The materials, stress state, and environment all synergize to contribute to the development and growth of cracks. With fatigue cracking, factors such as manufacturing flaws, stress cycles, and environment can all also have a major impact. Managing that threat and demonstrating fitness for service represent significant challenges.For Stress Corrosion Cracking (SCC),
In these situations, you can never have too much information. Anything that contributes to understanding the cause and severity of cracking helps to define appropriate methods for safe and efficient threat management.
In-line inspection, of course, is a great source of information. The advent of Ultrasonic Testing (UT) and Electromagnetic Acoustic Transducer (EMAT) crack-detection tools has revolutionized the management of cracking threats in recent years by enabling operators to identify and repair significant cracks. The contribution of other inspection technologies, such as material property inspections, geometry inspections, and metal loss inspections, can also provide substantial additional valuable information. However, all inspection systems have limitations; they cannot provide direct information on the history of operation (pressures, temperatures, products, etc.), they cannot give information on manufacturing and construction history and they cannot detect the local environment. Plus, it goes without saying, there are limitations on what can be detected and on accuracy.
Therefore, collecting additional data is vital to a comprehensive integrity management program. Cutting out samples of pipe, particularly with examples of defects present, can provide an enormous amount of valuable additional information of the kind that is extremely hard to generate from internal or in-the-ditch inspections or historical records.
Let us consider the example of an operator that experienced a failure due to a pipe body axially orientated crack. The cracking mechanism at the failure location was confirmed to be SCC. Managing the threat safely, efficiently, and effectively became their top priority.
Following two extensive inspection campaigns using multiple internal inspection technologies, including Electromagnetic Acoustic Transducer (EMAT), Magnetic Flux Leakage (MFL), and geometry mapping tools, many features were reported, verified in-ditch, and remediated.
Operators already had a robust crack threat management process in place, they were aware of the need to gain a deeper understanding of the contributing factors and demonstrate to stakeholders that the approach they were taking was conservative. Therefore, instead of installing structural repairs, they are currently removing the pipe joint for further analysis and sending it to a testing facility for the following purposes:
A condition assessment, material testing program, and burst test followed by an in-depth post-test analysis in a laboratory environment were proposed to provide confidence in the physical and causal attributes of the feature.
The deepest crack-like feature is confirmed by magnetic particle inspection (MPI) and characterized as a longitudinal and transverse colony.
The depth of the peak is measured using PAUT and determined to be consistent with existing deformation bands due to cold field bending.
A 3D laser scan is performed to generate a representative model of the actual spool geometry for finite element analysis simulations. This is intended to have a complete set of geometric data for the pipe, recognizing that very few pipes are actually straight and round. In addition, it will be possible to identify minor local stress variations or any other geometric features that may contribute to the formation of SCC at specific locations.