Shallow Water Drilling With Adeep Water Drillship

Abstract

ABSTRACT A working procedure has been developed and successfully used to enable a deepwater drillship to drill in shallow water. The paper describes how by a combination of recessing the wellhead and B.O.P. below the sea floor, in a sea-floor cellar, and by shortening the slip joint the deep water drillship CHANCELLORSVILLE was used to drill two wells in the Gulf Coast, in approx. 55 ft. of water. Experience gained during the first well was used to develop a more efficient procedure which was adopted for the second well. INTRODUCTION Until now, offshore drilling in shallow water depths has been greatly dependent upon availability of bottom supported drilling units, principally jack-ups or submersibles. The concept that a drillship or drill barge employing innovative techniques, could be used economically in place of a jack-up in shallow water was not readily accepted until recently when Samedan Oil Corp. initiated its program to drill in shallow waters. With the demand for bottom-supported rigs greater than the available supply, AOI proposed to use its drillship CHANCELLORSVILLE with a seafloor cellar as portrayed in Figure 1. The typical minimum water depth for a drillship has been generally considered to be about 100 ft. This limitation is based upon factors requiring adequate spacing between drill floor and sea floor to physically accommodate the wellhead. BOP, ball joint, and slip joint. Sufficient length of the riser system from ball joint to drill floor is necessary to allow movement (offset) of the vessel in the horizontal plane resulting from the combined actions of wind, waves, and current. Typical acceptable offset has been about 5% of water depth, or an offset which would limit the ball joint angle to about 5°. Ball joints are capable of angles up to 10°. However, other considerations restrict allowable to a significantly lesser angle. Passage of drilling tools through the ball joint, and wear at the ball joint as drill pipe is rotated must be considered. Sufficient vertical stroke of the slip joint should be present to enable the vessel to heave under the action of waves, and also for tidal changes. Sufficient clearance between the vessel's hull and the BOP fixed to the sea floor should preclude contact when the vessel exhibits motions both vertically and horizontally from wave action. As shown in Fig. 2, placement of the BOP and slip joint for CHANCELLORSVILLE in the normally designed manner would require a minimum water depth of about 99 ft. Modification Needed To work in water depths less than 88 ft. required certain modifications to the normal placement of equipment. It would be possible to accept reduced water depth by merely setting the slip joint for reduced but acceptable downstroke. For example, 10 ft. downstroke would enable operation in water depths of about 76 ft.