Heavy wall drill pipe has found increasing use for weight on the bit in directional holes and smaller vertical holes and as a transition member to reduce damage in the drill pipe run near the drill collars.

In these severe applications, proper use, inspection and maintenance is essential. Heavy wall drill pipe should be run in a property tapered drill string above the correct size drill collars. Proper breaker in procedures and make up torque should be used. Periodic inspection and maintenance, using the correct procedures, is essential. Hardbanding will help prevent wear on the tool joints and center upset and should be repaired using the correct procedures when it wears down. Finally, accurate records should be kept of heavy wall drill pipe service and maintenance.

Application of these recommendations will greatly extend the useful service life of heavy wall drill pipe and reduce overall drilling costs.

Heavy wall drill pipe has found increasing use as a key member in the drill string. In directional holes and smaller vertical holes, heavy wall drill pipe can supply much of the desired weight on bit. Heavy wall drill pipe has also found successful use as a transition member, designed to reduce damage in the drill pipe run near the drill collars. By the nature of its use, either as weight for the bit or as a transition pipe, heavy wall drill pipe receives some of the most severe services of any drill stem member. As will occur with any drill stem member, severe service can adversely affect its useful life. But its usefulness can be greatly extended by proper use, inspection, and maintenance.

How Heavy Wall Drill Pipe is Used

When heavy wall drill pipe is used for weight in directional holes, the typical assembly consists of four to six stabilized drill collars, 30 to 70 joints of heavy wall drill pipe, followed by conventional drill pipe to the surface. Usually in a directional well, the slope of the hole and the weight of the thick walled pipe causes it to lie on the low side of the hole which prevents it from buckling when run in compression.

When used for weight in vertical holes, the heavy wall tube loses the support afforded by the slope and hole curvature. In large diameter vertical holes, buckling and fatigue can occur if excessive weights andhigh RPM's are employed. Field experience has shown that the effect of weight and rotary speed is negligible when the hole size is no larger than the tool joint outside diameter plus four inches Figure 1.

For example 4½″ heavy wall drill pipe with a 6¼″ OD tool joint should not be used for weight in vertical holes larger than 10¼″ (6¼″ OD + 4″ = 10¼″)∙ To do so invites possible fatigue damage to the thick wall tube.

Heavy wall drill pipe can also be run as a transition member to reduce drill pipe damage. When used in this manner, the heavy wall drill pipe is run in tension and replaces the standard drill pipe in the critical cross-over area from drill collars to drill pipe. Experience indicates that 15 to 21 joints of heavy wall drill pipe will greatly reduce fatigue failures in this transition zone.

Improving the Design of the Drill Stem

Tapering the drill string to avoid sudden changes in stiffness between various sections can increase the fatigue life of all members. Use of the theory of bending strength ratios (Section Modulus or 1/C, where I is the Moment of Inertia and C is the external radius of the tube) between the various sections of the drill collars and drill pipe, has helped to extend the fatigue life of all members. Field experience has shown that, by setting a limit of 5.5 ratio, the failure rate in the crossover areas can be greatly reduced. In larger hole sizes and in severe drilling conditions, a further reduction of this ratio to 3.5 is often necessary to adequately reduce the frequency of failures.

Figure 2 is a graphic representation of data to provide a quick reference for sizing the top stand of drill collars to run below various sizes of heavy wall drill pipe. When larger drill collars are required, at least one stand (three drill collars) of the "suggested upper limit size" should be run between the heavy wall drill pipe and the larger drill collars.

Tapered drill collar strings are being used with greater frequency in hole sizes of 9- and larger, because the larger diameter collars are required on bottom for efficient drilling. Any idealized design using bending strength ratios would require too many different sizes for practical application.

When a tapered drill collar string is used, collars are usually limited to two or three sizes because of the time required to change handling tools for each size. Expereience indicates that reduction in size should never exceed 2″ in diameter between sections, and that at least one stand (three joints) of the smaller diameter drill collar should be used at each crossover point. These limitations are especially critical when using drill collars 9″ in diameter or larger.

Use of stress relief features on the box and pin connections, as well as cold working the thread roots, will reduce working stress levels and increase the life of the connections.

Tips to Follow at the Rig

Increasing service life also includes correct make up procedures for the connections on heavy wall drill pipe. The same break in procedures as used on drill collars are recommended for maximum connection life. For the initial make up connections should be thoroughly cleaned, dried, and well lubricated with a proper drill collar thread compound. It is good rig practice to slowly walk in the connection using chain tongs and then make it up to the recommended torque with the rig tongs. The connection should then be broken out, inspected for minor damage, relubricated, and make up again to the proper torque.

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