How Offshore Drilling Works
How Offshore Drilling Works
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Once the drill hits petroleum, a final bit of casing called a production casing goes down to the bottom of the shaft. This section of casing terminates in a solid cap, closing the well off from the surrounding petroleum reservoir.
It may seem a bit odd to seal up the prize once you've finally reached it, but the goal isn't to just vent pressurized oil and gas up to the surface, but to control its flow.
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Engineers send down explosives to perforate the production casing at different depths to allow petroleum into the well. This allows the oil and gas to reach the surface under less pressure, and not as a blasting geyser.
Initially, the natural pressure from the subsurface petroleum reservoir is sufficient to push fluids and gas to the surface. Eventually, however, this pressure declines, and the use of a pump or injections of gas, oil or water are required to bring the petroleum to the surface.
By adding water or gas to the reservoir, engineers are able to increase reservoir pressure, causing the petroleum to rise again. In some cases, compressed air or steam is sent down a well to heat the remaining petroleum, which also increases pressure.
If what came up from the wells was pure petroleum, it would just be a matter of barreling it up at this point. But this isn't usually the case, and it's why offshore drilling platforms often boast full production facilities as well. The liquid that rises up to the platform is a mixture of crude oil, natural gas, water and sediments. Most oil refinement takes place onshore, but oil companies sometimes use converted tanker ships to treat and store oil at sea. This process removes unwanted substances from the oil, prior to refining.
Wet vs. Dry Natural Gas
Natural gas falls into two categories: wet and dry. Wet natural gas contains various vaporized liquids, and these have to be filtered out before it can be transported elsewhere. Dry natural gas, on the other hand, is free of these pollutants. At this point, undersea pipelines and oil tankers transport the separated oil and natural gas to onshore storage and treatment plants.
Eventually, a well will either run dry, or the costs of further development will outweigh potential future profits. When this happens, petroleum companies plug and abandon the well.
At this point, operators remove platforms from their moorings — with explosives if need be — and either relocate them or drag them back to shore for scrap. Divers then cut the well casing off below the surface of the ocean floor and seal it in with concrete. In some cases, however, portions of the oil rig remain and are slowly overtaken by sea life.
A Slippery Subject
Despite our culture's dependence on petroleum, not everyone's a huge fan of offshore oil drilling. Criticism ranges from dire environmental warnings to concerns over how oil rigs obstruct the view on a holiday beach trip.
The extraction of natural gas reserves via offshore drilling, particularly with the use of Mobile Offshore Drilling Units (MODUs), carries significant implications for climate and greenhouse gas emissions. Although natural gas is considered a relatively cleaner-burning fossil fuel in comparison to coal and oil, the extraction process can unintentionally release methane, an exceptionally potent greenhouse gas, into the atmosphere.
This methane leakage can manifest during various phases, including drilling, production and transportation. Because methane possesses a significantly higher heat-trapping capacity than carbon dioxide over a short time frame, even minor emissions can make substantial contributions to global warming.
Efforts to mitigate methane emissions, such as the adoption of improved drilling practices and equipment, are crucial in curtailing the climate impact of offshore natural gas extraction, particularly when utilizing MODUs.
Rigorous monitoring and effective regulation are vital to ensuring that the advantages of natural gas as a transitional energy source do not exacerbate the challenges associated with climate change.
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How Oil Drilling Works
Not all oil is accessible on land or in shallow waters. You can find some oil deposits buried deep under the ocean floor. Reaching these oil deposits is dangerous, but if done correctly, it can also be rewarding. If things go wrong, however, the results can be deadly both for oil workers and the surrounding environment.
Using sonic equipment, oil companies determine the drilling sites most likely to produce oil. Then they use a mobile offshore drilling unit (MODU) to dig the initial well. Some units are converted into production rigs, meaning they switch from drilling for oil to capturing oil once it's found. Most of the time, the oil company will replace the MODU with a more permanent oil production rig to capture oil.
