Deepwater Drilling in High Current Environment

Abstract

Abstract Early this year, Exxon moved the dynamically positioned SEDCO 472 to a location about 270 miles north of the Amazon Delta off Brazil. Th1s site, in about 1,400 feet of water, is in an intense and persistent part of the Guiana current. The current flows in a northwesterly direct ion and has been measured in excess of four knots at the surface and over one knot at the bottom. Such intense current poses major challenges for an offshore drilling operation. Simply holding a dynamically-positioned rig on station and headed into the current required careful review of stat ion keeping procedures and hardware. Beyond that, equipment lowered into the water was subject to high drag forces and potentially violent vortex-induced vibrations necessitating modification or redesign. Exxon, in conjunction with Sedco, took extensive measures to prepare for this operation. The ship's dynamic-positioning system was modified to increase its station keeping efficiency in high currents and to enhance its heading stabi1ity. Equipment was developed and tested to cope with the extreme drag forces which act on the casing and riser as they are deployed. The vortex-induced vibrations were suppressed by helical strakes on the 20-inch casing and by self-aligning fairings on the riser. With a unique elastic suspension system, the subsea television camera was isolated from vibration of the drill pipe and casing as the TV was run to the bottom. All wire lines exposed to the current were protected with either haired fairing or snap-on fairing: An instrumentation system measured, any riser or casing vibrations and calculated an estimate of the associated fatigue. Design verification for each new piece of equipment required simulation of the environmental effects by analysis and test. This was Exxon's second attempt to drill in this deepwater, high current area off Brazil. A previous attempt, in the summer of 1980, had to be aborted when severe vortex-induced vibrations caused failure of the 20-inch casing while it was being run. The paper will include a description of current measurements, the earlier experience with the DISCOVERER 511, and development of the new equipment and procedures which were used on the SEDCO 472. Introduction Early this year, Exxon moved the dynamically positioned drill ship, SEDCO 472, to Brazil to drill off the north coast. Throughout the previous year, personnel from Exxon Production Research, Esso Exploration and Sedco made major modifications to the SEDCO 472 in preparation for this operation. These measures were nesses Hated by the deep water and severe current present at the drilling site. The water depth in the is area is 1,200 to 1,400 feet. Extensive current measurements indicated that currents are persistent in the northwesterly direction and at times exceed four knots at the surface and one knot at the seafloor. These high currents create high drag forces on the drill ship, and anything lowered into the water. In addition, drill pipe, casing and riser may be subject to severe vibrations caused by vortex shedding. This paper describes what Exxon and Sedco did to cope with these two aspects of the high current problem - the high drag forces and the vortex induced vibrations.