This paper presents a case study of a hypothetical pipeline that operated without coatings or Cathodic Protection (CP) for nearly the first 20 years of its service. Metal loss anomalies with depths greater than 50% of nominal wall thickness will require excavation according to federal regulations. To reduce the number of unnecessary excavations, laboratory testing data combined with methodology from both API 579 and ASME B31.4 was applied to determine an optimized wall thickness (t′).
Based on hypothetical anomaly data considered in this paper, the optimized wall thickness resulted in a 70% reduction in the number of anomalies that required excavation, while still maintaining a safety factor of 1.39 (corresponding to 100% SMYS). This ensured that the anomalies that posed an imminent threat were targeted, excavated and repaired.
Following the good engineering practices specified in ASME and API codes, tempered by engineering judgement and some conservative assumptions, the optimized wall thickness will result not just in cost savings, but also in repairing anomalies that constitute a substantial risk to public safety.
Pipeline systems are designed to withstand various loading conditions such as internal pressure, external pressure, cyclic loadings, temperature differentials, axial force and bending moments. Reasonable tolerances are provided for unusual external conditions the pipeline may encounter in river crossings, areas of heavy traffic, long self-supported spans, unstable ground, vibration or forces from abnormal operating conditions. Some of the protective measures may include; installing casings, adding concrete protective coating, increasing the wall thickness, lowering the line to a greater depth or adding more line markers.
Wall thickness established during the design phase of a new pipeline includes tolerances such as mill tolerances, defect tolerances and time-dependent threats that compromise the pipeline’s integrity. However, after a pipeline has been in operation for several years or decades and with regular monitoring and maintenance, some of the uncertainties associated during the design phase, specifically external corrosion threat to the pipeline will be well understood. The increased wall thickness due to additional tolerances generally results in pipe operating at a lower operating stress while the remaining strength of the material is underutilized. This presents opportunity for determining a new optimized nominal thickness (t′) for a given set of operating conditions without reduction in safety and satisfying the ASME and API code requirements. As will be discussed in this paper, the optimized wall thickness will satisfy the mechanical integrity requirements and result in enormous cost savings by excavating anomalies that are truly necessary.