Abstract

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.

Introduction

API TR 5C3 (7th Edition, June 2018 with October 2019 Addendum) [1] provides equations for pipe body fracture performance in Annex D (informative). The API 579 [2] level-2 FAD curve is utilized to obtain the elastic-plastic fracture pressure equations. The equations address the protection against failure through fracture (brittle or ductile) and are applicable to brittle, ductile, or somewhere between the two extremes for a pipe body containing an infinitely long, internal axial surface crack-like flaw subjected to internal pressure.

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