Pipe Buckling At Surface In Underbalanced Drilling

Abstract

Abstract Underbalanced drilling (UBD) with jointed pipe has become routine in the UK sector of the southern North Sea and operators in the Norwegian sector are now introducing the same concept. Typically, the well is kept live during tripping and a rig-assist snubbing unit is employed to handle the drillpipe while pipe-light. Significant surface compressive loading is exerted on the pipe during the snubbing phase. The focus of this paper is two-fold. First we consider buckling of the drillpipe in the snubbing jack and on the drill floor when a static compressive load is applied. Thereafter, we analyze the behavior of ejected tubulars during well entry and exit under live well conditions. Introduction There are three tripping options available in UBD. If reservoir damage is not an issue, the well may be killed before pulling out of the hole and the drilling rig is used for tripping as in overbalanced drilling. A second option is to use a downhole deployment valve installed inside the drilling casing. With the drillpipe pulled above this valve, the device is closed and the pipe is tripped with the drilling rig in a conventional manner1. The third option of rig-assist snubbing is currently the preferred method in the North Sea2. Here the well is kept live at all times and the snubbing unit is used while tripping pipe-light. Refer to Appendix A for a review of terms that are relevant to rig-assist snubbing. Maximum compressive surface load on the drillstring is experienced when snubbing in the bottomhole assembly (BHA) and the first few joints of pipe. During normal operation of a rig-assist snubbing unit, the drillpipe may buckle in long unsupported intervals inside the jack unit itself. The greatest unsupported interval is between the snub slips on the traveling head and the active stripper blowout preventer (BOP). Typically, a telescoping tubing guide (buckling guide) is used to control the lateral deflection of the pipe should buckling occur. This paper presents useful equations for unsupported buckling and buckling inside the tubing guide. The theory is compared with the results from recent full-scale buckling tests. Also included in the paper is an analysis of the dynamic forces involved should the drillstring suddenly eject from the well. This situation may occur as a result of an operator error or equipment malfunction. Unsupported buckling In buckling we are concerned with the stability of the pipe. We need to ensure that the drillpipe is able to support the given compressive load without experiencing a sudden change in its configuration. Often the magnitude of compression associated with buckling is far below the yield load of the pipe. When the pipe is subjected to a compressive load in excess of its critical buckling load Fcr, the tubular suddenly becomes sharply curved instead of remaining straight.