Floating Drilling: The First 30 Years-Part 1
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.

Introduction

Two basic parameters make drilling from a floating vessel different from drilling on land. These parameters are motion and distance. The motion comes parameters are motion and distance. The motion comes from continuous movement of the ocean's surface. which affects the drilling vessel floating on it. The distance is the depth of the water between the drilling vessel and the ocean floor.

Vessel motion can be resolved into six components. As shown on Fig. 1, these are the linear displacements of heave, up and down motion alone, the verticalaxis of the vessel: surge, back and forth motion along the longitudinal axis; and sway, sideways motion along the transverse axis; and the angular displacements of roll about the longitudinal axis, pitch about the transverse axis, and about the vertical axis. These six motion components can, and generally do, occur simultaneously. This creates a whole series of problems not encountered in land drilling. Accommodations problems not encountered in land drilling. Accommodations had to be nude for these motion before floating drilling could become the reality it is today.

The distance between the drilling vessel and the ocean floor (or, more simply, the water depth) created another set of operational and equipment problems that have been solved to the degree that floating drilling has been accomplished successfully in water depths just short of 5,000 ft. Plans are now in the hardware stage for drilling in waters as deep as 7,000 ft, and the problems associated with drilling in waters in excess problems associated with drilling in waters in excess of 10,000 ft are being addressed. There is little doubt that when the need to drill in these greater water depths arises, the capability will be there.

This paper describes some of the floating drilling problems associated with motion and distance and how problems associated with motion and distance and how they were solved. Part 1 discusses basic steps in floating drilling and the drilling systems and vessels that have been developed to accommodate motion. Part 2 (to appear next month) discusses drilling and Part 2 (to appear next month) discusses drilling and station-keeping systems developed to accommodate increasing water depths.

Drilling From a Floating Vessel

Before going into the problems resulting from motion and water depth, it might be best to first describe how a well is drilled from a floating vessel. The sequence of events in this description is not necessarily that followed for every well drilled from a floating vessel, but it is a method that has workedin the past and will continue to be used in the future.

Before the start of drilling, the usual activities have to take place. Seismic lines are run and interpreted, a well site is picked, the drilling vessel is selected, a contract is written and executed, and the vessel is mobilized to the well site and surveyed in on location. Drilling is now ready to begin. Refer to Fig. 2 for the following sequence of events.

Step 1 in drilling from a floating vessel is to lower the temporary guide base to the ocean floor. The temporary guide base is generally 12 × 12 ft and is outfitted with a bullseye, observed by TV for levelness determination. This base is run on drillpipe detachably connected to it with a J-tool or hydraulic connector. Four wire-rope guidelines are attached to the sub-base before it is lowered. The base may be loaded with weighted rotary mud so that necessary tension can be pulled in the guidelines when drilling equipment is pulled in the guidelines when drilling equipment is lowered to the ocean floor. With the base on bottom, the drillpipe running string is marked at the Kelly bushing, averaging out the vessel heave, so that the water depth may be determined by measuring the pipe when it is pulled. This measurement, corrected for tide, is the water depth from the kelly bushing to the ocean floor used in all subsequent drilling, logging, casing, and testing operations.

Step 2 consists of running the drilling assembly, made up of the bit, drillcollars, bumper subs, and drillpipe, down the guidelines, through the temporary guide base, into the ocean floor to drill the hole for the structural casing. This hole must be drilled carefully to ensure that it is kept within 1 of vertical. because it later will control how vertical the blowout preventer(BOP) stack will be when it is landed on the wellhead.

JPT

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