Assessments That Work for You

Fitness-for-purpose assessments that provide accurate repair dates for corrosion anomalies

In a Nutshell:

Industry methods provided by ASME for assessing the remaining life of corrosion anomalies can yield results that are over-conservative. Advanced fitness-for-purpose (FFP) methods can be used to gain a better understanding of the actual failure dates of metal loss corrosion anomalies. ROSEN USA has recently used an advanced FFP method called the Plausible Profile Method (P²) to provide increased accuracy in repair dates to a client experiencing complex corrosion in one of their pipelines (in comparison to industry methods such as Detailed RSTRENG, Modified B31 G, Kastner, etc.). This method extended the remaining life of 10+ anomalies by more than three years, potentially saving the client hundreds of thousands of dollars in repairs.

The pipeline industry worldwide has witnessed an increasing emphasis on pipeline integrity management programs, particularly concerning pipelines traversing geotechnical and hydrotechnical hazards. This heightened attention stems from several notable incidents involving pipeline failures. Recognizing the importance of mitigating the risks associated with geohazard-related pipeline failures, several pipeline operators have engaged ROSEN as their integrity partner.

Effective area methods (EAM) have been widely used in the industry to determine the remaining strength of corroded pipe. Detailed RSTRENG is among the EAM that have proven to be effective for determining fitness for service of metal loss anomalies in pipelines. However, Detailed RSTRENG may provide over-conservative results when estimating the profile of the corrosion cluster if the anomalies are circumferentially elongated. This article highlights how a more detailed evaluation of local conditions using the Plausible Profiles Method (P²) can potentially improve the relevance of the effective area used in the remaining strength evaluation of such anomalies, resulting in a more favorable calculated failure pressure.

P²Methodology in comparison to detailed RSTRENG

In determining if a corrosion anomaly is fit for purpose, one of the most important considerations is how the profile of the anomaly is used to estimate the effective area for integrity assessments. The Detailed RSTRENG method uses a “deepest-to-deepest” path approach to estimate the profile of the corroded area within a corrosion cluster. This single profile is then used to deterministically calculate burst pressures of the corrosion cluster and calculate a date of failure. In the P² approach, numerous plausible profiles of the corrosion cluster are generated, and burst pressures are calculated for each of them. Then, the 5th percentile of these burst pressures is taken to be the failure pressure at a specified instance in time, which can be a date prior to their next planned ILI or a certain target date. Note that the calculations between RSTRENG and P² are the same, and the difference between the methods is how the profile of the corroded area is estimated. The P²method was found to yield burst pressure results closer to in-field results when compared to Detailed RSTRENG.

However, the P²method does have a few limitations, which include:

Little/no benefit for axially long and narrow anomalies
Anomalies that fail by depth rather than pressure
No benefit to individual anomalies not part of a cluster
Several iterations may need to be run to determine the burst pressure of the anomaly

In Figure 1 below, the Detailed RSTRENG method is shown (Image a) in which the “deepest-to-deepest” path approach is taken in estimation of the anomaly profile. Image b below shows numerous plausible profiles generated for the same cluster. Image b depicts just 3 “plausible” profiles, but in reality, there can be hundreds depending on the size of the corrosion cluster.

Figure 1: River Bottom Profile methodology (a) compared to Plausible Profiles methodology (b)

When evaluating these complex corrosion clusters, the first step is to build matrices based on the ILI box data assessment depths (reported depth + tool tolerance) of the individual anomalies within a cluster. In the images below, an example matrix model used in the P² assessment is shown (top) and compared against the box data from the ILI run (bottom).

Figure 2: Matrix model (top) aligned with ILI signal data (bottom)

As observed in Figure 2 above, the cluster matrix model in the top image corresponds to the boxed signal data from the ILI. In both the top and bottom images, dark red areas indicate deeper metal loss anomalies, while lighter red or yellow areas represent shallower anomalies. Once the matrices are built and assessment parameters decided upon, the P² method is then iteratively carried out in order to determine the burst pressure and the failure date of the cluster.

Case study

A client in the U.S. was experiencing complex corrosion in 1 of their pipelines and requested ROSEN to perform a standard FFP assessment. Upon delivery of the assessment result, the client became aware that they had many corrosion anomalies coming up for repair in the near future when evaluated with traditional industry assessment methods. In order to further support this client, ROSEN suggested the anomalies be assessed with the P² method to gain a better understanding of the anomalies in question.

Plausible Profile assessments were performed on 13 corrosion clusters. The lLI was run in January 2023, and all 13 of these anomalies originally had repair dates prior to 2025. With the implementation of the P² method, ROSEN was able to analyze these corrosion clusters with increased accuracy and provide an updated repair date to the client, extending the remaining life for all 13 clusters assessed. This can be seen in the figure and table below.

Figure 3: Repair dates before and after P2 assessment was performed

In Figure 3 above, the red bars represent the repair dates of the clusters when assessed with industry standard methods (i.e., Detailed RSTRENG, Modified B31G and Kastner). The blue bar represents the same 13 clusters when assessed with the P² methodology.

Following the standard integrity assessment process would have resulted in hundreds of thousands of dollars in repairs for this client. By extending the repair times of these 13 clusters, the client avoided unnecessary repairs, optimizing their maintenance campaign efforts for the current and upcoming years. This method is now considered a conceivable desktop solution for extending the remaining life of qualifying anomalies that fail traditional metal loss assessment methodologies.

References

[1] Technical Report, Plausible Profiles (Psqr) Model for Corrosion Assessment, Authors S. Kariyawasam, S. Zhang, J. Yan, T. Huang, M. Al-Amin, E. Gamboa, Final Version 03, September 23, 2019.

[2] ASME B31G, “Manual for Determining the Remaining Strength of Corroded Pipelines,” 2012.

[3] Peer Review of the Plausible Profile (Psqr) Corrosion Assessment Model, Project Number EC‑2‑9, Prepared by John Kiefner et al., Final Version, August 9, 2019.