How does deep water drilling work?

The first offshore drilling platform was built in 1897 in California at the end of a wharf, according to How Stuff Works. This was followed by the first mobile platform for drilling in wetlands, developed in 1928 by a Texas oilman. It was simply a barge with a drilling rig mounted on it, but the start of a much larger offshore drilling industry that continued to accelerate. A platform that was far enough off the shore it couldn’t be seen from land was built in 1947 by a consortium of oil companies. These platforms weren’t just built to see if it could be done - as it turns out, most of the world’s petroleum is deep down, 500 to 25,000 feet below the surface of the Earth. New, giant oil reserves are being found in locations like the Gulf of Mexico at ever greater depths. As these deep water reservoirs are discovered, they are sometimes shrugged off as too deep to tap, but new drilling technology has allowed engineers to drill deeper and find huge amounts of hydrocarbons. Thousands of feet down, millions of years of decaying plant and animal matter has turned into three different forms of petroleum: liquid (oil), gas (natural gas), and solid (oil shale, tar sands).

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Semi-submersible like those used for deepwater drilling. Image from Transocean.

How do drillers find petroleum deposits?

There are a few ways petroleum deposits are found: sniffers, magnetic surveys, gravity surveys, and seismic surveys. Sniffers detect natural gas traces in seawater, but are uncommonly used as they only detect deposits that leak, not those that are sealed hundreds or thousands of feet underground. Magnetic surveys allow a ship to map magnetic anomalies in rock density that affect the Earth’s magnetic field. These discrepancies can also be found via aircraft, which allows huge areas to be surveyed quickly. Canada even offers gravity and magnetic survey data from the government for free. When performing these surveys, surveyors are looking for nonuniformities in the rock that mean there could be an anomaly like oil and gas deposits. Geomagnetic field information has be updated as it is frequently changing, while the gravity field is vertical and goes one direction, making gravity surveys much more simple. Both of these survey types are great for large areas and almost always require other forms of surveying to compliment them, such as seismic surveys. For this reason, sometimes magnetic and gravity surveys are skipped in favor of seismic surveys alone, but technology like a new microelectromechanical (MEMS) gravity survey device the size of a postage stamp could revolutionize this area. A gravity survey might require $100,000 for equipment, plus ship or airplane time to use gravimeters, but this new technology could reduce the cost of the equipment to a few thousand dollars, while making it light and small enough to create a whole fleet of small autonomous survey ships or drones to cover large areas.

MEMS gravity survey device, the size of a postage stamp. Image from IEEE Spectrum.

Seismic surveys are by far the most common type of survey for petroleum. Acoustic shock waves are sent down into the ocean with air guns and read on hydrophones as they travel back to a moving vessel. The sound waves travel at different speeds through different types of rock and other subterranean materials, allowing scientists to then analyze that data to find deposits. Once deposits are detected, offshore areas can be leased from the governments that own them, and exploratory drilling can begin.

Exploratory drilling

The initial wells to explore drilling for oil are called “wildcat” wells and can be drilled by different types of vessels containing drill rigs. In shallow waters 20 to 400 feet deep, a jack-up would most likely be used, which is a vessel with legs that can be extended below the vessel onto the sea floor, raising the hull above the waves to create a stable platform. In deeper waters, a semi-submersible platform can be either moored to the bottom of the ocean with cables or a tower, or stay in place with dynamic positioning. Although they may have thrusters, they don’t usually travel under their own power and need to be towed to new locations. Drillships operate in deep water and relocate quickly under their own power, good for drilling multiple exploratory wells in less time. Drillships usually have dynamic positioning systems as well, and similar to DP systems on a semi-submersible, the system is computer-controlled and uses azimuth thrusters to keep the vessel on station without the use of anchors or mooring. OneStep Power tests the electrical systems on dynamically positioned vessels to ensure fault ride-through, something you want to know if you’re relying on those systems for station-keeping while drilling a well!

“The Deepwater Proteus, a newbuild drillship that operates on hybrid-power systems which increase fuel efficiency. Source: Transocean”. Image from Journal of Petroleum Technology.

