ETA Engineers, Inc.
A riser can be simply described as a conduit from the platform to the ocean floor which transmits the drilling mud and serves as a guide for the drill string. There are two broad classes of risers, those used for exploratory drilling operations and those used for production operations.
In exploratory drilling, the riser tube running from the vessel down to the wellhead is normally called a riser while the fust casing joint running from the wellhead slightly above the mud-line down through the template to 30-100 feet below the mud-line is called a conductor or surface pipe. In production operations, the riser tube running from the platform deck down to the wellhead is also sometimes called a conductor.
In exploratory drilling operations conducted with a jack-up rig, the riser runs from the drilling deck down to the wellhead (Figure 2-10). The drilling fluid is pumped down the drill string. The mud returns in the annulus between the string and the riser in the water or between the string and the casing for that part of the drill string below the subsea wellhead. Ordinarily, when oil is struck in exploratory drilling the well is tested and capped and the rig moves on to another drilling site. In most areas the regulations require that all equipment be removed from the ocean floor. The riser string can then be reused.
For floating drilling operations using a drillship or semisub-mersible, more hardware is required to accommodate the vessel's heave, sway, and surge (Figure 2-10). Attention in this article is directed toward floating drilling operations since the riser-related problems are more complex.
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