Achieving well integrity demands knowing how much loading your tubulars can withstand, but what happens when near-yield loading causes unexpected deformations that are not predicted by code design and conventional elastic design theory? It begs the question, are harsh operating conditions and narrowing design safety margins increasing the potential for well failures? Alternatively, can tubulars be safely pushed even harder without compromising well integrity?
This paper raises industry awareness, demonstrates material test results, and proposes steps to account for possible near-yield load effects so that well designs can fully use the available tubular material capacity and be cost effective without compromising the long-term integrity of the well. Examples of near-yield load scenarios leading to substantial plastic deformation observed in laboratory testing are presented, and ways to determine when those effects need to be considered in elastic design methods are proposed.
The paper illustrates the effects of near-yield loading on tubular deformations using results of mechanical testing on a P110 material sample under cyclic uniaxial and multiaxial load. Uniaxial load cycles with stress amplitude below the rated yield strength showed substantial growth of plastic strain in only a few cycles. Combined-load tests on small-scale tubular P110 specimens loaded with axial force and internal pressure cycles also demonstrated pronounced near-yield load effects.
We discuss the significance of material behaviors that trigger plasticity at stresses below the measured yield strength and contribute to severity of near-yield load effects; such as changes in the stress-strain response due to elevated temperature exposure, directional dependence of material properties, and cyclic softening. We draw conclusions on how tubular material properties and operational loading extremes need to be assessed and balanced to maintain structural integrity of wells in anticipated operational scenarios.
To maximize capacity of completion tubulars, the complexity of near-yield load effects needs to be characterized with technically rigorous yet practical means. This paper points to relatively simple material tests for determining the propensity for near-yield loading effects of common OCTG materials, and suggests initial considerations for incorporating near-yield load considerations in tubular design.