EVOLUTION OF TRIAX MAGNETIC FLUX LEAKAGE INSPECTION FOR MITIGATION OF SPIRAL WELD ANOMALIES

With an overall objective for broad and confident integrity management of the PetroChina Pipeline Company's pipeline network, we illustrate the impact of a collaborative effort between PetroChina and GE Oil & Gas for the inspection and mitigation of spiral weld anomalies, particularly for new advanced assessments of features oriented along and within the spiral weld. Tool configuration, sensor types and the role of novel data analysis techniques including magnetic vector component measurements, is presented as a set to address a broader variety of spiral weld threats, while ensuring a high level of operational robustness and reliability. This paper describes some of the science behind the art, and describes the fundamentals of MFL magnetics and it's evolution as an ILI technology into the 21 st century for spiral weld anomaly inspection. INTRODUCTION The PetroChina Pipeline Company has a progressive structured integrity program for maintenance and operation of it's pipeline network. The abilities and challenges of pipeline failure assessment, risk management, mitigation as previously reported have progressed within this strategy including specific mitigation strategies and efforts for primary pipeline threats and a structured integrity management plan and strategy for successful ongoing operations. [1,2,3] Spirally welded pipe is widely used around the world and in particular, PetroChina has an installed base of over 40,000 km. The key threat identified within the failure history and risk analysis was anomalies related within or near the spiral weld. As previously disclosed [4], the integrity consideration of spiral weld defects lies in the correct interpretation and consideration of the type of defect and potential failure mode. The need then is a reliable information source for distinguishing inherent cracks and "crack-like" features (toe, lack of fusion) apart from corrosion features (HAZ area body or weld dressing) as related to the weld integrity itself. In 2008, PetroChina approached GE to collaborate on inspection capabilities and performance specifically towards the spiral welded pipe concerns raised in the failure history and PetroChina's risk assessment. PIPELINE CONSIDERATIONS AND COLLABORATION The particular pipeline network of interest has the following characteristics:  Pipeline Diameter 28”  Nominal Wall thickness 8mm ,  Pipe Grade X52 spiral-welded  Design pressure 43 bar (53% SMYS),  MAOP 36 bar (45% SMYS)  Pipe Age: 1974 construction In 2007, PetroChina and GE performed a trial inspection run to determine field experience in the capabilities of MFL and triax sensing for corrosion and particularly for spiral weld anomaly assessment. Subsequently in 2008, PetroChina approached GE with the challenge to advance the capabilities regarding quantifying and interpreting spiral features. These included  Lack of fill/under-cut,  Weld porosity, Incomplete penetration,  Lack of fusion,  Roof topping, Weld misalignment & "Hi-Low"  Corrosion GE accepted the challenge and the PetroChina-GE collaboration evolved over the next 18 months to include manufactured defect sets, multiple pulltest studies. and field evaluation work. The studies have focused and been applied to inspections and efforts for the NEOGF network of PetroChina, providing crude oil transportation in Northeast China With conventional MFL inspection, detectable features such as on typical long seam-welded pipe, we classify and describe crack or metal loss defects in terms of it's length (down the pipe) and it's width (around the pipe). Pipeline inspection tools for crack detection rely on a given defect's orientation to be axial (parallel) or circumferentially (perpendicular) to the main pipe axis. In the case of spiral welds, the nature of the defects and their related signals now changes relative to the angle and to the weld itself. MFL, and in particular multi-axis magnetic sensing (known as Triax), was determined to be the best option to progress. PetroChina and GE have been working together to advance the interpretation and analysis of spiral weld anomaly signals as seen by MFL technology of which we provide an overview. Overall, a common objective was shared to increase inspection confidence to provide for increased pipeline operation and throughput. APPLICATION OF MAGNETIC FLUX LEAKAGE TECHNOLOGY As noted, magnetic flux leakage (MFL) techniques have evolved in the pipeline inspection industry since the 1960s. Since 1978, GE has provided high resolution MFL inspection to the global industry. Since the 1990s, GE has offered and advanced the technology with the use of magnetic vector sensing or "Triax". In conventional MFL, only the main magnetic (axial) field component is measured and has been proven as robust over the decades. However, magnetic fields are vectors consisting of 3 field components. In "Triax" sensing, we capture all three field components which offers further resolution, insight and ultimately confidence in the interpretation of the feature. As illustrated in Figure 1, the three components are known as "Axial", "Radial" and "Transverse" to match the natural cylindrical coordinates of the pipe. The axial field is commonly measured by all MFL tools and is representative of the volume of the defect and disruption of magnetic field through the pipe wall. "Radial" and "Transverse" fields similarly occur when a defect is present and tend to characterize the profile of the feature. Figure. 1 Overview of Signal Responses from multi-axis sensor MFL inspection Radial Measured Outward/ inward from center of pipe Transverse Measured circumferent ially around pipe Axial Conventional MFL – Measured down pipe Axial Radial Transverse