Quality assessment and deviation analysis of three-dimensional geometrical characterization of a metal pipeline by pulse-echo ultrasonic and laser scanning techniques

Highlights

• The inner surface of a metal specimen with known flaws was inspected.

• Ultrasonic presented errors with smaller magnitude than laser scanning technique.

• The presence of artifacts near the defect edges affected laser technique performance.

• Ultrasonic technique demanded more time for inspection than laser scanning technique.

Abstract

Although ultrasonic-based techniques are the most widely used ones for the assessment of pipeline integrity they have some limitations, and alternative methods show potential to overcome them, such as laser scanning techniques. This paper presents the results of an experimental evaluation of pulse-echo ultrasonic and laser scanning techniques for pipeline inspection using a metal specimen which represents a damaged pipeline. Both techniques were able to detect all the defects on the inner surface of the specimen, but the defect geometry was important to define the accuracy of each technique. For almost all the evaluated defects, the differences between the reference and the three-dimensional representations created from the experimental data showed that the ultrasonic technique presented errors with magnitude around 0.2 mm, which is in general half the error observed for the laser scanning technique. However, ultrasonic technique demanded 8 h for specimen inspection, while laser technique required only 10 min.

Introduction

The transportation of oil and gas is mainly done by means of a network of thousands of kilometers of pipelines, which interconnect production areas, refining units and consumers. Although there are high capital investments for building and maintaining the required infrastructure, the preference for pipelines rather than road and rail systems is motivated by safety and financial gains due to a simplified process of loading and unloading, which also reflects as lower storage and labor costs [1], [2], [3]. However, this kind of transportation system is subject to damage, especially due to wall degradation, which may lead to serious consequences, such as property damage, environmental damage, business interruptions, and injury or death. Failures in oil and gas pipelines are mainly caused by internal and external corrosion, dents and weld defects [4].

In order to ensure the best performance possible during oil and gas transportation, the pipeline integrity assessment by means of periodical inspections is required [5]. Within this context, ultrasonic-based nondestructive tests are widely used to identify possible defects present on pipeline surfaces [4], [6]. The operating principle of ultrasonic technique is based on the propagation of sound waves with frequencies above the hearing level (20 kHz), which are called ultrasonic waves. In pulse-echo technique, one transducer is used for both emitting and receiving mechanical waves. The wall of the pipeline or the presence of defects can cause a reflection provoked by a sharp change of the acoustic impedance between two mediums – for instance steel to water. The analysis of the runtime among multiple reflection echoes can be used for measuring pipeline thicknesses or identifying defects [7].

The literature presents several studies on which ultrasonic technique plays a key role concerning pipeline assessment. Feng et al. [7] presented a literature review of typical girth weld defects of pipelines and the application of nondestructive testing techniques in order to detect them. By means of ultrasonic inspection, the authors showed it was possible to detect circumferential cracks in pipeline girth welds. Another example is Demčenko et al. [8], in which an ultrasonic-based inspection was performed in deteriorated concrete specimens, which were used to represent a real buried pipeline. Experimental results showed the ultrasonic technique was able to estimate the specimen thickness.

Despite the widespread use of ultrasonic techniques for pipeline inspection, the conventional pulse-echo technique can present some limitations for this specific purpose. In order to perform the inspection, a device is normally inserted into the pipeline and pushed by the flow of the fluid inside it. Attached to this device, an ultrasonic transducer has to be moved on a circular trajectory due to the fixed shape of the ultrasonic beam emitted. Thus, a different position of the transducer is required for each measurement point. Additionally, pulse-echo ultrasonic measurements are affected by the pipeline surface irregularities, since this technique is sensitive to the angle of incidence of the beam. If the angle of incidence is slightly off-normal to the surface, reduction in the intensity of the backscatter signal will occur due to the dispersion of the reflected ultrasonic beam energy [9], [10], [11].

There are inspection techniques that try to overcome the limitations of the conventional pulse-echo ultrasonic and to improve the evaluation of pipeline integrity. Common inspection techniques for this purpose include magnetic flux leakage, eddy current and radiography [3], [12], [13]. In addition, researches and commercial solutions have demonstrated the potential of laser scanning techniques for non-destructive tests, including the detection and characterization of pipeline defects [14], [15], [16], [17].

Several works related to the development of a laser profiler can be found in the literature, as presented in [18], [19], [20]. In this case, a camera is used to capture a sequence of images from a laser light beam which is expanded by an optical diffuser and then projected as a thin stripe onto the pipeline surface. The acquired images are then post-processed to analyze each projected laser stripe and to generate a pipeline profile, from which geometrical information can be extracted. Another approach that has gained attention during the last years for three-dimensional measurements is the use of hand-held laser scanners, since there are numerous commercial systems available at reasonable prices. This alternative uses a lightweight probe which can be easily moved along the object surface, allowing measurements in areas that are difficult to reach. In order to create a three-dimensional model, first a set of scans from different viewpoints of the specimen has to be made by moving the probe around the object. Subsequently, the three-dimensional data obtained in this process is merged by an alignment process called point cloud registration [14], [21], [22].

Even though the literature presents several examples of studies that perform tests with pulse-echo ultrasonic and laser scanning techniques to evaluate pipeline integrity, most of them analyze these techniques independently. The aim of this paper is to evaluate and to compare the performance of pulse-echo ultrasonic and laser scanning techniques for pipeline inspection by considering different defect geometries, thus giving straightforward guidelines for selecting one or the other method for real inspection tasks. With this purpose, a specimen was designed and manufactured to represent a section of a metal pipeline used in the oil and gas industry. In order to simulate a real damaged pipeline, defects with known geometry were artificially inserted on the inner surface of the specimen. The analysis of the results was done in two steps: quality assessment of three-dimensional models generated, and a deviation analysis by comparing the models created from the experimental data and the conventional true value of the dimensions of the specimen.

This paper is organized in four sections, as follows. The design and the manufacture of the specimen, as well as details regarding the ultrasonic and the laser scanning techniques used in this work are described in Section 2. Afterward, results concerning the quality assessment and the deviation analysis are presented in Section 3. Lastly, the conclusions are shown in Section 4.