Deepwater drilling trends are moving towards deeper water depths as well as deeper exploration horizons and reservoirs below the mudline. These trends have created various operational and well design challenges. This paper addresses a serious problem pertinent to the design of landing strings and drill strings for deepwater operations. These strings are designed to run long and heavy casings, tiebacks, or liners (typically including deep intermediate and production tubulars). The total weight of these strings may approach or exceed 1,000 klbf at the mudline. Adding in the additional weight of the landing string back 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 bi-axial loading from the tension 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. This paper examines recent tests in the light of more advanced models and presents an improved understanding of slip crushing loads.
"As drilling depths continually increase and hydraulic efficiency demands the use of 4 1/2 or 5 in. OD drill pipe down to completion depth, the vastly greater hook loads now encountered are bringing attention to drill- pipe failures occurring in the slip area. " These words form the opening statement of a 1962 paper by Vreeland2. If the diameters are changed to 5 7/8 in., the preceding statement reflects the situation today.
The seriousness of drill pipe slip crushing due to bi-axial 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 drill string is reduced by slip loading must be known. In particular, due to the larger OD drilling and landing strings, and heavier weights, there is some uncertainty in the prediction of 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 Gulf of Mexico), there is a clear need to further understand the physics of slip crushing. The current understanding of slip crushing is largely based on a model developed in 19591 and tests reported in 19622. While it is clear that manufacturers of slips and vendors of such equipment have a clearer understanding of the slip crushing mechanisms, this knowledge is confined to unpublished internal company reports on tests performed for specific clients.
The purpose of this paper is to assemble all available data regarding slip crushing and place them in the perspective of current understanding. A secondary aim is to describe the physics of the slip-drill pipe interaction in the 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 proceeds to describe 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, currently used test procedures are reviewed. Finally, a more detailed analysis (developed in Appendix A) is used to illustrate the physics of slip-drill pipe interaction. An outcome of this analysis, a modified formula for slip crushing load is compared with existing data.