How Offshore Drilling Works

Undersea Drilling

An enormous offshore platform lights up the night off the coast of Norway.
An enormous offshore platform lights up the night off the coast of Norway.
Arnulf Husmo/Stone/Getty Images

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.

In the next section, we'll follow the well down to the petroleum itself.