PIGGING & ILI
Reducing conservatism in fatigue crack growth assessments through testing By Dominic Wynne, Technical Solutions Specialist, ROSEN Group, US
The pressure in an oil and gas pipeline is never constant: variations in the pressure are caused by operation conditions or changing customer needs. Repeat loadings cause cyclic stresses in pipelines, and these can create cracks, or cause existing cracks to grow by fatigue.
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ipelines can be subject to various types of repeat loading, for example from traffic loading, or vibration due to water flow. Variation in internal pressure is the most common concern for operators, and has been linked to a number of failures. This cyclic loading leads to progressive, accumulated damage in the steel, known as fatigue, which if of sufficient magnitude can lead to the initiation and propagation of cracks that can cause failure. Typically, pipeline fatigue is more of a concern for liquid pipelines where much larger magnitude pressure swings, and hence stress variations, are observed compared to those carrying a compressible gas product. There have been a number of high-profile pipeline failures caused by fatigue cracking; for example a 40-inch diameter crude oil pipeline in France in 2009[ ]. These types of failure are rare , but it is reasonable to expect that the fatigue problem will get more difficult to manage as pipelines age.
Assessment methods for quantifying fatigue damage in pipelines are well established in the industry (e.g., API 579 or BS 7910). Fatigue life is reduced by orders-of-magnitude in the presence of a stress concentration; e.g., a geometric deformation such as a dent, or an anomaly such as a weld flaw. Methodologies such as API 579 or BS 7910 can explicitly consider the presence of an anomaly or the magnitude of a localised stress concentration. A simpler ‘S-N’ approach is used to determine the fatigue life of a pipeline, usually at the pipeline’s design stage, which is based on empirical curves which link stress range (S) to predicted number of cycles to failure (N). The SD-N approach is limited to predicting the fatigue life of an anomaly-free seam weld or pipe body. More often than not, during an assessment of remaining fatigue life for an existing pipeline, the presence of an anomaly has to be considered explicitly. This could be an actual anomaly detected by ILI, or one that
Figure 1: 40-inch crude oil pipeline, long seam fatigue crack.
could be theoretically present in the pipeline, in the form of one that survived the pre-service pressure test, one sized below the ILI detection thresholds, or one that would have passed the pipe mill quality control requirements. When calculating the remaining life of a crack-like anomaly in the presence of cyclic loading, the fracture mechanics approaches in API 579 and BS 7910 are required. These methods have been successfully applied for determining remaining fatigue life, in multiple industries, including the pipeline industry; however, the methods are known to be conservative. This article outlines an approach for reducing the conservatism in these assessments, allowing operators to safely extend anomaly remediation dates and reassessment intervals, supporting economical operation by reducing unnecessary digs.
Fracture Mechanics Methods Fatigue crack growth rate can be described as the crack growth per stress cycle. The relationship between crack growth and applied stress is represented by a sigmoidal curve with three distinct regions, Figure 1, which is typical of crack growth in-air. The first region, at the left-hand side of the Figure, is where crack growth rates are expected to be extremely low, or there is no growth at all. The second region is characterized by a linear relationship between the crack’s stress intensity factor (stress intensity is related to the applied stress and the size of the crack) and the crack growth rate. The final region describes the rapid crack growth leading to failure. The central linear portion of the curve can be described by the Paris-Erdogan equation, which is a simple single slope relationship, and the most common way to model crack growth within industry. It is given by: da =A.∆Km dN Where da/dN is the crack growth rate and ΔK the applied stress intensity factor range. A and m are empirical constants that relate to the
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The Australian Pipeliner | March 2022
