Spyros A. Karamanos

Finite Element Analysis of the Mechanical Behavior of Mitered Steel Pipe Elbows under Bending and Pressure

Using finite element simulation tools, which account for both geometric and material nonlinearities, the bending capacity of mitered steel pipe bends is investigated. First, numerical results are reported on the elastic behaviour of two steel typical grade 40 steel mitered elbows with D t values equal to 192 and 240, with the purpose of calculating flexibility and stress intensity factors. Furthermore, the elastic-plastic response of the two bends is also examined towards identifying their ultimate capacity and the corresponding mode of failure. Internal pressure effects on the structural response are also examined. The present paper addresses some important features of mitered steel pipe elbow response that may arise in geohazard areas, towards safeguarding the structural integrity of steel water pipelines subjected to severe ground-induced actions. INTRODUCTION The present paper is motivated by the need for determining the deformation capacity of welded steel pipelines for water transmission, constructed in geohazard areas. In such areas, e.g. areas with significant seismic activity, the pipeline can be subjected to severe permanent ground deformations, resulting from fault rupture, liquefaction-induced lateral spreading and subsidence, or landslide action. Under those extreme loading conditions, the pipe deforms well beyond the stress limits associated with normal operating conditions, whereas the structural performance of welded joints constitutes a key issue for pipeline structural integrity (Lund, 1996; O Rourke and Bonneau, 2007). In such geohazard areas, the main action on the pipeline is bending loading. Previous work on the structural performance of steel pipe elbows has mainly focused on “smooth” elbows, fabricated mainly through “induction bending”, which are widely used in industrial piping applications (refineries, chemical industries or nuclear plants). This research has shown that smooth elbows exhibit a unique structural behaviour under structural loading, characterized by significant flexibility and stress concentration, and therefore, they may constitute “weak spots” of the piping system in the course of a severe seismic event. For an overview of smooth elbow mechanical behaviour, the reader is referred to the review paper by Karamanos (2016). On the other hand, the response of mitered elbows has received much less attention. It is worth mentioning the experimental-analytical work presented by Gresnigt (2002), as well as the review paper by Wood (2008), which offers an overview of the mechanical behaviour of mitered bends under various loading conditions. Pipelines 2016 1255

Mechanical Response of Steel Pipe Welded Lap Joints in Seismic Areas

Nonlinear finite element simulation tools are employed to investigate the bending capacity of internally-pressurized double-welded lap pipeline joints for a typical steel, with D t value equal to 191.5. The study constitutes the first part of a comprehensive investigation on welded lap joints, which comprises both numerical simulations and experimental testing. The on-going research project is aimed at providing better understanding of welded lap joint behavior under extreme bending loading conditions, towards developing efficient design guidelines and safeguarding the structural integrity of steel water pipelines imposed to severe ground-induced actions. Herein, numerical work is presented, focusing on the global behavior of the lap joints, as well as on the value of local strains developed at critical locations. The present numerical results can be considered as preliminary, aiming at the efficient preparation and design of the upcoming experimental work. INTRODUCTION Large-diameter steel pipelines for water transmission, designed with AWWA M11, often employ welded lap joints. They are used instead of straight butt-welded full-penetration joints, because of their lower construction cost and their proven history of use. Welded lap joints require the forming a “bell” at the end of each pipe, which is constructed at the pipe mill through a mandrel that expands the end part of the pipe, so that the other end of the adjacent pipe segment, often referred to as “spigot”, is inserted and welded to the bell with a single or double full circumferential fillet weld, as shown in Figure 1. The present paper describes numerical work, and is motivated by the need for determining deformation limits of welded steel pipelines for water transmission, constructed in geohazard (seismic) areas. In those areas, the pipeline may be subjected to severe permanent ground-induced actions, from fault rupture, liquefaction-induced lateral spreading, soil subsidence, or slope instability that may deform the pipe well beyond the stress limits associated with normal operating conditions, possibly well into the inelastic range of the steel material. In such a case, the structural performance of welded joints constitutes a key issue for safeguarding pipeline structural integrity with “no loss” of pressure containment. Previous publications on the structural strength of welded lap joints has been directed towards determining their axial load capacity. Failures of such joints have been observed on the construction stage (Moncarz et al., 1987; Eberhardt, 1990), as well as due to strong earthquake action (Meyersohn and O’Rourke, 1991; O’Rourke et al., 1995; Lund, 1996; Pipelines 2017 515