Summary

This paper addresses a problem pertinent to the design of landing strings and drillstrings for deepwater operations. These landing strings are designed to run long and heavy casings, tiebacks, or liners (typically deep intermediate and production tubulars), and the total weight may approach or exceed 1,000 kilo lbf (klbf) at the mudline. Adding the weight of the landing string to the rotary results in a serious design problem regarding both the landing string itself and the handling equipment. Slip-based handling systems work well in most instances but lead to biaxial loading from the tensile and radial loads exerted by slip inserts. As a result of biaxial loads, the axial load rating of the landing string is reduced.

The current understanding of slip-crushing phenomena is based on testing and modeling work dating to 1959 by Reinhold and Spiri1 and Vreeland.2 This paper assembles all available data regarding slip crushing and places it in the perspective of current understanding. A secondary aim is to describe the physics of the slip-drillpipe interaction in some detail (i.e., extend the Reinhold- Spiri analysis1 to the next level).

The analysis presented in this paper shows variations between the historical Reinhold-Spiri model and test results, and it indicates an unconservative margin of ~20%. These trends can be accounted for with a more accurate model of slip-crushing mechanics. The improved model supports a nonlinear stress distribution in the pipe, and its prediction of peak stresses is consistent with general test observations. With better quantification of the friction factors between the slip and the bowl, load uniformity, and equipment tolerancing and fits, this new model and its underlying approach provide an appropriate framework for a more accurate tubular rating for slip-crushing performance. Until such time as these refinements are made and available, drilling personnel should consider and implement appropriate safety factors when dealing with marginal designs for slip-crushing scenarios.

Introduction

"As drilling depths continually increase and hydraulic efficiency demands the use of 4 1/2- or 5-in. outer diameter (OD) drillpipe down to completion depth, the vastly greater hook loads now encountered are bringing attention to drillpipe failures occurring in the slip area." These words form the opening statement of a 1962 paper by Vreeland.2 If the diameters are changed to 5 7/8 in., the preceding statement reflects the situation today.

The seriousness of drillpipe slip crushing because of biaxial loading in the slips is well recognized by operators and drilling contractors. When the margin of overpull is small, the factor by which the tensile capacity of the drillstring is reduced by slip loading must be known. In particular, because of the larger OD drillstring, landing strings, and heavier weights, there is some uncertainty in predicting slip-crushing loads and in designing them with adequate safety factors. This uncertainty leads to the consideration of alternate surface-handling systems, such as elevators and dual-shouldered landing strings. While there is often a legitimate need to adopt alternate surface-handling systems, especially in deep water (as evidenced by the horizons in the Gulf of Mexico), there is a clear need to further understand the physics of slip crushing. The current understanding is largely based on a model developed in 19591 and on tests reported in 1962.2 While it is clear that manufacturers of slips and vendors of such equipment have a clearer understanding of slip-crushing mechanisms, this knowledge is confined to unpublished, internal company reports on tests performed for specific clients.

This paper assembles all available data regarding slip crushing and places it in the perspective of current understanding. A secondary aim is to describe the physics of slip-drillpipe interaction in some detail (i.e., extend the Reinhold-Spiri analysis1 to the next level).

This paper begins with a review of published work on this problem and later describes the results of recent tests made available to the authors by several organizations in the industry. These results are described, interpreted, and discussed in the light of predictions of the Reinhold-Spiri model. Based on this discussion, current test procedures are reviewed. Finally, a more detailed analysis (developed in the Appendix) is used to illustrate the physics of slip-drillpipe interaction. An outcome of this analysis is a modified formula for slip-crushing load, compared with existing data.

Review of Previous Work

The failure of drillpipe in the region of contact between the drillpipe and slips was first addressed by Reinhold and Spiri in 1959.1 The chief contribution of this paper was the recognition that drillpipe is subjected to biaxial loading in the slip contact area. By treating the slip as an immovable wedge between a rigid bowl and the hanging drillpipe, a relationship between the axial force (Fa) on the drillpipe and the transverse force (W) exerted on it by the slips was derived. The ratio of the transverse force to the axial force is known as the "transverse load factor" or simply the "K-factor" (see Theoretical Considerations, Eq. 1b). The average radial pressure on the drillpipe OD and the axial stress in the pipe at the slip toe are estimated. This "radial pressure" is, in turn, used to estimate the tangential stress at the drillpipe inside diameter (ID). Knowing the tangential and axial stresses on the ID of the drillpipe, the von Mises equivalent (VME) stress criterion was used to estimate the axial load at which the drillpipe ID begins to yield. This analysis resulted in the well-known Reinhold-Spiri formula currently used in the drilling industry for the slip-crushing load of drillpipe.

Additionally, significant work on this problem was performed by Vreeland through a set of carefully engineered experiments.2 In his experiments, Vreeland studied the behavior of eight joints of 5-in., 19.5-lb/ft, Grade E drillpipe under slip loads. The test fixture was designed such that the upper part of the test specimen (i.e., the drillpipe joint) was placed in the slips while the lower part was connected to a piston that exerted axial load by means of an end piece. Standard- and extended-length slips, in conjunction with standard API bowls, were used to hold the drillpipe in the upper fixture. Strain gauges were installed inside each specimen at various distances from the bottom of the slip to record hoop strain. In each case, the ID reduction was plotted as a function of the axial load. Before reaching the slip-crushing load, the ID reduction is proportional to the axial load. Because yielding begins on the ID of the drillpipe, the load at which the slope of the axial load vs. the ID-reduction curve changes is a measure of the slip-crushing load. Based on eight tests, the following major conclusions were made.

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