ULTRASONIC IN-LINE INSPECTION : HIGH RESOLUTION CRACK DETECTION FOR PIPELINES USING A NEW GENERATION OF TOOLS
More than three million kilometers of high pressure liquid and gas pipelines are installed all over the world. Generally steel pipelines provide the safest means to transport large quantities of oil, oil related products and natural gas. However, just like any other technical component, they can deteriorate. As a result flaws can appear and grow until the pipeline fails. The appearance of cracks, especially Stress Corrosion Cracking is of increasing concern. These flaws must be identified, before they endanger the integrity of the line. This paper will introduce a new generation of high resolution inspection tools applying ultrasonic technology to detect, size and locate cracks and crack-like defects in the body and welds of transmission pipelines. These devices are pumped through the section of pipe to be inspected together with the medium being transported therein. They enable an inspection of the entire circumference and length of the pipe of up to several hundred kilometers in a single run. The paper will describe the non-destructive testing principles applied and introduce the major components of the inspection system, including the tool and defect specifications. The paper will also explain the data analysis process, including examples of data recorded by the tools. Introduction: It is generally accepted that high pressure pipelines provide the safest means to transport large quantities of oil and natural gas. It must be understood, however, that pipelines are exposed to a variety of environmental influences and loading conditions. As a result flaws can appear and grow. One challenging category of flaws are cracks and crack like anomalies such as stress corrosion cracking or fatigue. In order to ensure the integrity of a line cracks must be detected, sized and assessed, before they reach a critical dimension which can eventually lead to a failure of the pipeline. Specialized inspection tools, usually referred to as in-line inspection tools or intelligent pigs, have first been introduced nearly 40 years ago to detect and locate pipeline anomalies such as dents and metal loss. From technical generation to generation these inspection tools have become more refined and sophisticated, incorporating better and better defect specifications. Present day tools have reached a level of sensitivity and accuracy that allows the inspection data obtained to be utilized for advanced fitness-for-purpose calculations and integrity assessments. Detailed information on the history, physical principles used, technologies, tools and vendors is available in the literature [1-4]. Ultrasonic In-Line Inspection Tools: Ultrasonic tools for the in-line inspection have first appeared in the early 1980`s. The mission of these tools was the detection and sizing of metal loss. Crack inspection tools first appeared in the middle 1990 ́s . The major advantage of ultrasonic tools is their ability, unlike the competing magnetic flux leakage tools, to provide quantitative measurements of the pipe wall inspected. Experience has also proven that only the use of ultrasound provides a reliable means to detect cracks and crack like anomalies in pipelines [5]. This paper will focus on a new generation of ultrasonic crack detection tools. Ultrasonic inspection tools are in general fitted with a sufficient number of ultrasonic transducers to ensure full circumferential coverage of the pipe. They work in a pulse-echo mode with a rather high repetition frequency. Straight incidence of the ultrasonic pulses is used to measure the wall thickness and 45o incidence is used for the detection of cracks. While the wall thickness measurement is more or less established, in-line crack detection of pipelines has only been available for some years now. In terms of data processing, ultrasonic tools represent one of the most challenging tasks in ultrasonic non-destructive testing. Several hundred sensors have to be controlled, their echoes have to be recorded, on-line data processing has to be able to reduce the amount of data recorded and ensure that all relevant data are stored. The inspection speed of the tool depends on the medium and may vary within a certain range. The inspection process has to be fully automatic and cannot be supervised during a run. The data are stored on solid state memories that are the safest and most reliable means of storing data in such a hostile environment. The distance is measured using several odometer wheels. Hardware Introducing The Tools: Some design elements have been proven successful in the past and have been maintained in the new generation. Others have been completely modified. Figure 1 shows a photo of the newly developed tools in a 24" configuration. Figure 1 High Resolution Ultrasonic In-Line Inspection Tool for 24" pipelines. The tool is made up of two pressure vessels housing batteries, electronics, recording devices and a trailing sensor carrier housing the ultrasonic transducers. The tool is made up of a number of pressure vessels and a trailing sensor carrier. The front of the tool is covered by a protecting bumper unit, covering the transmitter housing shown here. The individual pressure vessels are connected through universal joints which allow the tool to negotiate bends. The first body contains batteries ensuring a safe supply of power to the tool for up to several days. The electronic and recording unit of the tool is housed in the second vessel and incorporates enough channels to cover pipeline diameters from 20" to 42" for crack detection application. The sensor carrier is made of polyurethane and houses the ultrasonic transducers. To adapt the tool to a different pipe diameter the polyurethane cups are exchanged, which can be done quickly. Tools sizes to cover pipeline diameters below 20" are also available, the current smallest diameter for crack detection being 10". In general there are two different sensor carrier designs for ultrasound tools. One for wall thickness or corrosion measurement and one for crack detection. The crack detection version, shown here, contains sensors orientated at a predetermined angle to the pipe wall which ensures that ultrasonic shear waves will travel under a 45o-angle within the metal. Figure 2 shows the arrangement of the ultrasonic sensors for crack detection. In order to avoid any blind zones in the vicinity of the longitudinal weld half the sensors are mounted in a clock wise orientation and the other half in an anti clockwise orientation. All sensors are mounted on metal plates. The polyurethane sled will ensure a constant stand-off. Figure 2 The assembly for the crack detection sensors The design will also have to ascertain that wax cannot clog the exposed parts of the sensor carrier. In the crack detection mode for this given diameter there will be 360 inclined sensors corresponding to a sensor pitch of 10 mm for either direction (clockwise and counter clockwise), together with 24 sensors for wall thickness measurement. These latter sensors are needed for the detection of girth welds, allowing a precise location of the defects. The sensor spacing is designed such that every part of the pipeline is covered by several sensors. Figure 3 shows the configuration of the ultrasonic sensors on the body of the sensor carrier for a crack detection sensor carrier.
Internal Inspection Device for Detection of Longitudinal Cracks in Oil and Gas Pipelines: Results From an Operational Experience
Pipetronix has developed a new generation of internal inspection device for the detection of cracks in pipelines. Since its commercial introduction in October 1994 the tool, UltraScan CD (Crack Detection), has successfully inspected nearly 1,000 km of operating oil and gas pipelines. The runs were followed by verification excavations at specific locations selected from the collected data. Comparison of the findings with corresponding verifications from excavations illustrate the sensitivity and reliability of the inspection method and the ability of the tool to accurately discriminate between different types of defects. The performance has proved the UltraScan CD to be a reliable internal inspection device for the detection of cracks (SCC, fatigue and other crack like defects) on pipelines. As a result, the German TUV (authorized by the German Government to verify the compliance with Regulations covering the operation and inspection of pipelines) has approved the use of UltraScan CD as a substitute for hydrostatic pressure testing of pipelines.Copyright © 1996 by ASME
Chemical Metallurgy: Principles and Practice
Foreword. Preface. Acknowledgements. Appreciation. 1. Acquaintance. 2. Mineral Processing. 3. Thermodynamics and Kinetics. 4. Pyrometallurgy. 5. Hydrometallurgy. 6. Electrometallurgy. 7. Energy and Environment. Subject Index.