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Abstract

Higher fracture toughness materials are more resistant to fracture than lower toughness materials under the same loading conditions. This fact is particularly important when considering potential drill pipe failures because there exists no industry standard which specifies fracture toughness for drill pipe. How fracture tough should your pipe be? This question is discussed through a comparison of recent fracture toughness criteria for high strength drill pipe. A Chevron Corp. criterion based on field observations has suggested 40 ft-lbf (54 J) at room temperature from 3/4 size Charpy impacts. Shell Canada Limited has developed fracture toughness criteria based on a fracture mechanics, leak before break concept for a pipe body containing fatigue cracking and on field observations of parted pipe and washouts in pipe. This has allowed the application of a room temperature, 80 J (59 ft-lbf), 3/4 size Charpy criterion in Shell Canada drilling contracts. A single, impact-type criterion has ease of application but flexible, fracture mechanics criteria may allow for better operational use of drill pipe in specific situations. An empirical relationship between fracture and impact properties was determined which allows the use of required toughness curves and leak before break curves based on operating conditions and drill pipe impact properties. Examples are given with respect to pipe impact properties. Examples are given with respect to tested properties of new, high quality drill pipe and used, failed drill pipe. These show it is possible to obtain drill pipe that will washout before parting but a criterion of 54 J pipe that will washout before parting but a criterion of 54 J will not guarantee it.

Drill pipe tube body failures make up a significant portion of downhole failures. A relatively common event is portion of downhole failures. A relatively common event is the drill pipe washout and a less frequent but often very expensive event is the catastrophic drill pipe twistoff. Both types of failures have been directly linked to fatigue cracking resulting from mean and alternating bending loads applied to the drill pipe. As an example, Figure 1 shows a portion of a drill pipe body with a fatigue crack that had portion of a drill pipe body with a fatigue crack that had just begun to washout and Figure 2 shows a full washout. In the case of the washout, the fatigue crack propagates through the wall of the pipe and allows pressurized drilling fluid to exit (leak), thereby allowing detection of the crack by mud pump pressure drop. In the case of the catastrophic pipe separation, the fatigue crack propagates until the pipe pipe separation, the fatigue crack propagates until the pipe can not sustain the applied loads and the pipe fails rapidly (break). For brittle fracture, the fatigue crack usually does not penetrate the pipe wall before failure occurs. Figure 3 shows such a catastrophic failure. This is the brittle fracture surface of a parted drill pipe failure which caused one million dollars of fishing costs for Shell Canada Limited (SCL).

Why do some drill pipes leak while others break under similar operating conditions? The answer lies in the behaviour of the drill pipe materials, specifically their fracture toughness. For an existing crack, a tougher material can sustain a higher load without catastrophic failure or, in other words, under a specific loading condition the tougher material can sustain a larger crack without parting. Unfortunately, no industry standard exists which specifies a fracture toughness requirement for drill pipe. pipe. In recent years, there has been increased interest within the oil and gas industry to produce minimum toughness criteria for API standard drill pipe. The most recent of these proposals comes from Charles Buscemi et al. of Chevron Corp. A minimum Charpy impact toughness requirement of 40 ft-lbf (54 J) in American Petroleum Institute (API) Specification 5D was suggested. Petroleum Institute (API) Specification 5D was suggested. P. 441