Markus Büchler, SGK, Switzerland, and Mark Glinka, EMPIT GmbH, Germany, draw conclusions from measurements and assessments of different coating integrity techniques on buried pipelines, namely the newly developed current magnetometry inspection.
For the assessment of external corrosion protection, coating integrity evaluation methods are often carried out on buried pipeline systems. In contrast to the commonly used direct current voltage gradient measurement (DCVG), the newly developed current magnetometry inspection (CMI) offers significant advantages. For the operator, the question arises regarding the comparability of the methods. Therefore, measurements were carried out with both DCVG and CMI to draw conclusions about the transferability of the respective measurement results. These results and an evaluation are discussed in this article.
Buried pipelines are usually protected from integrity-relevant wall thickness reductions with external corrosion protection consisting of a combination of a coating and cathodic protection (CP). The effectiveness of CP must be demonstrated based on ISO 15589-1 by means of the measurement of the IR-free potential (EIR-free) on individual coating defects by means of the intensive measurement (IM) that is based on a combination of a close interval potential survey (CIPS) and a direct current voltage gradient (DCVG). Additionally, the requirements of the ISO 21857, with respect to stray current interference and the ISO 18086 must be taken into consideration. The task now is that the determination of the EIR-free potential on modern pipelines is difficult due to the high quality of the coating and the associated small coating defects. The small voltage gradients usually prevent the calculation of the EIR-free on modern pipeline systems. As a result, demonstrating compliance with normative and legal requirements is often complex. This is further complicated by the often increased AC and DC interference.1
Given these difficulties with respect to the assessment of the EIR-free, a DCVG survey is often performed with increased potential swing (i.e. at a significantly more negative on potential) to demonstrate the absence of major coating defects on the external pipeline coating. By excavating and re-coating, a direct proof of the corrosion state and the restoration of the local defect-free condition is achieved. It is observed that only in very few cases the excavation of the thus identified coating defects was actually necessary from an operational perspective. In most cases the exposing of the coating defect reveals that CP was effective and that the metal at the coating defect did not suffer from active corrosion. Therefore, the assessment of pipeline integrity is often carried out using inline inspections (ILI) with magnetic flux leakage (MFL) or ultrasonic (US) measurement. These provide direct information on wall thickness reduction and are therefore able to deliver optimised integrity assessments. Correspondingly they reduce the number of unnecessary excavations.
However, ILI can only be applied on appropriately designed pipelines in the transmission network. In the distribution network or for unpiggable lines, no alternative methods exist for fault location and excavation to date. The technology developed in Germany by EMPIT for evaluating pipeline integrity based on CMI now offers an alternative and innovative possibility for a more in-depth assessment of pipeline integrity and the effectiveness of corrosion protection.
The corrosion protection by means of CP is based on electrical currents. These can be directly detected with electromagnetic measurements, which makes it possible to precisely locate coating defects. There is no need to increase the potential swing by shifting the on-potential (Eon) to more negative values. Therefore, it is possible to precisely locate coating defects without increasing the risk of alternating current corrosion and coating disbondment due to increased cathodic polarisation. Furthermore, the method fundamentally offers the possibility to capture additional relevant parameters, which are necessary for evaluating effective corrosion protection at each coating defect. Thus, the procedure has the potential for a comprehensive description of the corrosion-relevant characteristics of a defect. This article presents the measuring principle as well as field experiences with its application…
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