Pipeline dents fatigue life prediction is a subject of high interest for pipelines operating companies.
Empreinte is an in-house developed pre and post processor to ABAQUS Finite Element Calculations dedicated to pipeline integrity assessment. Empreinte was first developed and experimentally validated for dents assessments under static loading conditions.
As oil but also gas transmission pipelines are submitted to cyclic loading conditions (internal pressure variations, shutdowns, temperature variations …), it was decided to introduce a fatigue life criterion in Empreinte based on the Dang Van theory assuming that local mesoscopic stresses drive fatigue crack initiation.
Full scale tests performed for PRCI projects PR-201-927, PR-201-9324 and MD-4-2 were used to validate the proposed fatigue assessment methodology:
- the first full scale fatigue test was performed in 1994 on an X52 pipe. For this test, limited material and test data were available.
- the second full scale fatigue test was performed in 2007 on an X52 pipe. For this test, material characterization (in particular tensile tests with full stress strain curves) and test data (strain gages measurements, indenter geometry …) were available.
Fatigue life assessments were performed following three main steps:
1. using available data: non linear kinematic hardening constitutive laws were identified for the two pipes materials;
2. finite elements elastic-plastic modeling of the denting processes were carried out;
3. fatigue calculations were performed following a new approach using Dang Van criterion for which the parameters were determined from literature data.
The elastic shakedown assumption allowed the determination of the local stress cycle from the macroscopic stress cycle. The fatigue criterion integrating the combined influences of shear and hydrostatic stresses was checked on all points of the pipe.
Good agreement between experimental and calculated fatigue lives and fatigue crack initiation points was reached. This opens a promising way to assess pipeline defects fatigue life.
Efforts are now focused on the standardization of a testing method to identify the Dang Van criterion of a pipeline material at least in air environment.