone up the broken well, the last step is to pump concrete into the well and allow it to set.

This is yet another technology which has never previously been tried in water deeper than 100 metres. In shallow water, a relief well has been drilled in as little as thirty days under optimal conditions. Under the Gulf of Mexico, all drilling will be through hard shale rock, so conditions are far from optimal even without taking deepsea pressures into account. It is much more likely that drilling a successful relief well will take multiple attempts, if the water and oil pressures involved in deepsea drilling allow drilling a relief well at all.


Dealing with the surface slick

Dispersants

Dispersants specific to each particular type of unrefined oil are the first and most effective line of attack against oil slicks. The solvent part of dispersants increases the miscibility of oil in water, preventing the creation of persistent oil-water emulsions. The surfactant part decreases its surface tension. To maximise the benefit from these different roles, different combinations of dispersants are deployed underwater and at the surface. The two major dispersants used by British Petroleum to contain the oil slick resulting from the Deepwater disaster are Corexit EC9500A and Corexit EC9527A, which are at best 63.4% effective in handling southern Louisiana crude. Thus far, British Petroleum has purchased over a third of the available worldwide stockpile.

As their name suggests, the purpose of dispersants is to disperse oil throughout the water. They do not themselves actually remove or convert any oil, just break it down into much smaller droplets which are potentially more easily dissipated by normal ocean turbulence and subsequently biodegraded by ocean microorganisms. In regions with low wave action, dispersants are not properly circulated through the water, and so have very little effect on the oil spill.

Although the oil’s volatile hydrocarbons are more likely to evaporate when dispersed in this way, polycyclic aromatic hydrocarbons (PAH) and most other toxicity from the oil, as well as any toxicity from the dispersants themselves, remains in the water or, in less turbulent regions, on the seabed. The remnants of oil and dispersants are known to kill fish eggs. PAH, which can bioaccumulate in seabed organisms, has been linked to malformed fish embryos. The end effect of the Deepwater Horizon spill must also be measured against the pre-existing 22,000 square kilometre dead zone reaching out into the Gulf of Mexico from the mouth of the Mississippi river: which had already been impacting the shrimp fishing industry to the east.

The market is highly competitive, so the exact chemical composition of any given dispersant is a highly valued trade secret. Based on the manufacturer’s handling safely sheet, however, one known toxin in Corexit EC9527A is 2-butoxyethanol: which is known to cause headaches, vomiting, and reproductive disorders. Even so, modern dispersants are believed to be less toxic than those used to clean up the 1989 Exxon Valdez spill; and are generally believed to have a lower environmental impact than untreated oil slicks.


Other methods

Booms physically collect surface oil and keep it from spreading. Smaller booms are used to divide existing surface oil into sections, so that it can be skimmed or towed away for burning. Over 500 kilometres of floating boams have thus far been employed to attempt to contain the oil slick from the Deepwater disaster; and new booms are being purchased as fast as their manufacturers can supply them. Due to the unprecedented volume of the spill, the demand for booms directly resulting from this spill alone is estimated to continue at the present rate for up to a year after the wellhead is successfully capped.

Oil skimmers physically remove the thin sheen of oil floating on top of the water. Drum, disc, and rope skimmers collect oil as they are moved around on top of the oil-charged water. Many of these skimmers are also oleophilic, with surfaces that have been chemically treated to attract oil. As the oil collects on the drum or rope, it is systematically wiped off and collected, leaving the surface available for new oil. Weir skimmers do not move. Instead, they form low barriers so that oil can be skimmed away as water flows over them.

Controlled burns to remove oil from the surface of water were pioneered in the deep sea oil drilling off the coast of Newfoundland. Absolutely calm seas are not necessary, but if choppy waters have emulsified too much of the oil, it can no longer be burned off. For a successful burn, the surface oil must be fairly thick, and the wind must be reliable, reasonably light (lest the flames spread out of control), and (for reasons of thick, black smoke) be directed away from any inhabited areas. The amount of oil to be burned away during each individual burn is contained with booms and towed away from the main slick. Each individual burn takes about an hour and burns away several thousand gallons of oil. In ideal conditions, a controlled burn can remove as much as 90% of the oil in the burn area. What remains behind is a waxy film that can be skimmed off. This method cannot be combined with any form of biomediation.

Adding nitrate or sulphate fertilisers to oil slicks can encourage decomposition of crude oil by microorganisms. A biomediation accelerator quickly gels together oil hydrocarbons so as to create a bacterial bloom which can break down oil more than three times as quickly as in nature. This method is contra-indicated in the Deepwater disaster due to the pre-existing dead zone, which is already extremely low in oxygen due to heavy fertiliser outflow from the Mississippi river. This method cannot be combined with controlled burns.

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