How Offshore Drilling Works

Written by Administrator Thursday, 14 October 2010 22:47

Some people say money makes the world go round. Others insist the key ingredient is love or even music. But whatever drives humanity to carry on from day to day, our dependence on fossil fuels leaves one fact for certain: The axle of our spinning globe is greased with oil.

We consume more than 80 million barrels of the stuff every day [source: CIA]. To meet our ravenous demand for fossil fuels, petroleum companies constantly comb the planet for new reserves. Since oceans cover nearly three-quarters of Earth's surface, a great deal of those reserves wind up underwater. 
Reaching these undersea drilling sites poses quite a challenge. After all, drilling on land is an undertaking on its own. How do you drill in lightless ocean depths and transport all that liquid, gas and solid petroleum back to the surface? How do you keep from polluting the ocean? And how do you do all of this, with tons of special equipment, in the middle of rough seas?
To surmount these obstacles, petroleum companies have invested billions into the development of offshore drilling and offshore oil platforms. The first of these platforms was constructed in 1897 at the end of a wharf in California. In the years to follow, oil prospectors pushed out into the ocean, first on piers and then on artificial islands. In 1928, a Texan oilman unveiled the first mobile oil platform for drilling in wetlands. The structure was little more than a barge with a drilling outfit mounted on top, but it set the example for decades of advancements to come.

In the years that followed, petroleum companies moved even farther into the ocean. In 1947, a consortium of oil companies built the first platform that you couldn't see from land in the Gulf of Mexico. Even the North Sea, which endures nearly constant inclement weather, is currently home to many offshore drilling sites [source: The Guardian].

Today's oil rigs are truly gigantic structures. Some are basically floating cities, employing and housing hundreds of people. Other massive production facilities sit atop undersea towers that descend as far as 4,000 feet (1,219 meters) into the depths -- taller than the world's most ambitious skyscrapers. In an effort to sustain their fossil fuel dependency, humans have built some of the largest floating structures on Earth.
Hunting for Fossil Fuels
While fossil fuels have only become the driving force behind human civilization in the last couple of centuries, oil and natural gas have been making their way back to the Earth's surface for millions of years. Spanish conquistadors observed oil rising to the surface in the Gulf of Mexico in the 16th century, and the Chinese drilled for it in the ground as early as 347 A.D. [source: Totten]. To find even older evidence, you don't have to look any farther than the prehistoric animals unlucky enough to have been consumed by the world's tar pits.

However, most of the world's petroleum is trapped between 500 and 25,000 feet (152 and 7,620 meters) under dirt and rock. All of this oil began as tiny plants and animals called plankton, which died in the ancient seas between 10 and 600 million years ago. This decaying matter drifted to the bottom of the ocean and, over time, was covered with sand and mud. In this oxygen-free environment, a kind of slow cooking process took place. Millions of years of heat and pressure eventually transformed this organic material into vast deposits of liquid, gas and solid petroleum, all capped in traps under thick layers of rock. We call liquid petroleum oil and gaseous petroleum natural gas. Solid petroleum deposits often take the form of oil shale or tar sands
Needless to say, these fossil fuel deposits don't just start bubbling crude every time a hillbilly fires a rifle. Geologists study surface features and satellite maps, check soil and rock samples, and even use a device called a gravity meter to find subtle gravitational fluctuations that might indicate a subterranean flow of oil. Not all of these options are particularly viable, however, if the terrain you're canvassing is thousands of feet below pitching ocean waves.

When searching for fossil fuels at sea, oil geologists are able to use special sniffer equipment to detect traces of natural gas in seawater. But as this method can only help find seeping deposits, oil companies largely depend on two other means of locating traps.

When close to the surface, certain rocks affect the Earth's normal magnetic field. By using sensitive magnetic survey equipment, a ship can pass over an area and map any magnetic anomalies that occur. These readings allow geologists to hunt for the telltale signs of underground traps.

