Atteris performed defect assessments using the Pipeline Defect Assessment Manual (PDAM) and in accordance with DNV RP F101 to determine the requirements for repairs. Assessment of the defects was complicated by the fact that corrosion was occurring close to a weld location in an HDD section where the defect was externally inaccessible to allow verification by UT measurement. MFL sensor “bounce” due to passing over the weld bead was believed to be affecting the accuracy of the defect measurement, leading to a requirement to adopt a risk-based approach to determining the requirement and urgency of performing a repair.
One of the defects assessed had a reported wall loss of 74% with an accuracy of +/-15%. Taking into account the measurement tolerance the defect potentially exceeded the DNV RP F101 applicability limit of 85% wall loss. Through consideration of multiple sensitivity analyses and evaluation of the consequences of a leak (noting that the transported water had negligible hydrocarbon content), Atteris demonstrated to the clients and regulators satisfaction that repair of the defect could be postponed pending re-evaluation following future pig runs to assess whether the defect depth had stabilised or was continuing to grow.