1 Copyright © 2011 by ASME Proceedings of the ASME 2011 30th International Conference on Ocean, Offshore and Arctic Engineering OMAE2011 June 19-24, 2011, Rotterdam, The Netherlands OMAE2011-49583 CHARACTERISATION OF SLUG FLOW CONDITIONS IN PIPELINES FOR FATIGUE ANALYSIS Ahmed M. Reda Department of Mechanical Engineering, Curtin University Perth, WA, Australia Gareth L. Forbes Department of Mechanical Engineering, Curtin University Perth, WA, Australia Ibrahim A. Sultan School of Science and Engineering, University of Ballarat Ballarat, Victoria, Australia ABSTRACT Understanding the problem of slug flow induced fatigue damage is of particular importance to the reliable operation of pipelines. Slug flow across unsupported pipeline spans, pipeline crossings or vertical engineered buckle initiators, i.e. sleepers, produces dynamic motion in the pipeline resulting in cyclic fatigue stresses. In some cases, the dynamic effects will cause the pipeline to fail at a point of stress concentration. In other cases, however, these effects may be negligible. The current literature provides no guidance as to when the dynamic effects of slug flow must be considered. This paper gives guidance and describes how fatigue due to slug flow in pipelines, which would normally require dynamic analysis, can be quantified using simplified quasi-static analysis. The paper also presents a design process which could be used by pipeline engineers to determine the level of analysis needed, before embarking on more complex and expensive dynamic finite element. INTRODUCTION This paper presents analytical solutions for pipeline spans, which are subjected to a moving slug with a constant velocity, see Figure 1. It also presents some basic concepts of vibration, which will help to grasp some of the main issues involved in structural dynamics. The aim of this paper is to provide a simplified design procedure for calculating the stress ranges associated with the vibration of the span as the slug moves along the pipeline. In addition, the paper aims to provide pipeline engineers, who are faced with the moving load/mass problem, with a simplified design procedure. This simplified procedure will help to obtain the desired characteristics of the moving slug. The major focus of this paper is to present the dynamic load factor-speed parameter relationship at different damping ratios. This relationship enables the dynamic problem to be treated as a simpler quasi-static problem, which can be solved analytically without requiring complex dynamic finite element analysis. Previous research papers have discussed the dynamic response of pipelines under the influence of slug flow [1-5]. For instance, these papers have relied on the use of finite element analysis of specific pipeline systems with increasing complexity to account for the non-linear support of the seabed. However, the fundamental analytical solution of a simply supported beam under the influence of this loading type along with a discussion of the structural response form is still yet to be addressed with any level of detail necessary to help those working in this field to grasp the complex phenomenon involved. Some of the complexities which are present in slug flow (without delving into the area of support conditions), are demonstrated by the questions that a pipeline design engineer usually asks when faced with this problem. These questions include: i. What size does a slug need to be for a dynamic analysis to be undertaken? ii. At what speed does slug flow produce increased dynamic amplification over and above the static loadings? iii. At which location on the span does the highest bending moment exist? iv. Does a moving mass or moving load analysis need to be undertaken? These questions can be answered through discussion and analysis of the closed form solution of a simply supported