Time-domain simulation of vortex-induced vibration for deepwater marine risers

The engineering challenges of Vortex-Induced Vibration (VIV) for marine risers are of great concern as offshore operations move into deepwater. The VIV phenomenon creates fatigue issues and could cause permanent damages in risers. An accurate tool for analyzing riser VIV is important for the safe operation of offshore drilling and exploration. Risers typically have an extremely large length-to-diameter ratio which makes direct scaling nearly impossible in physical model testing - this naturally requires the use of numerical methods. -- In this work, a time-domain numerical model for simulating VIV of deepwater risers was developed. A novel forcing algorithm that utilizes the hydrodynamic coefficients of finite segments of a riser was adopted to predict the external forces on a riser due to fluid effects and riser motion. The predicted external forces are then applied on the riser. A global-coordinate-based finite element model was employed to compute the responses of the riser. -- The computation consisted of a two step process: static and dynamic analysis. The static analysis iterates to find the equilibrium profile of the riser(s), then the dynamic analysis solves the equation of motion at each user-defined time step. The program outputs the responses and the reaction forces (tension) of the riser(s) at any user-defined nodes. -- The numerical model was coded in Fortran 90 and validated by comparing program outputs with published experimental data. Validation studies were first conducted on mooring lines, and the comparison showed that the finite element method was able to capture the nonlinear behaviour of these slender structures. With confidence in the finite element model, the VIV forcing algorithm was implemented in the program. Validation studies were then extended to the VIV prediction of a 6 meter rigid pipe with spring boundary conditions and a 13 meter laboratory scale top-tensioned riser. The comparisons between the predictions in this study with the experimental results are generally in good agreement.