An oil riser (more properly a drilling riser) is part of the piping system of an offshore well. It is a large-diameter pipe that connects the drill rig on the surface to a complex on the seafloor that includes the blowout preventer (BOP) stack. Risers are typical of wells drilled from floating drillships such as the Transocean Deepwater Horizon.
Drilling an oil well is much different from drilling a hole in a board, and not simply in terms of size. A drilling rig resembles the cordless drill on your workbench only in that it creates a hole by rotating something sharp under pressure. Drilling a narrow (usually six- to twelve-inch) hole several thousand feet long requires more than just a power source and a way to clamp onto the bit.
Rotary drilling rigs operate by rotating a hardened steel drill bit on the end of a string of pipes that reaches all the way from the surface to the bottom of the hole. A major difference from hand drills is that the borehole must be kept full of fluids while the well is being drilled. This fluid is called drilling mud, and it serves several purposes. As the bit chews its way down, it creates small chips of rock. Drilling mud is pumped into the bottom of the well, under high pressure, through holes in the drill bit. This forces the mud to circulate, or rise to the top of the hole, carrying the cuttings with it. Additives in the drilling mud lubricate the drillstring to reduce friction, and the heavy fluids serve the critical function of maintaining pressure in the borehole to prevent oil and gas from forcing their way to the surface under extreme pressure.
As a well is drilled with this system, piping is necessary to contain the drilling mud in that portion of the well that is between the seafloor and the surface. The riser serves this function. The drillstring (drill pipe and bit) runs inside this pipe, through the seafloor stack that includes the blowout preventers, and into the casing that holds the well open. Mud circulates in the space between the walls of the pipe and the drillstring. Without the riser, mud circulation would not be possible.
Once a well has been completed and a production facility is established, the riser has served its purpose and is removed for recycling or use elsewhere. In that sense, a riser can be thought of as a temporary section of wellbore that connects the subsea stack to the surface. Risers are ultimately replaced by production tubing.