PIPELINE INTEGRITY ASSESSMENT


INDUSTRY NEWS


Pipeline Region (as per Fig. 2)


Region 1 Region 2 Region 3 Region 4 Region 5


Example: 0 deg inclination


was a factor within the pipeline to some extent, perhaps caused during hydrotest and/or wet lay-up.


Step 5 – Determine ongoing corrosion rate


The operator’s ongoing corrosion rate of 0.26 mm/yr was deemed overly conservative. Instead, an ongoing corrosion rate of 0.10 mm/yr was selected for subsequent fitness-for-service (FFS) calculations, based on the various interpretations of the available inspection data.


Step 6 – FFS assessment Maximum safe operating pressures were calculated for all of the recorded MLFs


Estimated Water Layer Height (m)


0.21 0.24 0.23 0.23 0.25


0.005 Table 4: Flow modeling results.


from the year 6 ILI based on their predicted sizes after an additional 25 years of operation (for 30 years total life). At an average corrosion rate of 0.10 mm/yr, the MLFs would have increased in depth by 2.5 mm, and length by 5 mm - a conservative assumption of feature length increase being 2x the depth increase. The results of the FFS assessment are shown in Figure 2.


Assuming the alternative constant ongoing corrosion rate of 0.1 mm/yr, it is clear that the pipeline would be fit for service by the end of its design life, not only for the normal operating pressure of 17 barg, but also for the de-rated pressure of 30 barg.


Step 7 – Recommendations A number of recommendations were made to the operator, including no requirement for immediate remedial activities, further pipeline de-rating, or repair, as corrosion rates were not as severe as first thought. It was also recommended that chemical injection philosophies did not need to be modified and monthly pigging should take place to alleviate water hold-up and disrupt biofilm.


Conclusion


This case study demonstrates how a thorough pipeline integrity assessment can be performed when operating data is lacking and highlights issues when


making operational decisions based on a simplified review of inspection results.


All available inspection data must be examined carefully to consider a range of possibilities, the validity of which can then be investigated. This assists in the calculation of realistic ongoing corrosion rates, and subsequently more cost-effective operational decisions and planning.


Ultimately, the detailed pipeline integrity assessment allowed the operator to save considerable costs regarding ongoing operational practices. Equally as important, they were reassured that corrosion rates were not as severe as first thought and that the integrity of their pipeline was not in jeopardy.


By Joseph Morris, Steve Hodges and Gareth John of Intertek


References 1. B. F. M. Pots et al, “Improvements on de Waard-Milliams Corrosion Prediction and Applications to Corrosion Management,” CORROSION/2002, paper no. 235 (Denver, CO: NACE, April 2002). 2. G. H. Booth, A. W. Cooper, P. M. Cooper, “Rates of Microbial Corrosion in Continuous Culture,” Chemistry and Industry 49, (1967): p. 2084.


© NACE International 2014


Figure 2: Maximum safe operating pressures of internal year 6 ILI recorded MLFs for their predicted condition after 30 years of service.


www.internationalmetaltube.com


This is a condensed version of the full paper presented at NACE CORROSION 2014, paper number 4050.


IMT August/September 2014 19 17


Estimated Water Wetted Perimeter (m)


0.66 0.73 0.72 0.72 0.76 0.10


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