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There are four main types of MODUs:
A submersible MODU usually consists of a barge that rests on the sea floor at depths of around 30 to 35 feet (9.1 to 10.7 meters). On the barge's deck are steel posts that extend above the water line. A drilling platform rests on top of the steel posts. These rigs are typically used in areas with calm water.
A jackup is a rig that sits on top of a floating barge. A ship tows the barge to the drilling site. Once positioned, the jackup can extend legs down to the sea floor. The legs are loaded in such a way that they don't penetrate the floor. Once each leg is secure, the jackup continues to ratchet the legs so that the platform rises above the water level. This keeps the rig safe from tidal motions and waves. Jackups can operate in depths of up to 525 feet (160 meters).
Drill ships are ships that have a drilling rig on the top deck. The drill operates through a hole in the hull. Drill ships can pilot to the drill site and then use a combination of anchors and propellers to correct for drift as the rig drills for oil. They can operate in deep water conditions.
Semisubmersibles float on the surface of the ocean on top of huge, submerged pontoons. Some have propulsion systems, which allow them to navigate to drilling sites under their own power while others require a second vessel to tow them to the right location. Most use several anchors -- up to a dozen -- that help maintain the structure's orientation. Computers control the tension on each anchor chain to correct for drift. Some can convert from drilling rigs to production rigs, reducing the need for a second rig to take its place once oil is found.
The MODU's job is to drill down into the ocean's floor to find oil deposits. The part of the drill that extends below the deck and through the water is called the riser. The riser allows for drilling fluids to move between the floor and the rig. Engineers lower a drill string -- a series of pipes designed to drill down to the oil deposit -- through the riser.
At the sea floor is the blowout preventer (BOP). The blowout preventer has a pair of hydraulically-powered clamps that can close off the pipe leading up to the rig in the case of a blowout. As the rig drills down, engineers must add more pipe to the drill string. Each section of pipe is about 30 feet long (9.1 meters).
To add stability to the well, the engineers use metal casings, much as they do with land-based oil rigs. These casings help keep the well from collapsing in on itself. Each casing is lined with cement walls. Casings get narrower as the well gets deeper. Oil companies use progressively smaller drill bits as the well's depth increases. At each annulus -- the spot where a narrower casing joins with a wider one -- engineers use a liner hanger O-ring to seal the two sections together.
When the MODU hits oil, engineers must seal the well to prepare it for a production rig. The engineers will use a pair of plugs to seal off the well bore. The bottom plug sits near the oil deposit. Drilling mud or seawater provides the pressure to hold the plug in place while the engineers place a top plug to cap the oil well. Then the well is ready for a production rig to take over. Offshore production rigs work in a similar way to land-based oil rigs.
With all of this oil-drilling technology in use, and new methods in development, the question remains: Will we have enough oil to meet our needs? Current estimates suggest that we have enough oil for about 63 to 95 years to come, based on current and future finds and present demands.
For more information on oil drilling and related topics, including oil refining, check out the links below.
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More Great Links
Sources
Australian Institute of Petroleum. "Offshore Oil Drilling." Earth Science Australia. (June 21, 2010) http://earthsci.org/mineral/energy/gasexpl/offshore.html
Diamond Offshore. "Offshore Drilling Basics." (June 22, 2010) http://www.diamondoffshore.com/ourCompany/ourcompany_offshorebasics.php
Minerals Management Service. "Mobil Offshore Drilling Unit (MODU)." (June 21, 2010) http://www.mms.gov/ooc/Assets/KatrinaAndRita/BackgrounderMODU.pdf
Oil Gas Glossary. "Barge Submersible Rig Defiinition." 2010. (June 21, 2010) http://oilgasglossary.com/posted-barge-submersible-rig.html
Ritzholtz, Barry. "Deepwater Horizon." The Big Picture. May 25, 2010. (June 21, 2010) http://www.ritholtz.com/blog/2010/05/deepwater-horizon-2/
U.S. Energy Information Administration. "Petroleum Basic Statistics." July 2009. (July 6, 2010) http://www.eia.doe.gov/basics/quickoil.html
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