An Exacting Wall-Penetration Pipe Analysis

Abstract

Existing piping systems sometimes need to operate at larger mechanical loads than they are originally designed to. Often, these beyond-the-original-design loads lessen the margin against piping failure so much that performing recalculations using original analysis methods cannot qualify the pipeline. To demonstrate suitability of the existing pipeline to increased loads and to avoid piping hardware modification, more advanced analysis methods need to be employed. In this paper, a single detail from the beyondthe-original-design analysis [1] of a buried thin-wall water pipeline in nuclear power plant (NPP) Krško is shown. That analysis, the description of which is beyond the aim of this paper, demonstrated that the most critical part of the system is the penetration of the buried pipeline into a power plant’s concrete building. Namely, because of a supposed beyond-theoriginal-design earthquake, relative displacements between the building’s basement wall and the surrounding soil evolve, which in turn imposes large deformation on a buried pipe at the site of the building wall’s penetration. The most significant mode of deformation is pipe denting at the pipe-to-concretewall seal location. Pipe denting strongly influence pipeline’s strength and fatigue life; therefore, the topic was addressed by researchers in the past [2] and [3] and is still attractive today [4] to [6]. The solutions to the problem are in the form of a pure theoretic analysis [2], or as guides extracted from recapitulation of many experiments [3], or, recently, from experiments supplemented with numerical analyses [4] to [6]. All researchers agree that the excessive size of the dent, the abrupt change of the dented surface curvature and the pre-existing pipe damage on the dent spot severely lessen pipe strength and fatigue life. Because the denting is a problem still under research, it is not included in piping structural design codes yet, therefore leaving the structural engineer on thin ice. The aim of this paper is twofold: describing the experimental-numerical analysis of the dented section of a pipeline at NPP Krško and demonstrating the pipeline qualification with the applicable American Society of Mechanical Engineers (ASME) Code [7] (which is the obligatory code for NPP Krško), although no rule regarding dents is given in [7]. An Exacting Wall-Penetration Pipe Analysis Koc, P. Pino Koc* University of Ljubljana, Faculty of Mathematics and Physics, Slovenia