Burst Strength of Oval API Casing with Hydrogen Sulfide Embrittlement

Summary In this paper, we present calculations and analysis based on Haigh (1936) and Austin and Swannell (1979) models for burst resistance of oval pipe. The tangential (hoop) stress is the dominating principal stress. The subject case is hydrogen embrittlement of 10¾ in., 65.7 lbm/ft, Q125 API casing. High-grade API oilfield tubing and casing become brittle when exposed to hydrogen sulfide (H2S). When H2S mixes with water, a chemical reaction releases free hydrogen. Because of their small size, these atoms penetrate steel and set up internal stresses. The steel now becomes brittle, and many failures in the oil industry have historically been caused by hydrogen embrittlement. The risk of bursting from inside pressure is increased if the pipe is not perfectly round after fabrication or if subjected to deformation during service. When pressurized from the inside, the oval pipe tries to restore roundness, imposing strain on the brittle steel tubulars. The traditional use of the maximum distortion energy criterion (von Mises yield criterion) no longer applies for brittle steel. Instead, for the purpose of determining the maximum allowable inside pressure, the tangential (hoop) stress is used as the governing stress acting on a brittle failure model. Useful formulas, one simple and one complex, are proposed for use by practicing drilling and well engineers. Total hoop stress is the sum of tensile hoop stress from the API burst formula and tensile bending stress from the changing ovality during pressurization. The effects of surface imperfection and material anisotropy are not considered. The absence of surface defects (fracture initiation points) is not necessarily a valid assumption, prompting the need for a detailed analysis of the casing material subjected to an equivalent sour environment. In a North Sea gas-condensate well, a steadily increasing level of H2S was observed in the production stream. The primary well barrier (production tubing and downhole safety valve) was verified and found undamaged. The secondary well barrier (production casing) is missing because the Q125 tubular is not qualified for sour service at low temperature corresponding to shallow depths. Therefore, one well barrier has failed, and the other is intact, implying that a single barrier failure may lead to a leak to surface. The well is currently shut-in and classified as having reduced barrier status according to recommended guidelines for well integrity. Published statistical data for a two-parameter Weibull distribution were used, indicating a 50/50 probability to exceed 0.2% ovality for subject production casing. Furthermore, suspected casing wear prompted a burst analysis for the maximum field case of 1% ovality. The innovative aspect of the paper is to present easy-to-use formulas for calculating the burst limitation of oval and brittle API casing, enabling the practicing drilling and well engineer to perform design calculations, either standalone or in combination with finite element modeling.