Pragmatic Study of Dent Restraint Conditions for ILI-Reported Dents Based on API RP 1183

Abstract The restraint condition of a dent significantly influences the fatigue life of pipelines subjected to mechanical damage. Two primary methods are commonly used to determine this condition: the clock position-based method based on dent orientation and the restraint parameter-based (RP-based) method using the restraint parameter (RP) developed by PRCI MD4-9 project. In the clock-based method, a dent is classified as restrained if it lies between 4 o’clock and 8 o’clock, and as unrestrained otherwise. The RP-based method calculates RP from the dent shape obtained via in-line inspection (ILI) caliper tools. Specifically, a dent is considered restrained if RP > 20, and unrestrained otherwise. Both methods are included in API RP 1183-2020. To evaluate effectiveness of the RP-based method, we established a comprehensive database of over 9000 dents. Each dent has a three-dimensional (3D) profile obtained from a caliper tool. These dents were collected from 20 pipelines with outer diameters ranging from NPS 6 to NPS 48, involving four ILI vendors. About 60 dents were excavated, and their actual restraint conditions analyzed and compared to RP-based method calculated from dent shape using caliper ILI.