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Fitness for Service Assessment of a Subsea Pipeline Dent


A fitness for service (FFS) assessment of a subsea pipeline dent in a spanning region identified by Magnetic Flux Leakage (MFL) in-line inspection (ILI) and Remotely Operated Vehicles (ROV) survey was carried out by TWI staff in South East Asia. The dented pipeline was located away from welds. API 579 level 1 and 2 FFS assessment was conducted showed that the dented location was not fit for service. The dented pipe caused further integrity concerns as to whether a repair or replacement was required, as any failure would cause significant consequences towards the environment and production loss. The detailed assessment was conducted using a combination of codes such as ASME B31.8 (2018), API 579 (2016) and DNV F105 (2006) to determine the integrity of the dented subsea pipeline. The API 579 level 3 assessment was performed using commercial finite element software.

The pipeline considered in the assessment is a 14-inch subsea pipeline that carried crude oil.  From ILI information and ROV survey results, a dent with a depth of 51.21mm was reported. The dent was in the middle of the free spanning, with a span length of 3m at the seabed (15.2m below sea water level). The ILI and ROV reports did not give a detailed profile of the denting, but the length of the dent was given as 560mm. No corrosion was found at the dent location (from weld to weld) in the ILI results.


API 579 Part 12 outlines the level 3 dent assessment procedures. The procedures required the plastic collapse, local failure, buckling collapse, cyclic loading, creep and creep failure modes to be considered for the dent assessment. In this assessment, the creep assessment was not conducted, as the pipeline was not operated in the creep range. The pipeline was operated under constant internal pressure. Fatigue due to operating cycle was not significant.

The pipeline dent was potentially located in a free spanning region. The cyclic loading due to in-line and cross-flow VIV and direct wave may affect the integrity of the dent. However, the free spanning fatigue assessment was not covered in API 579 procedures. Instead, the fatigue screenings in DNV-RP-F105 procedures were used to address the free spanning fatigue assessment.

Dent deformation process was conducted by using an elastic-plastic finite element (FE) approach to determine the magnitude of permanent plastic strain that had developed. A strain assessment using ASME B31.8 Appendix R was performed to validate the accuracy and conservatism of the assessment.

Figure 1. Maximum principal stress for the load case (protection against local failure) - cross section view
Figure 1. Maximum principal stress for the load case (protection against local failure) - cross section view

Results and Discussion

DNV fatigue assessment results

Fatigue screening was conducted in accordance with DN-RP-F105 rules. Three potential fatigue scenarios were assessed, and the results are as follows:

  • No cross-line VIV fatigue assessment was required
  • No in-line VIV assessment was required
  • In-line VIV and wave loading were required

Due to an unknown wave direction for spanning orientation, the worst case was assumed. Wave loading was estimated, and it was found that the wave loading was insignificant and very unlikely to cause significant fatigue damage for 20 years. As the dynamic response from environmental loads was insignificant, VIV was unlikely to occur.

API 579 and ASME B31.8 static loading assessment results.

The localised equivalent strain due to the dent was 13.3% against an ASME B31.8 allowable of 6%. In the API 579 level 3 assessment, the load case passed the plastic collapse and collapse from buckling assessment. However, acceptance for protection against local failure assessment was not proven. The maximum principal stress and plastic strain are shown in Figures 1 and 2.



The assessment results showed that the dent was not acceptable to API 579 level 3 requirements. The equivalent strain obtained from FEA was found to be unacceptable to the ASME B31.8 requirement. The DNV-RP-F105 VIV assessment showed that a dent in a 3m span was not significant with respect to fatigue.


API 579, “Fitness for Service,” American Petroleum Institution, 2016

ASME B31.8, “Gas transmission and distribution piping system,” 2018

DNV-RP-F105, “Free Spanning Pipelines,” 2006

Figure 2. Equivalent plastic strain distribution of the dent
Figure 2. Equivalent plastic strain distribution of the dent