Abstract

Current models of slip crushing treat the rotary slip as an axisymmetric wedge that generates an axisymmetric radial load on the drillpipe lateral surface. However, recent tests on strain gauged drillpipe specimens suggest that this model does not adequately capture the mechanical response of the drillpipe. The tests show that drillpipe response is a complex and non-axisymmetric function of slip geometry, friction between the slip and the bowl in the rotary table, and the mechanics of load transfer between the slip and drillpipe.

This paper presents a new model of slip behavior that represents the slip system as a series of line loads. Analysis of the suspended tubular under these forces and axial tension leads to a limit load that characterizes slip crushing. A corollary of this analysis is the minimum slip length required to support a given axial tension.

Background

The failure of drillpipe in the region of contact between the drillpipe and slips was first addressed by Reinhold and Spiri in 19591. This paper recognized that drillpipe is subjected to bi-axial loading in the slip contact area. By treating the slip as an immovable wedge between a rigid bowl and the hanging drillpipe, a relation between the axial force on the drillpipe and the transverse force exerted on it by the slips was derived. The ratio of the transverse force to the axial force known as the "K-factor" is given by (1)

The angle of taper has been standardized by the API to 90, 27′, 45″.

The average radial pressure on the drill pipe outer diameter and the axial stress in the pipe beneath the slip toe are estimated. This "radial pressure" is used to estimate the tangential stress at the drillpipe inner diameter. Knowing the tangential and axial stresses on the inner diameter of the drillpipe (where the radial stress vanishes), the von Mises equivalent (VME) stress criterion is used to estimate the axial load at which the drillpipe begins to yield. The Reinhold-Spiri formula for the slip crushing load is,(2)

The predictions of Eq. (2) were compared with a limited series of meticulous tests on 5 in., 19.5 ppf, Grade E drill pipe loaded in standard and extended length manual slips2.

Until recently, Eq. (2) has been used to calculate slip crushing loads for drillpipe and casing hung in rotary slips, and sometimes to calculate the loads exerted by packers on tubing. Eq. (2) is based on a statically determinate analysis of a conical wedge, a consequence of which is that the peak stress is always on the drillpipe inner diameter in the vicinity of the slip toe. An unstated consequence of the assumptions leading to Eq. (2) is that the drillpipe stress distribution is axisymmetric. This theory thus implies that the peak stress region is a circle on the drillpipe inner diameter in the plane of the slip toe.

Following the work by Reinhold and Spiri1 and Vreeland2. in the late 1950s, slip crushing analysis received little attention until 1985. A 1985 paper by Hayatdavoudi discusses the principles of slip insert design to reduce probability of yielding in the pipe3. The work includes representation of strain gage data from slip crush tests on 9–5/8 in., 53.5 ppf casing. Though the paper does not contain the details of testing, the data presented in this paper indicates that the deformation of the casing inner diameter above the toe of the slip is larger than the deformation near the top of the slip.

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