New Design Limits Plot for Overview of Load-Resistance Relationship in Wellbore Tubular Design

Abstract

Abstract In the tubular design of oil and gas wells, visualization of the loads and design limits (resistance over design factor) in a single 2D plot can help provide well designers a safety-factor overview of a particular string section. This plot is called a design limits plot (DLP). This paper illustrates the theoretical foundation and construction procedure of a new DLP, which is believed to more accurately represent safety factors. The new DLP was built by plotting differential pressure, ΔP, vs. equivalent axial load Feq (= Fa + min(pi, po)×AS, where Fa is the axial force, AS is the cross-sectional area, and pi and po are the internal and external pressures, respectively), for both envelopes (design limits) and load points. Temperature-deration effects were applied to load points instead of envelopes so that the safety factor could be neatly represented by the relative load-point location with respect to the room-temperature envelopes. For comparison, the triaxial yield envelope was plotted together with API envelopes (axial, burst, and collapse). A field example is presented that demonstrates the application of the new DLP for wellbore tubular design. It is shown that, for load cases in which counter-load pressures (which are defined as pi for collapse loads and po for burst loads) are high, the new DLP represents the safety factors more accurately and comprehensively. Using the equivalent axial load instead of the axial force in the new DLP allows for better consistency with both the von Mises triaxial yield criterion and the API 5C3 2015 addendum. The new DLP not only represents a more accurate overview of safety factors, but also facilitates direct reading of pipe burst/collapse ratings.