According to statistics compiled by the U.S. Office of Pipeline Safety, external mechanical damage is one of the primary failure mechanisms for transmission pipelines. Mechanical damage can be described as a pipeline deformation with or without secondary features such as gouging or cracking. These types of features are a major concern of pipeline integrity engineers. This is why inline inspection tools specifically designed to detect geometry deformation have been used in the industry for many years. A pipeline dent of sufficient depth can reduce the burst pressure of the pipeline. Some of the most severe pipeline conditions possible occur when a dent is in coincidence with another defect such as a gouge or a crack.
Since the development of Tri-axial Magnetic Flux Leakage (MFL) inspection tools, a number of pipelines have been inspected with both caliper and MFL inspection tools. A large database of features was created that contained detailed information from both the caliper inspection tool as well as signal information from the MFL tool for dent like features. An empirical study of the relationship between the MFL signal of a dent and the recorded mechanical deflection of a dent from a calliper tool was conducted and the results from this study indicated it is possible to accurately predict the size (Depth, Length, and Width) of a dent from tri-axial MFL inspection tool data. The methodology of sizing dents and the accuracy of dent size predictions will be demonstrated in this paper.
Magnetic Flux Leakage technology has been used for the in-line inspection of pipelines for over 40 years. MFL inspection tools were developed to detect metal loss resulting from corrosion. During the early stages of MFL inspection technology, these tools could only detect large areas of corrosion or corrosion clusters. Advancements in tool design, sensor type and configuration, as well as advancements in electronics have resulted in a greater ability to accurately identify and characterize smaller anomalies. Today's advanced MFL tools can identify many different types of pipeline features and this paper will specifically describe the identification and sizing of geometric deformations or dents.
By today's standards, the MFL tools of the 1970s, and early eighties were of low resolution and weak in magnetic strength. Advancements in technology, mainly in electronics, enabled pipeline inspection companies to vastly improve in many ways.
The number of magnetic field sensors that could be placed around the tool has increased dramatically over the years. The first generation of MFL tools measured the magnetic flux leakage field in only a single direction, where as most of the modern high resolution tools have sensors in 2, or all 3 orthogonal directions. In a Tri-axial in-line inspection tool there are three separate sensors orientated perpendicular to each other. This triad of sensors contains an axial sensor that records the magnetic field parallel to the pipe, a radial sensor, which records the field perpendicular to the pipe, and a circumferential sensor which records the field directed around the pipe.
