Pipe fracture performance often becomes one of the governing factors in the tubular design and material selection for OCTG (Oil Country Tubular Goods) steel pipes used in the sour (H2S) service environments. API TR 5C3 Annex D [1] provides equations for calculating pipe internal pressure at which a pipe will fail due to crack propagation of existing crack-like flaws. The fracture pressure equations in API TR 5C3 was derived based on API 579 [2] Failure Assessment Diagram (FAD) methodology.

The load ratio equation in API TR 5C3 Annex D uses the combined von Mises equivalent stress as reference stress and includes the crack depth (a) in the calculations of the hoop and axial stresses, which are inconsistent with the API 579. In contrast, API 579 uses the component stress distribution normal to the crack face (e.g., hoop stress due to internal pressure for a longitudinal crack) for the un-cracked geometry. The crack depth (a) is then incorporated in the reference stress (σref), which is then used in the load ratio equation.

In this paper, a general load ratio equation is first derived and then compared to the API TR 5C3 load ratio equation. It is found that the API TR 5C3 equation underestimates the load ratio by approximately 15% to 25% depending on the pipe OD to wall thickness ratio (i.e. Do/t) as well as the crack depth to pipe wall thickness ratio (i.e. a/t). This can potentially lead to an over-prediction of pipe internal pressure at fracture.

Subject
External surfaces, Pipe surfaces, Tubular goods, Materials, Piping, Internal pressure, Internal stress, Stress cracking, Stress, Crack depth, Flaws, Internal surfaces, Fracture
Keywords:
OCTG Pipe, Fracture, Failure Assessment Diagram (FAD), Load Ratio, Sour Service
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2021
Association for Materials Protection and Performance (AMPP)
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