High Precision With Ultrasonic Pigging

Being subject to appropriate inspections, pipelines and piping/pipework offer an almost unlimited service life. Strength and actual service capabilities must be assessed at regular intervals. Especially for thin-walled pipes made of high-strength steel, it is of particular importance to conduct regular inspections and examinations to detect any flaws and defects before they actually start to cause problems. Here, ultrasonic and magnetic flux leakage (MFL) pigs have been established as proven and suitable tools for inspection applications.

The utilization of these technologies requires interdisciplinary expertise and know-how. Bearing in mind that such “inspections” shall provide reliable measurements for the assessment of a pipeline‘s condition, several aspects related to material quality, mechanical properties and corrosion conditions must be considered.

The target to obtain reliable results for the establishment of the best techno-economical concept for a pipeline’s service life and condition makes it indispensable to have a broad range of engineering disciplines and experience involved. This involves raising the necessary questions on starting to assess a pipeline’s operational condition, applying the appropriate inspection technology, and the interpretation of recorded data on site (preliminary report) in detail based on determined threshold values (final report), as well as assessment of individual results. Finally, a reliable and safe concept for proper operation and/or rehabilitation has to be developed. This approach will be described later on a crude oil pipeline on which TÜV SÜD provided services for a well-known energy supplier.

Reliability of pipelines is of paramount importance in the energy sector. Verification of the integrity and strength of buried pipelines is aimed at preventing unacceptable defects from occurring on pressurized parts, which under continuous operational loads, may result in leakage or fractures causing costly and unscheduled service interruption.

Assessment Of Defects
Anomalies such as wall thinning which affect the safety and integrity of a pipeline are referred to as "direct" defects. By contrast, indirect defects such as damage to the insulation of a buried pipeline do not involve any direct hazards, although they may evolve into direct defects over time, such as corrosion leading to wall thinning.

Corrosion in welded area.

Thinning of a pipe wall results in increased stresses in the base material, which, in turn, may limit the service life of the pipeline. Wall thinning is defined as a reduction in wall thickness on the inner or outer wall surface of the pipe, which may have been caused at an early stage during metal sheet manufacturing or later during pipeline operation. It may lead to leakage caused by corrosion or to cracking as a result of material fatigue or because maximum stress limits are exceeded. Wall thinning is frequently caused by milling in the course of pipe production.

Stress analysis is based on the assumption that the actual load-bearing capacity and fatigue strength of a defect can be determined by means of a strength analysis. By taking into account the consequences of dynamic operations such as pipeline startup and shutdown, conclusions can also be drawn as to the remaining service life of pipelines with these defects.