Graduate Thesis Or Dissertation

 

Nonlinear dynamic response of bottom-land deep ocean pipelines Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qv33s005h

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  • A finite element formulation is developed for the prediction of the large displacement response of pipelines which are placed upon irregularly contoured marine sediments and are subsequently subjected to episodic hydrodynamic loads. The equations of motion during each episode of extreme wave activity are obtained from a variational form of Hamiltons principle and this form of the equations of motion is integrated using the Newmark method of implicit integration. Within each time step a Newton-Raphson iteration scheme is used to achieve equilibrium while accurately tracking the nonlinear path-dependent response. An updated Lagrangian formulation shifts the pipeline reference configuration and separates rigid body movements from pipeline deformations. This provides a large deflection, small strain transient analysis of the pipeline response. Nonlinear, elastic-plastic springs simulate transverse and axial bottom resistance from both cohesive and cohesionless sediments. Inclusion of a pressure differential across the pipe wall modifies tensile stresses and influences the flexural stiffness of the finite element model through the geometric stiffness matrix. Hydrodynamic added mass and nonlinear, viscous drag forces are included by means of the relative-motion form of the Morison equation. The formulation results in a three-dimensional finite element model of a bottom-laid pipeline that is constrained to follow the irregularly contoured ocean bottom. A finite element method computer model is developed and used to predict the pipeline response to both monochromatic and random wave loads. Numerical examples presented demonstrate the validity of the formulation and illustrate the results obtained from sample simulations. It is found that nonlinear structural behavior is a dominant factor in predicting pipeline response and that inclusion of nonlinear effects greatly reduces predicted pipeline displacements. It is further shown that both the directionality of wave attack and the wave length of incident waves significantly alters the magnitude and location of the maximum pipeline response. Computational times for the random wave simulations analyzed were substantial with an approximate ratio of CPU to real time ratio of 105 to 1 being typical for the finite element analysis of a prototype marine pipeline having 150 degrees of freedom.
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