Floating Drilling: The First 30 Years-Part 2
Authors: Silcox W.H. 
Published: December 2007

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.

Water Depth Accommodators-The Drilling System

For safety and well control, the BOP stack used in floating drilling is mounted on the wellhead at the ocean floor. The distance between the BOP stack and the fig floor presents operational situations not encountered in land drilling. For example, one cannot simply jump down from the rig floor to turn the hand-wheels to shut in a BOP when the stack is some 2,000 ft (600 m) underwater. A number of systems and components have been developed to solve problems related to water depth.

Re-entry Systems. Re-entering a 3-ft (0.9-m)-diameter hole in the ocean floor in shallow waters without too much current, say less than half a knot(0.9 km/h), isn't too difficult. Put that same hole under half a mile of waterin an area with 1- to 2-knot (1.8- to 3.7-km/h) currents and the problem obviously is more difficult.

Almost from the beginning of floating drilling, wire-rope guidelines have been used to guide drillstrings, casing, BOP stacks, and riser pipe into oronto subsea wells. In most instances the guidelines are anchored to the ocean floor by the temporary guide base. In some cases when the hole for the structural pile is spudded without a temporary guide base, the mud pumps we rerun at full capacity as the bit entered the ocean bottom. This washed a largeconical hole in the ocean floor which, with luck, could be re-entered without guidelines. However, under these conditions, when the structural casing and the permanent guide base are run, the guidelines are attached to the permanent guide base for subsequent re-entry operations.

With the advent of dynamically positioned drillships. guidelineless re-entry systems were developed. These systems still used temporary and permanent guide bases; however, instead of using guidelines and guideposts, they we refitted with guide cones that provided a large target for the tools or casing being run. provided a large target for the tools or casing being run. TV cameras run through the drillpipe, casing or riser, and BOP stack (depending on what was being run) provided guidance into the hole or back onto the BOP provided guidance into the hole or back onto the BOP stack. Combinations of TV and sonar also have been used for r-e-entry guidance. With the dynamic positioning system the driller can take control of the drilling positioning system the driller can take control of the drilling vessel from his station and position it as required for re-entry. Re-entry by means of these systems has been made in waters as deep as 4,900 ft (1500 m).

Marine Risers. The first floating drilling systems did not use marine risers for mud returns. Hoses connected below a rotating packer mounted on top of the BOP stack served to bring mud returns back up to the drilling vessel.

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