The Effect of Stress Relief Features on HWDP and Drill Collars: Are SRF's Necessary, and When Are They Most Beneficial?

Abstract The benefit of stress relief features (SRF's) on heavy weight drill pipe (HWDP) has often been questioned. Much more may be at stake than the cost of adding stress relief features and the impact cutting stress relief grooves has on connection refacing and recuts. The fatigue benefits provided by SRF's may prevent a costly failure and will provide improved service life. The fatigue performance of HWDP and drill collars (DC's) cut with and without pin stress relief grooves and API boreback boxes was evaluated using Finite Element Analysis (FEA) and a strain life prediction model. Both 3 1/2 inch (NC38) and 6 5/8 inch (6 5/8 FH) HWDP were modeled in various hole curvatures and while buckled in straight hole sections of various hole diameters. Similarly, 4 3/4 inch (NC38) and 8 inch (6 5/8 REG) drill collars were evaluated. Although typical ‘small’ and ‘large’ HWDP and drill collar sizes were analyzed, the trends and results observed are thought to apply across the size spectrum. The results of the analyses show that stress relief features improve fatigue life significantly and should be included on all HWDP and drill collar components with NC38 and larger connection types. Additionally, heavy weight drill pipe is proven more fatigue resistant than comparably sized drill collar alternatives because most of the bending in HWDP occurs in the tube instead of the connection. Conversely, in drill collars significant bending occurs in the highly stressed connection. When possible and practicle, replacing drill collars with heavy weight drill pipe (eliminating tapered drill collar strings) is recommended. The fatigue benefit of stress relief features on HWDP and drill collars decreases as dogleg severity increases or as the hole size in which the components are buckled increases. Nevertheless, the stress relief features still provide a significant improvement in fatigue life (relative to HWDP and drill collars without stress relief features in the same hole) while drilling across severe doglegs or when buckled in holes with large radial clearances. Introduction The question as to whether or not SRF's should be machined on HWDP has long been debated. Rental companies and drilling contractors often shy away from cutting SRF's on HWDP because of the obvious commercial benefits associated with being able to recut the connection more times. In addition, in applications where most of the weight on bit (WOB) is applied using drill collars, fatigue failures of drill collar connections have dominated HWDP fatigue failures. Typically, HWDP has been observed to be more forgiving than drill collars from a fatigue standpoint. This has led to the notion that SRF's might not be necessary for HWDP. The problem is further complicated because current public specifications (e.g. API) do not address HWDP. The increased use of HWDP for bit weight application and the use of HWDP in larger hole sizes have generated a renewed interest in assessing the fatigue benefits associated with cutting SRF's on HWDP. Questions like "how much better, from a fatigue standpoint, is 6 5/8 inch HWDP compared to 8 inch drill collars" have become critical as more HWDP is being used to replace drill collars for bit weight application. Any improvement in the fatigue performance of connections on HWDP and drill collars provides tremendous benefit in terms of cost savings by increasing the service life and by reducing the number of failure instances. In turn, the drilling contractors and rental companies will also benefit from the increased tool life. In an effort to quantify the benefits of SRF's on HWDP and drill collars and to compare the performance of HWDP and drill collars, an analysis was performed that compares these configurations on a relative basis.