When drilling starts, core samples are removed and taken back up to the vessel, where geologists look for signs of petroleum, called a “show”, according to How Stuff Works. When a deposit is found, a production well is drilled and an offshore platform is moved into place to produce oil. Wells usually last about 10 to 20 years, so these platforms have to survive storms and all sorts of inclement weather without moving. The North Sea is a popular area for drilling and has very harsh weather with waves over 20 feet and wind speeds over 100 mph, according to Energy Voice. They’re usually attached to the sea bed with cables or metal legs. Multiple wells can be attached to one platform, and directional drilling means wells do not have to be drilled vertically but can be drilled at an angle miles away from the platform.

Huge waves and high winds at a platform in the North Sea. Image from Energy Voice.

How are deep water wells drilled?

Deepwater wells are usually drilled above the deposit, if possible. The first step is lowering a drill bit down to start the hole, also called “spudding in” a well. The drill bit spins inside a casing that ensures the surrounding soft sediment doesn’t cave in. The pipe is jetted into place with water or drilling fluid so the well head is above the sea floor, acting as a base for drilling. The drill bit continues down into the sediment and rock and drilling mud is pumped down through the bit to cool and lubricate it while also raising cuttings and particles from the drilling process to the surface through the hole and casing. The fluid also equalizes pressure between the inside and outside of the pipe and helps to keep fluids from flowing into the well bore, according to Shell. Mud is made up of water, clay, and fine rock, and is environmentally friendly.

Drill bit for drilling into hard rock. Image from AGU Advancing Earth and Space Science blog.

At a specified depth, a new casing pipe is run down into the well and concrete is pumped down into the pipe, flowing up around the outside of it to secure it in place. Mud is pushed down behind the cement and they are separated with a plug. In extremely deep wells, a second, smaller casing string is used and the same cementing process done to ensure strength. Each time a new casing is cemented in place, the bore is smaller than before. Before oil and gas is reached or pressures get too high, a blow out preventer, also called a BOP stack is secured to the sea floor with a riser on top. It contains valves to protect the rig and environment if the pressure is too great for the mud. The BOP stack is then tested and drilling continues with drilling done through the stack, with a final casing string installed once oil and gas are reached. Check out the video from Shell below for some great visuals on this process!

Video from Shell.

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Offshore pipelines

Once all the petroleum is flowing, it has to get where it needs to go. Pipelines deliver oil and gas to production platforms and to shore. Infield pipelines transport fluids inside the field to the platform and move processed water to injection wells. Export pipelines transport processed oil and gas to the shore from the platform in a multi-phase pipeline with both oil and gas, or a single-phase pipeline with just one. Transmission pipelines can carry oil and gas from one coast to another for trading.

Diagram of a deepwater drilling operation. Image from Wired.

Deep water drilling has gone far from where it started on a dock in California to platforms like small cities fixed in the ocean. Crews of over a hundred people work to pump hydrocarbons from 40,000 feet below the sea bed where oil is over 400 degrees Fahrenheit, according to Wired. Technology will continue to advance and previously unreachable deposits of oil and gas will be tapped by new rigs. We look forward to keep these rigs safe!

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Sources:

Using a drilling rig to bore holes for petroleum extraction in deep sea

Deepwater drilling,[1] or deep well drilling,[2] is the process of creating holes in the Earth's crust using a drilling rig for oil extraction under the deep sea. There are approximately 3400 deepwater wells in the Gulf of Mexico with depths greater than 150 meters.[3][full citation needed]

Deepwater drilling has not been technologically or economically feasible for many years, but with rising oil prices,[when?] more companies are investing in this sector.[4] Major investors include Halliburton, Diamond Offshore, Transocean, Geoservices, and Schlumberger. The deepwater gas and oil market has been back on the rise since the 2010 Deepwater Horizon disaster, with total expenditures of around US$35 billion per year in the market and total global capital expenditures of US$167 billion in the past four years.[5] Industry analysis by business intelligence company Visiongain estimated in 2011 that total expenditures in global deepwater infrastructure would reach US$145 billion.[6]

A HowStuffWorks article explains how and why deepwater drilling is practiced:

Not all oil is accessible on land or in shallow water. You can find some oil deposits buried deep under the ocean floor. ... 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. ...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.[7]

In the Deepwater Horizon oil spill of 2010, a large explosion occurred, killing workers and spilling oil into the Gulf of Mexico while a BP oil rig was drilling in deep waters.

