FSI Methodology for Analyzing VIV on Subsea Pipeline Free Spans with Practical Boundary Conditions
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The objective of this thesis is to develop a more realistic numerical model than current methodologies for free span stability of submarine pipelines based on fatigue analysis. A general assumption in performing vortex-induced vibration (VIV) analysis of pipeline free spans is that both ends of the free span are fixed and/or pinned in order to simplify computational simulations; however, Det Norske Veritas (DNV – translation to The Norwegian Truth) Recommended Practice F105 states that these boundary conditions must adequately represent the pipe-soil interaction and the continuality of the pipeline. To adequately simulate the free span’s response to VIV, three-dimensional fluid-structure interaction (FSI) simulations are performed by coupling the computational fluid dynamics (CFD) codes from STAR-CCM+ with the finite element (FE) codes from ABAQUS. These FSI simulations in combination with separate coupled Eulerian-Lagrangian (CEL) simulations are modeled to mimic real world conditions by setting up the boundary conditions to factor in the effects of pipe-soil interaction at the ends of the span. Computational design of experiments (DOE) is utilized to determine the sensitivity of several input variables on the maximum stress response of the free span from VIV. The variables considered in this investigation include the soil density (1700-2000 kg/m3), length of pipe contact with the soil (20-200 inches), and the pipe embedment depth within the soil (0-10 inches). A Box-Behnken surface response design was used to capture the non-linear responses throughout the design space. These simulations show a mitigation of overall stresses to the free spans; as a result, the integration of pipe-soil interaction in free span assessment may aid in the prevention of unnecessary corrective action.