A casing head forms part of the wellhead of an oil or gas drilling well. This wellhead represents the portion of the well that is visible above the ground, and the casing head sits just at the base of this device. Casing heads rest just atop the main operating components of a well, including the drivepipe or conductor pipe. They are generally fixed in place, along with the rest of the wellhead. The majority of moving components within the well system are located underground below the casing head.
Each casing head consists of a heavy-duty steel flange. It may be constructed from galvanized steel or special alloys to increase corrosion-resistance in the often extreme conditions found on an oil field. This device is welded or bolted to the drivepipe mechanism below to create a stable and permanent bond. Each of these steel flanges must be designed to accommodate extreme levels of pressure and temperature ranges, which are common in oil and gas drilling.
The casing head on a well is primarily used to house a series of blowout preventors (BOP) units. Many wells include two types of BOP units, including both ram and annular models. Blowout preventers are critical to the safety and performance of the well. These specialty valves are equipped to handle extremely high levels of pressure, and are designed to self-adjust to regulate pressure changes. Each of these valves may be bolted or welded to the casing head.
Casing heads also house a variety of surface equipment needed to control and regulate drilling operations. This may include special monitors or panels that measure pressure, temperature, and drilling depth within the well. It also includes pressure-control equipment, including controllers for the blowout preventers and related devices. By serving as a transition point between subsurface and above-ground well components, the casing head often acts as a link between these two areas to hang equipment, wires, and sensors.
Depending on the application, this casing device may also serve as a base for a specialty assembly of controls and valves used to operate the well. This assembly is often referred to as a Christmas tree because of its resemblance to a decorated holiday tree, with pipes and valves extending at many different angles. The Christmas tree is used to control the flow of oil or gas to a nearby processing plant, and may also be used to shut off the well when production has halted or ceased.