Advances in Drillpipe Fatigue Management

Abstract

Abstract Recent improvements in manufacturing quality control have dramatically reduced drillpipe failures caused by insufficient material properties. However, preventable fatigue-induced failures continue to occur at a high frequency. Increased drilling activity, rising rig spread rates and the technically challenging nature of today's ultra deep and extended reach wells dictate the need for an effective drill pipe fatigue management system. This paper discusses the cumulative fatigue damage approach as applied to drill pipe and presents a field-implemented fatigue management strategy that can significantly improve the overall fatigue performance of the drill string. Drill pipe fatigue analysis has been approached through both total fatigue life and fracture mechanics models. The fracture mechanics model was used to develop the concept of comparative fatigue design introduced in a previous paper (IADC/SPE 87188). Comparative fatigue design allows for a quantitative comparison of drill string design and well bore geometry alternatives based on normalized fatigue crack propagation and various known well bore parameters like dogleg severity, hanging tension, drill pipe size, hole size, etc. However, this "snapshot" technique only provides an insight into the fatigue performance of the drill pipe under one set of operating conditions. A joint of drill pipe experiences substantial variations in loading conditions at different depths along the well bore trajectory. Hence the comparative fatigue design technique was further refined to consider all significant operating conditions and allow quantitative comparison of cumulative drill pipe fatigue damage. The cumulative fatigue damage technique has been successfully applied as a design tool to compare different drill string configurations, well bore trajectories and drilling operations. This technique can also be applied in calculating relative fatigue damage within different well bore sections and the entire well bore, thus identifying sections of the drill string that will accumulate maximum fatigue damage after a series of operations. Sections of drill pipe may then be repositioned within the string or pulled and reinspected to minimize the risk of fatigue-induced failures. Introduction Drill string components can fail in a number of ways. Tensile overload, torsional overload, combined mode failure, fatigue and corrosion fatigue, and environmentally assisted cracking are some of the common mechanisms associated with drill string failures. Fatigue and corrosion fatigue account for the majority of the drill string failures [1] [2]. The fatigue damage starts when the component goes into service and is subjected to some form of cyclic stress. As the material is subjected to repeated cyclic loads, additional fatigue damage occurs. This fatigue damage is cumulative (non-reversible) in nature and progresses until a fatigue crack forms. If the crack is not detected by inspection and the affected equipment is placed back into service, the crack grows until the component fails by leak (typically seen as washouts on drill pipe tubes), brittle fracture, or gross plastic deformation due to overload of the remaining cross sectional area. When constructing and designing a drill string, the designer balances a number of conflicting factors. Hydraulic requirements, steering and measurement, hole cleaning, rate of penetration, stuck pipe avoidance and recovery, and perhaps most critical, structural soundness of the drill string all have to be considered. To ensure structural integrity of the drill string, the designer must address two failure modes: overload and fatigue. Often, though, prevention of overload failures, not fatigue failures, is the principal focus of drill string design. Overload failures are much easier to understand and prevent, and consequently simpler to address at the design stage.. Conversely, the complex variables and low threshold stresses that create fatigue damage make designing for fatigue mitigation a more challenging task.