A Client owns and operates a high pressure gas pipeline. The pipeline is 12 diameter, approximately 44km long. The pipeline is constructed from API 5L X52 grade steel and has a maximum operating pressure of 75barg. The wall thickness values of the pipeline are 6.35, 12.7 and 15.9mm.
The pipeline was commissioned in October 1999 and has been operating safely since that time.
As part of the operational procedures the Client commissioned an in-line inspection in late 1999 and then in 2000 prior to commissioning and a further inspection in 2003. The next in-line inspection was originally scheduled for 2010, with further inspections undertaken in accordance with the recommendations of the design code.
One of the requirements of the design code is that the time interval between in-line inspections should not normally exceed 10 years. However, the design code allows the time interval to be exceeded if justification can be demonstrated using risk based techniques.
In view of the above, and noting the safe operating record to date, the Client were seeking to justify extending the time interval to the next in-line inspection (originally scheduled for 2010) using a risk based methodology.
The general approach adopted by AFAA for setting the time to the next ILI comprises four stages:
Stage 1: Data Gathering
Stage 2: Run Comparison with Previous Inspections
Stage 3: Deterministic Integrity Assessment
Stage 4: Probabilistic Integrity Assessment
Based on a conservative analysis of available information using the AFAA approach, the next ILI of the pipeline was not required until 2012 in order to maintain the risks at acceptably low levels.