A Feasibility Study on the Use of Subsea Chokes in Well Control Operations on Floating Drilling Vessels

Abstract

Summary The use of a subsea remote-operated adjustable choke between the blowout preventer (BOP) stack and the marine riser was considered as a means for eliminating the need for long subsea flowlines. Well behavior was predicted through computer simulations and showed/that existing problems could be reduced but not eliminated entirely. Introduction During the past 20 years, offshore drilling in water depths up to 350 ft (107 m) has become routine. Deepwater exploratory drilling from floating vessels in water depths as great as 5,000 ft (1524 m) has been achieved in the past few years. Currently drilling vessels are being constructed that are capable of drilling water depths of more than 6,000 ft (1829 m). As drilling extends to such great water depths, the blowout control problem becomes much more difficult. Well control equipment and procedures were developed first for drilling operations on land. With only minor changes, these well control techniques have been applied to bottom-supported drilling rigs such as submersibles, jackups, and rigs operating on offshore platforms. More extensive changes in BOP equipment and procedures were required for floating vessels, which are used almost exclusively for deepwater operations in water depths of more than 350 ft (107 m). The most significant change was the location of the BOP stack at the seafloor rather than at the surface. This paper considers the need for development of additional subsea well control equipment as the search for petroleum is extended to greater water depths. The special well control problems for floating drilling vessels stem primarily from the reduced fracture resistance of the marine sediments and the need for long vertical subsea chokelines between the subsea BOP stack and the drilling vessel. The shallow marine sediments often are unconsolidated and undercompacted, having a relatively low bulk density up to several thousand feet below the mudline. The fracture problem is aggravated because the mud column extends far above the mudline to the flowline above sea level, and mud density generally exceeds seawater density. Thus, when a kick occurs during deepwater drilling operations, formation fracture usually will occur at a lower equivalent mud density than for a similar situation on land. For the example shown in Fig. 1, the fracture gradient computed at a casing seat 3,000 ft (914 m) below the mud line in 3,000 ft (914 m) of water is only 10.6 lbm/gal (1.27 kg/m3). The use of long vertical subsea chokelines in deep water between the subsea BOP stack and the drilling vessel has two disadvantages. The increased chokeline length can result in unacceptably high circulating frictional pressure losses in the chokelines. This can be extremely important because current well control procedures are based on the assumption that frictional pressure losses held against the well annulus are small and can be applied as a convenient safety margin when circulation of the kick is begun. In addition, since the subsea chokelines extend vertically over a considerable length, the density of the fluid in the chokelines contributes directly to the hydrostatic pressure in the well. When the top of a slug of low-density kick fluid reaches the subsea BOP stack, the hydrostatic pressure of the drilling fluid in the chokeline is lost quickly during the rapid elongation of the slug as it exits the large casing and proceeds upward through the small-diameter chokeline. This too is extremely important because current well control procedures are based on the assumption that significant well pressure changes will occur more slowly than the unsteady-state readjustment time of the surface drillpipe pressure, on which choke manipulation is based.