Predicting Consolidation Settlement in Very Soft Clay; Comparison of Conventional 1-D Theory and 3-D Numerical Simulation
Authors: Lee Dan , Liu Shulong , Cooper Philip , Nash Ian , Evans Nigel 
Published: July 2010

The foundations for deepwater flowline termination assemblies (FTAs) are subject to complex load conditions in all six degrees of freedom during their operational life. The loads are generated from self-weight of the structures, pipeline and spool loads, and loads from pipeline thermal movements.

One-dimensional (1-D) conventional analytical method has generally been employed to determine the consolidation settlement of these structure assemblies. This analytical method has the tendency to overestimate the consolidation settlement resulting in an unduly conservative estimate. It has the further disadvantages of not being able to account for eccentric loadings, permeability and complexity of the structure assemblies.

To refine the 1-D settlement estimates and to reduce the level of conservatism, numerical analysis based on Abaqus' three-dimensional (3-D) finite element analysis (FEA) has been adopted. The FEA employed the " modified Cam-clay?? soil model based on the critical state plasticity theory to determine the consolidation settlement of typical heavy FTAs. Two case studies where the structures are supported by very soft clay are examined. The results indicate that the FEA gives a lower consolidation settlement as compared with the 1-D analytical solution. The FEA is able to address eccentric loadings from the complex FTA and also provide consolidation settlement at different locations on the FTA for use in the design of spools connected to it. Parametric studies are conducted to identify the critical parameters in the evaluation of consolidation settlement and to provide recommendations for analysis and design optimisation.

Introduction

The estimation of FTA's consolidation settlement is typically determined by analytical method based on Terzaghi's (1925) theory of one-dimensional settlement, which generally overestimates the magnitude of settlement and may result in an overdesign of structural components. With the advancement in computational technology and progress in numerical analysis, this conservative estimate can be refined by the use of finite element analysis (FEA) to incorporate realistic loading conditions, complex structural assembly and soil variations. Abaqus' three-dimensional (3-D) FEA is employed to determine the consolidation settlement of the FTA in order to reduce the level of conservatism.

The FEA employed the " modified Cam-clay?? soil model developed by Roscoe and Burland (1968), based on the critical state plasticity theory to give an accurate prediction of the consolidation settlement. A 2-D FEA model is initially constructed to " calibrate?? the mesh density and other pertinent parameters. This has the major advantage of low computational time in determining the sensitivity of the parameters and to ensure that the results obtained are comparable with those determined from the 1-D analytical solution before embarking on the rigorous 3-D FEA. Parametric studies are conducted to identify the critical parameters in the evaluation of consolidation settlement and to provide recommendations for analysis and design optimisation.

Two case studies where the heavy FTAs in deepwater are supported by soft clay with very high void ratios are examined. The results indicate that the 2-D FEA give a good agreement when compared with the 1-D analytical solution, whereas the 3-D FEA gives a much lower consolidation settlement due to the more realistic stress distribution representation over the threedimensional nature of the consolidation problem. The FEA is capable of addressing eccentric loadings and associated rotational settlement from complex structures and able to provide consolidation settlement at different locations on the FTA for use in the design of spools.