PSI - Issue 33

Vitor E.L. Paiva et al. / Procedia Structural Integrity 33 (2021) 159–170 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Pipelines are exposed to external factors that could eventually cause damage to their structural integrity. Dents in pipelines are considered to be one of the most severe yet common forms of mechanical damage that are commonly associated with a loss of integrity. Dents may be caused by the impact of external elements such as construction equipment and rocks during a pipeline’s construction and operation, resulting in plastic deformation of the pipe wall. The presence of a dent causes a local reduction in the diameter of the pipe, and consequently, strain and stress concentrations are introduced. Dents affect a pipeline’s load-bearing capacity and reduce its operating life (Cosham (2004)). The shape of the dent is a very important factor that influences the fatigue life of pipelines, since fatigue cracks tend to start at points of strain or stress concentration. Consequently, ascertaining the strain distribution induced by the introduction of a dent plays an important role for determining the structural integrity and safety of the pipeline. Thus, the predicting, detecting and monitoring of mechanical damages in pipelines has been the subject of research for decades. Some examples of articles that deal with this subject are listed in the References section of this paper (e.g., see Cosham (2004), Bolton (2008), Bolton (2010), Pinheiro (2009), Rosenfeld (2002), Fowler (1993), Akbari (2015) and Garbatov (2017)). More specifically, the fatigue behavior of pipeline specimens with dents under cycling internal pressure loading with and without restriction constraint to free deformation were studied as part of an extensive research program. The investigation encompassed the fatigue testing of thirty-three full-scale dented pipeline specimens divided in three groups. One group comprised nine dented specimens tested in air (no restrictions to the free dent deformation). A second group comprised eight dented specimens tested while buried in soil. The third group was composed of sixteen dented specimens repaired with composite material reinforcement layers that have been tested in air. This paper presents thoroughly the results achieved for the group of buried specimens and briefly mentions the results of the specimens tested in air for comparison purpose. The results of the third group comprising the repaired dented specimens will be presented opportunely. The results achieved for the nine non-buried specimens were fully analyzed and presented in previous publications (e.g., Paiva (2019), Freire (2020) and Paiva (2020)). The nine pipeline 3m-length specimens were constructed with low carbon steel pipes API 5L Gr. B. The specimens had 324mm diameter and 6.35mm wall thickness. They 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 Cosham et al. (2004)) and by the API 579-1/ASME FFS-1 Level 2 et al. (2016) methods described in its parts 12 (Dents) and 14 (Fatigue). 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 Coffin-Manson predicted fatigue lives very close to the test results (Freire et al. (2020) and Paiva et al. (2020)). Eight out of nine test results are given in the Results section of the present paper. Since pipelines are usually buried in the ground to provide protection and support, a dent may experience significant resistance as a result of the pipe-soil interaction. Accordingly, the aim of the present paper is to present the fatigue test results of dented specimens under similar to the above test conditions with the difference that those tests were fully completed while the specimens were buried, simulating the same conditions of a real buried pipeline. One of the objectives is to investigate if the contact with the soil-environment where the pipeline is to function will play an important role in the mechanical behavior of a dent and in its fatigue life. The tests carried out with this second group of specimens were divided into two phases. The first phase was performed prior to the dented specimens being buried. During this phase, the specimens were freely placed (not buried) in the laboratory and loaded with five hydrostatic internal pressure cycles so that hot-spot cyclic strain amplitudes at the dent area and at nominal locations could be measured by using two experimental techniques: Digital Image Correlation (DIC) and Fiber Optic Bragg Strain Gages (FBSG). The full-field measurements were taken using DIC to