Fatigue Assessment of Dented Pipeline Specimens

Abstract

This paper reports results from an investigation program launched with the objective of assessing fatigue lives of actual pipeline specimens with dents. Nine pipeline 3m-length specimens were constructed with low carbon steel pipes API 5L Gr. B. The specimens had 323mm diameter and 6.35mm wall thickness. The specimens were loaded with hydrostatic internal pressure pulsating at a 1Hz rate. Six specimens had 15% deep longitudinal smooth dents (ratio between dent depth and outside specimen diameter) and three specimens had complex longitudinal 6% deep dent shapes. Nominal and hot spot stresses and strains were determined by experimental techniques (Fiber Optic Bragg Strain Gages - FBSG, and Digital Image Correlation - DIC) and by a numerical technique (Finite Elements - FE). The stresses and strain fields determined from nominal loading conditions or from experimental measurements and from the finite element analyses were combined with different fatigue assessment methods. The estimated lives were compared with the actual test results. The fatigue assessment methods encompassed those proposed by the Pipeline Defect Assessment Manual (PDAM) and by the API 579-1/ASME FFS-1 Level 2 methods described in parts 12 (Dents) and 14 (Fatigue). Most of the predicted lives exhibited high level of conservatism. A Level 3 method that employed experimentally and numerically determined hot-spot strains in conjunction with a fatigue strain-life equation proposed by Coffin-Manson predicted fatigue lives very close to the test results.

Fatigue Tests on Buried or Repaired Dented Steel Pipeline Specimens

“…Most of the predicted lives using these methods exhibited high level of conservatism. A Level 3 method that employed experimentally and numerically determined hot-spot strains in conjunction with a fatigue strain-life equation proposed by Manson-Coffin predicted fatigue lives very close to the test results [11,12]. Eight out of nine test results are reported in Section 3.…”

“…Moreover, Table 1 gives fatigue test results for eight out of nine other but similar specimens that were tested in air (not buried specimens and not repaired) and previously published [11,12].…”

“…The results achieved for the nine non-buried specimens tested in air conditions were fully analyzed and presented in previous publications (e.g., [10][11][12]). They were loaded with hydrostatic internal pressure pulsating at a 1 Hz rate.…”

“…Figures 6 and 7 show the 3D shape of the dent areas of two specimens here named Specimen #20 (buried) and Specimen #29 (repaired). It is important to note that these actual deformed shape measurements can be exported to the Solid Models and Finite Element software programs to perform numerical elastic or elastoplastic stress analyses in order to assess the static and cyclic integrity of the pipeline with dents, as described in [9][10][11][12], for example. The dented specimens # 20 and # 29 depicted in this section were subjected to internal pressure cycles with maximum pressure of 6.2 MPa, equivalent to a maximum 0.65 SMYS nominal circumferential strain.…”