FATIGUE MONITORING

Pressure Cycling Monitoring is a vital part of a Cyclic Fatigue Management Program

Pressure cycle fatigue is a major risk factor for pipeline in cyclic service. Fatigue due to cyclic pressure loading is the primary driver to cause seam welds to grow to  critical size as well as to cause cracking to develop and grow to critical size on dents. Fatigue and environmental corrosion are the drivers for stress corrosion cracking growth.

An effective fatigue management program needs to be built upon an effective baseline analysis that will assess the cracking threat and predict remaining life values due to fatigue crack growth and then a program to monitor pressure cycling to ensure that the pipeline is operating within those safety windows.

Yet often fatigue is only screened using qualitative assessments of pressure spectrums that do not provide helpful or often even correct assessments of the amount of pressure screening. The remaining life for dent and seam weld anomalies are directly proportional to the amount of fatigue loading. Therefore relying on qualitative analyses to classify pipelines into arbitrary aggressiveness categories is doing a disservice to effective pipeline integrity management.

This is easily remedied by utilizing the pressure data that is being gathered by SCADA systems to perform quantitative analyses. This process can be automated by integrating SCADA historians with engineering software that performs rainflow and equivalent cycling counting.

The equivalent cycling results are a simple but powerful tool that not only provides data for engineering pressure cycle fatigue analyses but also many other inputs into an integrity management program.

  • Equivalent cycle indices can be trended on a monthly basis to monitor the amount of fatigue loading. Changes in loading can be flagged to allow for either pipeline operations to be modified or inspection intervals reviewed to ensure safety margins are maintained.
  • Equivalent cycle indices are a valuable input to risk models. The values are directly proportional to remaining life values and vastly improve the understanding of risk versus qualitative aggressiveness categories.
  • Equivalent cycle indices can be used in threat assessment as low values would indicate remaining life values that may vastly exceed the pipeline life. A quantitative assessment provides an authoritative process for threat assessment.

Here are a few elements to keep in mind when considering cyclic fatigue:

  1.  Fatigue loading is proportional to the absolute pressure changes and not relative pressures. This means that if there are two pipelines operating  at 72% SMYS, with all other conditions being the same, the higher grade pipeline will have shorter fatigue life for flaws.
  2.  For liquid pipelines pressure decays impact the amount of pressure cycling. Anomaly location matters as the proximity to a pump station is a  major factor in remaining life results.
  3.  A complex pressure spectrum can be converted into equivalent cycles to make analysis of cyclic pressure loading easier. It is only one more step  to turn the equivalent cycles into an index value that can be used to trend and compare pipelines. Cyclic index values can be implemented into  either qualitative or quantitative risk models.

Whether you are developing or fine tuning your cyclic fatigue management program the implementation of pressure cycling monitoring, when tied in to a fracture mechanics based baseline assessment, will play a vital role in maintaining the integrity of the pipeline for cracking threats.

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Integrity without knowledge is weak and useless, and knowledge without integrity is dangerous and dreadful

– Samuel Johnson