A New Load Rating Criterion Avoids Overdesigning Downhole Tools

Summary The traditional approach to designing completion tools can result in the inefficient use of materials and/or inaccurate or overly conservative load ratings for some applications. In this paper, we present a new load-rating criterion, the onset of global nonlinearity, which can more accurately represent the actual strength of downhole tools in those scenarios. Using the onset of global nonlinearity may allow for higher load and pressure ratings for a component than would be produced by traditional design approaches. This new load-rating criterion has been demonstrated through the first principal analysis, laboratory experiments, and field results in applications such as hydraulic fracturing, sand control tools, subsurface safety valves, and injector-producer cycles. The increased load and pressure ratings are most striking in high-pressure and high-temperature applications. The prevailing load rating criteria for metallic tools are based on material yield strength. The initial yield criterion using von Mises stress, which dictates the load limit that a downhole tool can carry before exhibiting plastic deformation, is often used to design completion tools. However, for some applications of downhole tools, this load rating criterion can be overly conservative, limiting the design options unnecessarily. Several load rating criteria are evaluated and discussed considering the fundamentals of engineering mechanics. Based on our investigation, we propose a mechanics-based load rating criterion for applications where the number of peak load cycles is small and where small-scale yielding is allowed to occur in a localized area without causing major nonlinearity in the global scale. The load rating criterion is termed the onset of global nonlinearity. Engineering mechanics for several widely used load rating criteria are discussed and their application ranges and limitations, if any, are identified. The proposed onset of global nonlinearity for applications with low numbers of peak load cycles was validated through experimental tests under burst and collapse pressures as well as tensile and torque loads. The global onset of nonlinearity has been successfully applied for applicable downhole tools in real projects. A new load rating criterion for applications with low cycles of peak loading helps to use the load-carrying capacity of the material fully and safely. Applying the onset of nonlinearity approach typically yields a higher load rating, which may be as much as 1.5–2 times more than that defined by the initial yield criterion, when the tools operate in noncorrosive environments, the required peak load cycle number is small (e.g., less than 200), and the tool materials are ductile.