Shallow Dents: Updates to the UKOPA Dent Management Strategy

Abstract

Pipelines can be dented, but shallow dents with depths less than 2% of the pipe diameter have only recently begun to be reported reliably by high resolution in-line geometry inspections. Most thin-walled onshore pipelines around the world are found to contain these shallow dents, many on welds of unknown toughness, or subject to severe pressure cycling. Much of the existing guidance for dent management was published before such shallow dents were being reported, and did not necessarily consider them. Furthermore, recent failures in Canada have demonstrated that the existing guidance can be non-conservative when a shallow dent is combined with fatigue loading or other undetected damage. The United Kingdom Onshore Pipeline Operators Association (UKOPA) is developing a strategy for the management of dents to provide guidance to operators based on published best practice. The aim of the work is to ensure that dents now identified but not sized by MFL inspection tools are appropriately inspected, investigated, assessed and repaired. UKOPA’s methodology allows shallow dents to be screened and assessed without the requirement for numerous feature investigations. This management strategy is: Stage 1: Use previously published UKOPA guidance on the prioritization of dents. This involves following a series of flow charts, leading the operator from dent discovery, through decisions affecting assessment and possible repair. Stage 2: This Stage provides a series of criteria to indicate whether a weld is likely to be of sufficient toughness to withstand shallow denting, then gives a method to carry out an engineering assessment of a dent based on finite element analysis. This paper presents the background and justification of ‘Stage 2’, and updates ‘Stage 1’. It includes a review of recent published work covering dents on welds, including analytical studies, finite element analyses, testing and failures. The results of this work by UKOPA will form an input to the planned updates to the Pipeline Defect Assessment Manual (PDAM). The paper then applies the updated guidance to operational dent assessment problems provided by UKOPA members. Finally, an example of a dent assessment under the previous and updated guidance, including a finite element analysis, is given to illustrate how a shallow dent on a weld of unknown toughness may be re-categorized as not requiring repair.