Assessment of fatigue of a subsea pipeline including analyis of dents
A major operator was planning to re-develop two of their existing fields in the United Kingdom continental shelf (UKCS). It was anticipated that the re-development would result in greater production yields and increase the life of the field. Two import/export trunklines transported produced gas and, as a result of the re-development, the throughput in both pipelines was expected to increase, resulting in higher operating pressures.
The operator asked us to perform a study to determine the fatigue lives of the two pipelines at the current and higher operating pressures. Furthermore, a previous anchor strike to one of the trunklines had resulted in damage to the external coating, raising concerns that the damage could extend to the steel pipework. We were asked to include this in our analysis and determine what effect any potential structural damage would have on the fatigue life.
We initially determined the fatigue lives of both pipelines based on operational pressure data collected since commissioning. Fatigue damage was calculated to DNV-RP-C203 using the rainflow method, with the subsequent remaining fatigue life calculated in accordance with DNV-OS-F101. The forecast operational pressure data was then re-scaled to represent the higher maximum allowable operating pressure (MAOP), allowing the same calculations to be re-run using the re-scaled pressure data.
Additional analysis was required to assess structural damage caused by the anchor strike. No definitive sizing of the existing damage had been performed so a number of sensitivity checks were conducted for a range of assumed dent sizes. The fatigue damage was calculated using Fowler's method and repeated for the range of assumed dent sizes. Fatigue lives were then calculated using DNV-OS-F101.
Our analysis confirmed that the predicted fatigue lives of both pipelines at the increased operating pressure exceeded their respective design lives.
Our analysis highlighted that pipeline fatigue life is significantly affected by the presence and size of a dent. The maximum dent depth for the new pipeline MAOP meant that the remnant fatigue life exceeded the design life. On this basis we recommended that our client review and accurately size the existing dent to allow for comparison against the dent sizes assumed in the study.
We also confirmed that it was unlikely that the historic dent from the anchor strike posed an immediate threat to the integrity of the pipeline.
As a result of our analysis, our client was confident it was safe to operate both pipelines at the increased operating conditions. Subsequently, the client was able to progress with the field re-development project with the technical assurance that the integrity of existing subsea infrastructure would not be compromised.