Surveyors can also detect possible traps through the use of seismic surveying. This method, known as sparking, involves sending shock waves down through the water and into the ocean floor. Sound travels at different speeds through different types of rock. If the shock wave reaches a change in rock layers, it bounces back up toward hydrophones dragged behind the survey ship. With the aid of computers, seismologists can then analyze the information to pinpoint possible traps in the Earth.
Survey ships use both compressed air guns and explosives to emit shock waves. Of these two methods, air guns are far less of a threat to sea life, but even acoustic pollution poses a threat to such seismically aware sea animals as the endangered blue whale.

What happens once survey teams detect undersea oil deposits? Well, it's time to mark down the GPS coordinates, plant a buoy and obtain a government lease to begin a little exploratory drilling and see what you've got.

Exploratory Drilling 
You can send shock waves down to the ocean floor all day, but ultimately you're going to have to drill a little if you want to know if you have a potential gusher on your hands. To handle this job, oil companies send out a mobile drilling platform to perform exploratory drilling on a site. Some of these platforms are ship-based, but others have to be towed to the drilling site by other seagoing vessels.

An exploratory drilling rig will typically drill four temporary exploratory wells over a suspected deposit, each taking 60 to 90 days to complete. Geologists initially drill to obtain a core sample. The principle is the same as if you stuck a hollow cylinder into a birthday cake and then removed it. You'd then be able to examine the cylinder to discover what varying layers of icing and cake existed inside the cake. Will there be ice cream? This is one method of finding out without cutting yourself a whole slice.

Of course, oil geologists aren't hoping for ice cream. They're looking for signs of petroleum, which they call a show. Once a show has occurred, drilling stops and geologists perform additional tests to make sure oil quality and quantity are sufficient to justify further action. If so, they then drill additional wells to substantiate the findings. 
Once geologists have established the worth of a petroleum deposit, it's time to drill a production well and begin harvesting the riches. An average well lasts a good 10 to 20 years before it's no longer profitable, so offshore production platforms are built with a long stay in mind. The platforms are typically fixed directly to the ocean floor using either metal and concrete foundations or tethering cables. As you might imagine, the platform has to remain as stationary as possible during all this drilling, no matter how severe the weather becomes.

One platform can boast as many as 80 wells, though not all of them go straight down. Directional drilling allows oil platforms to sink production wells into the ocean floor at an angle in order to reach deposits miles away from the drill site. If you've seen the 2007 film "There Will Be Blood," then you may know this as the "I drink your milkshake!" method. In the film, a maniacal, mustached oilman boasts that, through directional drilling, he's managed to drain all the oil beneath a nearby parcel of land. This issue also arises in the offshore drilling industry. For instance, in California, the state is authorized to drill new wells if it can prove that wells in adjacent federal waters are draining California-owned oil deposits.

Even after its wells have run dry, offshore production platforms often find renewed life as a central hub for other nearby oil platforms. The other platforms pipe petroleum over for processing and/or storage.

So you've sunk millions into erecting your oil rig. Now it's time to get busy drinking that offshore milkshake.
Undersea Drilling
 You've established your multimillion-dollar offshore drilling platform and, miles beneath you, there's a fortune in untapped petroleum deposits. The challenge in undersea drilling is transferring all that precious oil and gas from point A to point B without losing it and polluting the ocean. How do you tunnel into the Earth without water flowing into the hole or all the oil surging up into the sea?

To ensure accurate drilling, engineers connect the drill site to the platform with a subsea drilling template. On a very basic level, this serves the same purpose as the templates you might have used to trace a pattern or carve a jack-o-lantern design into a pumpkin. While the design may vary depending on the exact ocean floor conditions, the drilling template basically resembles a large metal box with holes in it to mark the site of each production well.


Since production wells often have to sink miles into the Earth's crust, the drill itself consists mostly of multiple 30-foot (9.1-meter) drill pipes screwed together, called a drill string. They're much like tent poles in this respect. A turntable on the platform rotates the drill string and, at the other end, a drill bit grinds through the Earth. The drill bit generally consists of either a rotating bit embedded with industrial diamonds or a trio of rotating, interlocking bits with steel teeth. In the weeks or months it takes to reach the oil deposit, the bit may dull and require replacement. Between the platform and the ocean floor, all of this equipment descends through a flexible tube called a marine riser.

