We view and analyze the total fatigue life cycle of a crack in four phases

Typical Steps for Performing Fatigue Crack Growth Analysis of Pipelines 

1. Determine Initial Crack Size
  • If initial crack sizes are taken from ILI Crack or Seam Weld Integrity assessments, include the ILI sizing tolerances 
  • Initial crack sizes can be those that survived the latest pressure test 
  • For dents, the time to for 
2. Gather and Evaluate Representative Operation Cyclic Pressures
  • From historic operational pressures and assuming that such pressure spectra are representative of future operating conditions 
  • Ensure that the time stamp in pressure data accounts for potentially higher pressure changes 
  • Simplify complex pressure spectrum into equivalent pressure ranges and associated number of cycles (ASTM E-1049) 
3. Choose Pipeline Material Properties
  • Representative of the pipeline at the location of the crack or seam weld anomaly and expected fracture behavior (brittle or ductile) 
  • Fracture toughness, yield and tensile properties 
4. Select Fatigue Crack Growth Model
  • Most commonly Paris relation with appropriate values for parameters C & n 
  • Others available such as Walker, Forman and NASGRO 
5. Decide on Failure Condition Setting the Termination of the Fatigue Analysis
  • When growing crack size reaches a size expected to fail at MAOP or Safety Factor X MAOP and chosen material properties 
  • When it reaches a specified number of cycles or time 
6. Validate Fatigue Life Prediction

Dent Fatigue Behavior

Dent fatigue behavior is another aspect of our fatigue life assessment services. We perform fatigue life analysis of dents and dents with damage using ILI or in-ditch deformation measurements, based on Fracture Mechanics principles, enhanced by computational finite element modeling and verified by customized full-scale dent fatigue testing. When pipe samples are not available from the pipeline system of interest for full-scale testing, we exercise caution when using dent fatigue test data obtained from similar pipelines, cyclic loading spectra dent shape parameters and crack or gouge dimensions, and dent restraint conditions. Fatigue testing is known to have inherent scatter and therefore we apply probabilistic and statistical principles to understand the data and use them to make fatigue life predictions.