A Study Of Dynamic Response And Fatigue Analysis Of Marine Risers Resulting From Vortex Induced Motions Of Spar Platforms

Spar platforms are widely used in deep water engineering for their small water areas, low gravity center, good stability and low responses to wave excitations. However, within a certain range of Reynolds number, the main deep-draft columnar structures of a spar platform may undergo Vor tex induced motions (VIM) while the current flows pass by. When the VIM occurs, the platform would move periodically, resulting in the platform's fatigue damage. Meanwhile, as the boundary condition at the top of the riser, the platform's motion will cause the riser's fatigue.In this thesis, the wake oscillator model is employed to predict the spar platform's VIM. ANSYS FEM software is used to establish the model of the steel catenary riser (SCR). The dynamic response and fatigue damage of the SCR caused by the Spar platform's VIM is obtained by using the platform's motion as the bo undary condition.First, a literature overview of local and international research of Spar platform's VIM and the riser's fatigue damage caused by the VIM of the Spar platform are given. Some problems demanding further study are mentioned, providing a guideline for subsequent work of this thesis. The mechanism of the VIM and key fundamental research method s of them are introduced in detailThe Matteoluca wake oscillator model which is suitable for the rigid column subject to elastic constraint is used in this thesis. The Spar platform's hull is simplified as a rigid cylinder, and flow field behavior of the near wake is modeled by a nonlinear oscillator satisfying the van der pol equation. Several conclusions of VIM are obtained by analyzing the interaction of the platform and the fluid.At last, the dynamic response and fatigue analysis of SCR resulting from motions of Spar platforms are discussed. The structure features of the SCR and research methods of fatigue damage are introduced. Then the spar platform's motion responses are used as the boundary condition for FEM analysis. The displacement and stress curves in different cases are obtained. Finally, based on the Palmgren-Miner fatigue law, the SCR's fatigue damage life caused by the platform's VIM is calculated.