Rig-Assisted Snubbing: Drillpipe Pressure Control in MPD/UBD

Abstract

An important pressure control issue during live well work is the expulsion force acting on the drillpipe. Wellhead pressure multiplied by drillpipe sealing area in the blowout preventor must be overcome before entering the well. The highest snubbing force (compression) is applied when running the first joint into the well since pipe weight is at its lowest at this time. Focus of this paper is the mechanical analysis of the drillpipe as a well barrier element during live well entry. We look at normal operation (primary well barrier) and contingency (secondary well barrier). Load cases include critical unsupported buckling, helical buckling inside tubing guide, collapse and burst. In critical unsupported buckling there is no radial confinement of the pipe and the critical buckling limit is determined at the onset of lateral deflection. On the other hand, inside the tubing guide the pipe is allowed to buckle into a helix and the buckling limit is related to permanent corkscrewing of the pipe. Technical contributions in this paper include engineering design formulas for unsupported buckling and helical buckling. Also presented are experimental buckling results from pipe up to 3½ inch (8.9 cm) diameter. Design calculations for primary and secondary well barriers are explained and analyzed using a field case. All load calculations are based on zero neutral axial stress as stress reference datum, which produces a yield circle that is conveniently deployed for three-dimensional well tubular design. The use of the dimensionless yield circle is found to be an efficient method to assess helical buckling loading and the combined effects of pressure and axial stresses.