NON-LINEAR SOIL MODELS FOR PIPELINE AND RISER ANALYSIS
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- Institutt for marin teknikk 
This thesis describes the development and application of non-linear soil models in pipeline and riser design. A non-linear soil model is typically employed when investigating a complex pipe-soil interaction problem. Two main pipe-soil interactions are frequently studied: the vertical pipe-soil interaction at the touchdown point of the steel catenary riser (SCR) during cyclic heave motion, and the lateral pipe-soil interaction during the pipeline s lateral buckling. Mathematical models for non-linear pipe-soil interaction are presented. The interaction typically is represented using a force-displacement curve found by fitting a high number of test data points. In the lateral direction, the soil model developed by Verley and Sotberg is widely used in industry for assessing pipeline walking. In the vertical direction, Aubeny proposed a fairly good soil model which can capture important soil behaviour such as suction force and pipe-soil separation. The formulation and implementation of these models are the primarily focus of this thesis. New FORTRAN subroutines are created based on presented models. In lateral pipe-soil interaction, the soil subroutine, which is known as DNVMODEL-Y, includes mathematical models from DNV, PONDUS, and SAFEBUCK JIP. In vertical pipe-soil interaction, the soil subroutine, AUBENY-Z, includes the non-degradation vertical soil model developed by Aubeny. The soil subroutines are implemented inside the SIMLA software package to allow complete pipe-soil interaction analysis.Validation of the soil subroutines is conducted and presented in detail. Two validation methods are employed: comparing the results with related works/software and ascertaining the soil behaviour by various test cases using forced displacement load. Some applications of the developed soil subroutines in practical pipeline and riser analysis are presented in this thesis. Pipeline walking analysis using a non-linear soil model and Coulomb model is presented and discussed in detail. In regard to vertical pipe-soil interaction, deepwater SCR s response under forced heave motion is discussed. Sensitivity analysis of key parameters for several types of soil are also presented in respect to lateral and vertical pipe-soil interaction.