Pipeline constructed in rocky terrain is vulnerable to damages such as denting, gouging and other mechanical damages. In-line inspection (ILI) of these pipelines often reported several hundreds or even thousands of dents. Although most of these reported dents are well below 6% outside diameter (OD) depth limit as per ASME B31.8, few dents (sharp rock dents) with high strain could pose threat to integrity of the pipeline. Recently, strain-based models have been proposed to assess mechanical damage severity in pipelines. Attempts have also been made to characterize cracking susceptibility in rock dents using the critical strain based ductile failure damage indicator (DFDI) model. The objective of this study is to validate this model using full-scale denting tests conducted at the laboratory. Additionally, validation also extends to against the simplified DFDI model without finite element analysis (FEA). In this paper, the existing ASME strain limit and strain limit damage models are reviewed. The critical strain based strain damage model known as Ductile Failure Damage Indicator (DFDI) is then presented. The theoretical aspect of this model, including early work by Hancock and Mackenzie on strain limit (εf, reference failure strain) for ductile failure, is reviewed. The experimental aspect of material critical strain and its measurement using uni-axial tensile testing are then described. An elastic-plastic finite element analysis is employed to calculate DFDI, which is used to quantify the accumulated plastic strain damage and its susceptibility to cracking, and is validated using six full scale denting tests. Finally, the simplified strain limits for plain dent is proposed and validated.
Today’s in-ditch laser scan inspection technology provides pipeline operators with relatively accurate 3D dent profile data. Some of the benefits of using laser scan data are to accurately calculate dent strain and quantify its severity. However, there are some concerns regarding the scan parameters used such as; scanner resolution settings, scan coverage over the dent, and its surrounding area as well as repeatability and reliability of scanned dent data that could affect the accuracy of the calculated dent strains. Therefore, it is important to understand how these parameters affect the accuracy of calculated dent strains, which could lead to either over- or under-estimating equivalent strains and result in unnecessary repairs or leaving critical dents in the pipeline without mitigation. The benefit of this study is to help the pipeline operators to reduce in-ditch dent inspection time without compromising on the accuracy of dent geometry and its strain. In this paper the effect of different scan resolutions on the calculated strain is studied first. Then, using high resolution, the effect of scan coverage on the dent strain is studied. In particular, the difference in the calculated strain among 60°, 90°, 180°, 360° scans coverage circumferentially. The repeatability of dent scan with two different resolutions is then evaluated with two real life dent samples. Finally, the findings from this study are summarized. This serves as the basis for developing an optimal procedure for dent laser scanning with acceptable level of scan parameters for a reliable strain assessment. The benefits and limitations of 3D laser scanning technology from this study are also presented.
Mechanical damage, such as gouge, is the damage to the pipe surface caused by external forces and is usually caused by third-party damage during construction and excavation. This normally results in a highly deformed, work hardened surface layer with possible metal removal. In many cases, dents are coincident with gouges. Industry standards and regulators treat this type of mechanical damage as critical and require immediate investigation. Therefore, from a pipeline operator perspective, distinguishing between plain dent and dent with gouge is a great challenge for topside dents because quite often they are caused by un-authorized third party activity and contain gouge. In a previous study[1,2], the present authors developed an approach that combines dent strain-severity criterion with MFL signal recognition to identify dent with gouge and crack. In this paper, an extension of the previous study to topside dents is presented. The enhanced approach for distinguishing between plain dents and dent with gouge/crack for topside dents is summarized. Case studies are given to demonstrate the effectiveness of the approach for identify topside dent with gouge.
Dents containing crack fields (colonies) were often observed in liquid pipelines. A recent PRCI research “Study of the Mechanism for Cracking in Dents in a Crude Oil Pipeline” showed evidence of corrosion fatigue cracking mechanism in dents and estimated the crack growth rate as a function of stress intensity factor using the measured spacings of fatigue striations from fracture surfaces based on the assumption that the formation of fatigue striations on a cycle-by-cycle basis. However, due to the lack of full-scale fatigue crack growth data, the success was limited in this study. This gap prompted PRCI to launch a full-scale experimental investigation of cracks-indents under cyclic pressure load in the simulated groundwater (NS4 solution) environment. The objective of the study is to determine the crack growth rate in dent as a function of stress intensity factor, the number of cycles to failure, and the failure modes of crack-in-dent. The investigation is aimed at establishing a framework for the remaining fatigue life prediction of cracks-in-dents in liquid pipelines. This framework would benefit liquid pipeline operators to manage the integrity of dents associated with corrosion fatigue cracking exposed to groundwater in a timely manner. A total of six pipe samples containing cracks in shallow dents excavated from a 24-inch diameter liquid transmission pipeline were selected for full-scale fatigue tests. The test system developed under the project consisted of (1) a computer-controlled hydraulic pressure cycling system, (2) an environment chamber containing NS4 solution mounted on the dent region to provide a simulated field environment condition, (3) real-time crack growth monitoring systems including direct current potential drop (DCPD) system, Clip gage, and Strain gage, and (4) a data acquisition system. The cyclic pressure range used in the fatigue test was between 78 psig (7.2%SMYS, minimum) and 780 psig (72%SMYS, maximum) with R = 0.1, which was based on historical operational pressure fluctuation data. A constant frequency of 0.0526 Hz was selected for the testing to ensure the frequency requirement for corrosion fatigue was met. In this paper, the objective, along with the background of this research, is discussed first. Then, the pipe sample preparation, experimental setup, and test results are presented. The fatigue crack growth rate as a function of the stress intensity factor is then discussed. Following this, the fatigue crack growth coefficients were estimated using the full-scale test data and FEA. Finally, the fatigue test results are summarized and presented the framework for the life prediction of corrosion fatigue cracks in shallow dents.
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