On April 20, 2010 a methane bubble escaped into a drilling pipe, got past the malfunctioning blowout preventers and shot up the drill pipe to the Deepwater Horizon, causing an series of explosions that killed 11 workers and toppled the rig over 5,000 feet to the seafloor below. The effect has been a catastrophic crude oil leak into the Gulf of Mexico, from three different places on the subsea formation. One of them is a riser.
The riser, also known as a drilling riser, riser pipe or marine riser, is a pipe rising from the gulf floor over a well. When used, the riser connects the well with the rig enabling crude oil to be pumped to the surface. The riser could be considered a temporary extension of the well rising to the surface. Sometimes the fittings as well as all the sections of pipe that ultimately connect the well to the rig are lumped together into the term “riser”. The riser pipe often serves as a guide for the drill stem from the rig to the wellhead and as a conductor of drilling fluid from the well to the rig.
Two Kinds of Risers
Marine Riser: a marine drilling riser is used with underwater blowout preventers (BOP) which are large valves used to close the well in an emergency, and is generally used by floating drilling rigs. This kind of riser is comprised of a very large tube that functions under low pressure. It has auxiliary lines that circulate fluids to the BOPs located near the seafloor as well as other auxiliary lines that contain the power and controls to the BOPs.
Marine risers used in depths greater than 75 to 100 feet require a marine riser tensioner for stability. The tensioner is located on the drilling platform above water and keeps constant tension on the riser to offset undersea currents and waves. The amount of tension needed depends on the weight of the riser equipment, the buoyancy of the riser, the forces from waves and currents, the weight of the internal fluids, and also has to have an allowance for equipment failure, requiring a complex amount of engineering and analysis.
Tie-back Riser: a tie-back drilling riser is used with a surface BOP and generally is used with fixed platforms. This kind of riser has either one very large pipe functioning under high pressure or several concentric pipes extending to the surface BOPs.
The use of, and movement around, a riser is an intricate process that requires alot of engineering and analysis. Besides the complexity of using a marine riser tensioner, many drilling platforms are not stationary and can be moved around on an oilfield from one well to another. With risers reaching up from the seafloor waiting to be connected to drilling platforms, more complex engineering and analysis must be performed to insure the rig does not run into and damage a riser.
Thus far in the 2010 Gulf of Mexico oil spill the leak has not been contained though some sources believe that the riser which is leaking is bent and crimped from the force of the rig falling on it, stemming some of the flow of oil. However, as sand and oil continue to leak through the pipes, the sand will erode the riser and eventually the leak will become even larger. Though an integral part of undersea oil drilling, the riser has the potential to cause catastrophic disasters if it is damaged severely.