Numerical modelling of undrained vertical load- displacement behaviour of offshore pipeline using coupled analysis Senthilkumar M , Kodikara J & Rajeev P Monash University, Melbourne, Vic, Australia ABSTRACT Predicting the initial embedment of an offshore pipeline is one of the important design considerations for on bottom stability analysis. This paper reviews the current state of the art to predict the initial embedment, and presents numerical modelling to perform the coupled analysis of pipe-seabed system. Two dimensional plane strain, axis- symmetry model was developed using ABAQUS computer program. Soil was modelled using both Mohr-Coulomb with uniform undrained shear strength and modified Cam-clay obeying Biot consolidation while the pipe was assumed to be rigid. Both undrained and drained loadings were performed in displacement control manner to develop the load- displacement behaviour. The results were benchmarked to the analytical solutions available in literature. RÉSUMÉ Prédire l'ancrage initial d'un gazoduc sous-marin est l'un des considérations de conception important pour la stabilité sur l'analyse de fond. Ce document passe en revue l'état actuel de l'art de prédire l'ancrage initial, et présente la modélisation numérique pour effectuer l'analyse couplée de réseau de conduites-sol marin. Deux souche plan bidimensionnel, modèle de l'axe de symétrie a été développé en utilisant le programme informatique ABAQUS. Du sol a été modélisé en utilisant à la fois de Mohr-Coulomb avec la résistance au cisaillement non drainée uniforme et modification de Cam-clay consolidation obéissant à Biot tandis que le tuyau était censée être rigide. Les deux chargements non drainés et drainés ont été effectués de la manière de commande de déplacement à développer le comportement charge-déplacement. Les résultats ont été comparés aux solutions analytiques disponibles dans la littérature. 1 INTRODUCTION Pipelines are increasingly becoming an important part of the offshore infrastructure as more petroleum resources explored further away from shore. The design measures of the pipelines have given growing attention following the increased usage of extra long pipelines on deep seabed conditions. The pipeline embedment has became an important design parameter to control pipe vulnerability to external interactions such as heat expansion, free spanning, wave oscillation and submarine slides etc (Bruton et al. 2008; Randolph and White 2008; White and Randolph 2007). However, much attention is paid to understand the influence of initial embedment to control the expansion related challenges: (1) axial walking (when expands along the axis of the pipe); (2) lateral buckling (when buckles laterally). Predicting the initial embedment could be simplified to static loading of a pipe on cohesive soil medium. In the past, undrained pipeline penetration in cohesive soil was simplified to traditional bearing capacity theory for flat surface on a shallow embedment (Ghazzaly and Lim 1975; Karal 1977; Small et al. 1971; Wantland 1979). However, by means of classical plasticity approach, Randolph (1984) estimated the limiting pressure of a laterally moving cylindrical body fully embedded in cohesive soils. Later Murff et al (1989) adopted and extended these plasticity solutions to analyse partially embedded pipe penetration in cohesive soils where the presence of free surface is prominent. Finite element method has been widely employed to study the nonlinear behaviour of pipe- seabed interaction recently. The vertical load- displacement behaviour of pipe-cohesive soil with linearly increasing shear strength profile and no weight was reported by Aubeny et al (2005). Prior to the analysis, the pipe was placed in a predefined depth called wished in pipe (WIP) as shown in Fig .1.a. Merifield et al (2008) conducted FEM analyses on pipes subjected to combined loading of a WIP in horizontal and vertical directions on weightless uniform soil. Later, Merifield et al (2009) reported that the vertical collapse load is higher in the pushed in pipe (PIP) (Fig. 1. b) case due to the passive influence of heave as additional work is required to displace the soil around the pipe periphery. Fig.1.a. Wished in pipe (WIP) b.Pushed in pipe (PIP) The reliability of the undrained approach to predict the accurate initial embedment is questioned over time. In reality, loading of a pipe on cohesive soil will initially be taken by the soil pore pressure, keeping the load carried by the soil skeleton to minimum. But there is substantial time lag between pipe laying and operational stages