As the boring hole descends deeper into the ground, operators send a constant flow of drilling mud down to the drill bit, which then flows back up to the platform. This thick, viscous fluid consists of clay, water, barite and a mixture of special chemicals. The drilling mud lubricates the drill bit, seals the wall of the well and controls pressure inside the well. Also, as the drill bit shreds rock, the resulting fragments become suspended in the mud and leave the well in the rising, return flow. On the surface, a circulation system filters the mud before sending it back down the well.

The drilling mud acts as the first line of defense against high, subterranean pressures, but there's still a high risk of a blowout of fluid from the well. To handle these events, petroleum companies install a blowout prevention system (BOP) on the seafloor. If pressurized oil and gas gush up the well, the BOP will seal the well with hydraulic valves and rams. It will then reroute the surging well fluids into specially designed containment systems.

The drilling process itself occurs in phases. The initial surface hole, with a diameter of about 18 inches (46 centimeters) descends from several hundred to several thousand feet. At this point, engineers remove the drill string and send down hollow segments of metal pipe called casing. Once cemented into place, this conductor pipe barrier lines the hole and prevents leaks and caving. For the next phase, a 12-inch (30-centimeter) drill bit digs the well even deeper. Then, the drill string is again removed so surface casing can be installed. Finally, an 8-inch (20-centimeter) bit bores the rest of the way to the petroleum deposit. This final stretch is called the bottom hole, and is lined with intermediate casing. Throughout this process, a device called a packer travels down the well, expanding against the walls to ensure everything is sealed.
 Striking Oil
 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. 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.
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.

Mobile Drilling Platforms
 
 During the exploratory drilling phase, the goals are simple: Get in, find out if there's oil and then move on to the next site. If a location proves prosperous, then the company can bring in a more permanent structure. But for the months it takes a crew to size up a location, a mobile drilling platform provides everything a team needs with minimal investment. Jack-ups, the most common rigs, typically cost between $180 million and $190 million to build [source: Offshore Magazine]. There are five varieties of mobile drilling platforms.

Drilling barge: Mostly used for shallow drilling in non-ocean waters, thise platform is exactly what it sounds like: a floating barge with drilling equipment. Tugboats tow the platform out to the site, where anchors hold it in place. However, given that drilling barges basically just float on the surface, they're only suitable for calm waters.

Jack-up: This rig resembles a drilling barge, but with one exception. Once this platform reaches the drilling site, it can lower three or four massive legs into the water until they touch the bottom. At this point, they lift the platform out of the water. This provides a much more stable environment from which to drill, as the legs stabilize the platform against winds and lift it above pitching waves. The design has its limits, however, as deeper waters require impractically large legs.


Submersible rig: This drilling platform combines some of the properties of drilling barges and jack-ups. Only in this case, the production facilities are elevated on stilts hundreds of feet above pontoonlike barges. After reaching the drill site, the crew floods the barges with water. The barges sink until they rest on the sea or lake floor, while the platform remains elevated above the water on stilts. In effect, the crew sinks the rig into order to anchor it. When the time comes for the rig to relocate, the crew pumps the water back out of the barges, causing them to float back up to the surface and push the stilted platform back up into the air. Like the jack-up, this platform is limited to shallow waters.


Semisubmersible rig: This platform is much like a submersible rig, except it's designed to work in much deeper waters. Instead of sinking until its lower hull rests on the seafloor (which, in deeper waters, would drown everyone), it simply lets enough water in to lower it to appropriate operating heights. The weight of the lower hull simply stabilizes the drilling platform, while massive anchors hold it in place.


Drill ships: This is essentially an oceangoing vessel with, you guessed it, a drilling platform in the middle. The drill string extends down to the ocean floor through a moon hole. Drill ships operate in very deep water and often have to ride out rough sea conditions. They use dynamic positioning equipment to keep aligned with the drill site. This equipment uses satellite information and sensors on the subsea drilling template to keep track of the drilling location. Using this data, electric motors on the underside of the hull constantly move the ship to keep it lined up with the well.

When it's time for these temporary platforms to move on, the really big rigs enter the picture. On the next page, we'll take a look at the different types of offshore production platforms.