| Popis: |
Dents in buried pipelines either caused by third party mechanical damage or introduced during pipeline construction remain a leading contributor to reportable pipeline releases. API Recommended Practice 1183 (API RP 1183) provides a modern, shape-based fatigue life assessment of pipeline dents that can be incorporated into a pipeline operator’s integrity management program. Specifically, in API RP 1183, dent shape information is obtained by analyzing in-line inspection (ILI) caliper data and is expressed using characteristic lengths and areas. Once obtained, these characteristic lengths and areas define not only the dent restraint condition, but also various fatigue growth parameters. API RP 1183 provides multiple screening techniques that are intended to identify dents that are non-injurious and therefore do not require additional detailed assessments or response actions. These screening techniques both increase in complexity and decrease in conservatism. Three screening processes provided in API RP 1183 are: (1) a table with lower bound, conservative estimates of fatigue life, (2) a pipe geometry and spectrum severity indicator (SSI) approach, and (3) a pipe geometry and operational spectrum approach. When combined with the shape-based fatigue life assessment, multiple analysis approaches are described. However, as more dents are being analyzed with the methods from the first edition of API RP 1183, discrepancies between the screening methods and the shape-based approach are being observed. The aim of this paper is to discuss those cases where the conclusions from the screening processes and the shape-based assessment are inconsistent. In other words, there are cases where the screening process “passes” a dent indicating the dent is non-injurious and can be monitored while the shape-based assessment dictates the dent is injurious and a response should be taken. After discussing the inconsistencies in these cases, the authors make recommendations on how operators should use API RP 1183 in its current form. |
