Assessment of the Remaining Strength Factor and Residual Life of Damaged Pipelines

Abstract

The transmission pipelines intended for the transportation of hydrocarbons (petroleum, liquid petroleum products, natural gas, liquefied petroleum or natural gas) or other fluids are subjected, during their operation, to corrosion processes that may generate on the steel pipe surface local metal loss flaws/defects (also named volumetric surface defects, VSDs). The evaluation of these defects represents an important issue regarding the pipelines exploitation and maintenance. Typically, such assessment aims at the evaluation of the remaining mechanical strength (that can be expressed most conveniently by the remaining strength factor, RSF) of the damaged steel pipes. Sometimes, for pipelines susceptible to low cycle fatigue failure in the stress concentration areas such as VSDs, their residual life should also be assessed. Both these issues are covered by the present chapter. First, the pertinent modalities for the definition and determination of RSF for the transmission pipelines on whose steel pipes volumetric surface defects/flaws have been detected are presented and discussed. In the case of hydrocarbon transmission pipelines containing flaws of the local metal loss type, we consider that it is important to assess their remaining strength not only in relation with the attainment of the serviceability limit state (as defined in international Norms presently used for such assessment), but also in relation with the achievement of an ultimate limit state (pipe failure/bursting). Therefore, the modalities proposed by the authors for the evaluation of a RSF related to the attainment of an ultimate limit state of the pipeline (bursting) are also briefly presented. Second, the results of some cyclic hydraulic tests performed on a full-scale pipe sample with VSDs are described. Their main goal was to define a modality to evaluate the residual life of a pipeline by taking into account the features of the relative position of the defect with respect to the weld seam of the steel pipe.