Deterministic Assessment of Minor Mechanical Damage on Pipelines Available to Purchase

In-line inspection (ILI) of the Trans Alaska Pipeline System (TAPS) using high resolution metal loss and caliper tools indicated 77 locations with suspected minor mechanical damage features (MDFs). The tools used are able to detect the presence of a suspected feature, and measure indented dimensions, but are insufficient to detect the presence of cracks or gouges needed to reliably assess feature severity based solely on the ILI data. Excavations of 42 sites deemed most severe provided important field data characterizing residual deformation dimensions, revealed the occurrence of generally surficial gouges or cracks, and allowed a reliable field assessment of defect severity. Upon completion of the excavations, 35 possible MDF locations remained unexcavated. An engineering evaluation was undertaken to assess whether or not the remaining MDFs pose a threat that is significant enough to warrant excavation. Multiple assessment methods were utilized including deterministic, probabilistic, and risk assessment methods. A deterministic mechanics model was developed to estimate the safe operating life of the pipeline at each of the remaining uninvestigated locations considering the characteristics of previously observed damage, the perceived severity of uninvestigated damage, material properties of the pipe, and the fatigue environment resulting from expected modes of pipeline operation. The results strongly suggested that 33 of 35 damage features were extremely minor, with remaining life well in excess of the remaining project life cycle. None of these features were judged to threaten the immediate integrity of the line, and are unlikely to do so in the foreseeable operating life of the facility. The results also were found to support the outcome of the operator’s risk-based evaluation process.