A numerical procedure to solve the coupled motion of a flexible riser and a viscous incompressible fluid is developed. In this study, the former is solved by a multi-modal response model and the latter is solved by a sectional solution (strip theory) method. In every fluid section the 2D-vorticity equation is solved by a Vortex-in-Cell method and the different fluid sections are coupled through the boundary conditions from the motion of the riser. This procedure is used to solve the vortex-induced vibration (VIV) response of flexible risers. Both vertical risers and steel catenary risers are predicted by the present method. The method can also be used to solve the coupled motion of two or more flexible risers. A fatigue analysis module is included in the numerical package. The present method is validated against experimental tests and other numerical tests, and the riser response and fatigue life is found to be in good agreement. The numerical results are visualized to obtain better understanding of the computed results and the phenomenon.
As the oil industry is moving into deeper and deeper waters, the structures are becoming more and more slender and hence prone to vortex induced vibrations and large displacements. When one or more risers are exposed to a sheared current there will almost always be induced some kind of vortex-induced vibration. In order to be able to predict the fatigue damage caused by these transverse vibrations it is crucial to know which modes that will dominate the response of the risers and at what amplitudes they are responding. In addition, if more than one riser is analyzed, it is important to be able to predict the coupled response of the riser system. And numerical tools with this feature included will be of great support to the oil industry. In order to help the engineer to understand the motion of long and flexible cylinders (e.g. risers and anchoring cables) in fluid flow, a computer program with the capability of visualizing these motions is developed. A modal analysis is used to predict the cylinder motion, and the fluid domain is solved by the vortex in cell method. General physical equations are utilized in order to make an effort of making the computer program general and being able to solve a number of different classes of VIV problems (such as collision of multiple risers, steel catenary risers, deepwater risers, surface waves, etc.).
The remainder of this paper can be outlined as follows: the next section describes the fluid solution procedure, then the structural response prediction is reviewed, and then the fluidstructure interaction procedure is discussed. This is followed by some numerical results and some concluding remarks.
The fluid domain is modeled by a so-called strip theory in which the fluid is divided into 2D sections (Fig. 1) along the span of the riser. On each of these sections the fluid flow is found by solving the vorticity equation.