The expansion of deepwater drilling is happening despite accidents in offshore fields ... Despite the risks, the deepwater drilling trend is spreading in the Mediterranean and off the coast of East Africa after a string of huge discoveries of natural gas. ... The reason for the resumption of such drilling, analysts say, is continuing high demand for energy worldwide.[8]

History

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Some of the earliest evidence of water wells are located in China. The Chinese discovered and made extensive use of deep drilled groundwater for drinking. The Chinese text The Book of Changes, originally a divination text of the Western Zhou dynasty (1046 -771 BC), contains an entry describing how the ancient Chinese maintained their wells and protected their sources of water.[9] Archaeological evidence and old Chinese documents reveal that the prehistoric and ancient Chinese had the aptitude and skills for digging deep water wells for drinking water as early as 6000 to 7000 years ago. A well excavated at the Hemedu excavation site was believed to have been built during the Neolithic era.[10][11] The well was cased by four rows of logs with a square frame attached to them at the top of the well. 60 additional tile wells southwest of Beijing are also believed to have been built around 600 BC for drinking and irrigation.[10][12]

Types of deepwater drilling facilities

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Drilling in deep waters can be performed by two main types of mobile deepwater drilling rigs: semi-submersible drilling rigs and drillships. Drilling can also be performed from a fixed-position installation such as a fixed platform, or a floating platform, such as a spar platform, a tension-leg platform, or a semi-submersible production platform.

1. Fixed Platform - A Fixed Platform consists of a tall, (usually) steel structure that supports a deck. Because the Fixed Platform is anchored to the sea floor, it is very costly to build. This type of platform can be installed in water depth up to 500 meters (1,600 feet).
2. Jack-Up Rig - Jack-up rigs are mobile units with a floating hull that can be moved around; once they arrived at the desired location, the legs are lowered to the seafloor and locked into place. Then the platform is raised up out of the water. That makes this type of rig safer to work on because weather and waves are not an issue.
3. Compliant Tower Platform - A compliant tower is a particular type of fixed platform. Both are anchored to the seafloor, and both workplaces are above the water surface. However, the compliant tower is taller and narrower and can operate up to 1 kilometer (3,000 feet) water depth.
4. Semi-Submersible Production Platform - This platform is buoyant, meaning the bulk of it is floating above the surface. However, the well head is typically located on the seafloor, so extra precautions must be taken to prevent a leak. A contributing cause to the oil spill disaster of 2010 was a failure of the leak-preventing system. These rigs can operate anywhere from 200 to 2,000 meters (660 to 6,560 feet) below the surface.
5. Tension-Leg Platform - The Tension-leg Platform consists of a floating structure, held in place by tendons that run down to the seafloor. These rigs drill smaller deposits in narrower areas, meaning this is a low-cost way to get a little oil, which attracts many companies. These rigs can drill anywhere from 200 to 1,200 meters (660 to 3,940 feet) below the surface.
6. Subsea System - Subsea Systems are actually wellheads, which sit on the seafloor and extract oil straight from the ground. They use pipes to force the oil back up to the surface, and can siphon oil to nearby platform rigs, a ship overhead, a local production hub, or even a faraway onshore site. This makes the Subsea system very versatile and a popular choice for companies.
7. Spar Platform - Spar Platforms use a large cylinder to support the floating deck from the seafloor. On average, about 90% of the Spar Platform's structure is underwater. Most Spar Platforms are used up to depths of 1 kilometer (3,000 feet), but new technology can extend them to function up to 3,500 meters (11,500 feet) below the surface. That makes it one of the deepest drilling rigs in use today.[13]

2010 Deepwater Horizon oil spill

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On 20 April 2010, a BP deepwater oil rig (Deepwater Horizon) exploded, killing 11 and releasing 750,000 cubic meters (200 million gallons) of oil into the Gulf of Mexico. With those numbers, many scientists consider this disaster to be one of the worst environmental disasters in the history of the US.[14]

A large number of animal deaths have resulted from the release of the oil. A Center study estimates that over 82,000 birds, about 6,000 sea turtles, and nearly 26,000 marine mammals were killed from either the initial explosion or the oil spill.

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