CFD simulations of a top-tensioned riser experiencing VIV and varying axial tensions are conducted through viv-FOAM-SJTU solver. The varying tension reflects the effect of heave motions of the platform. Strip method is used to simplify the calculation and PimpleDyMFoam module in OpenFOAM is used to solve the fluid field. When the axial tension changes over time, natural frequencies changes over time leading to the changes of VIV response. Internal resonance of the first mode has great impact on the in-line vibration. The multi-modal vibration and modal transitions are illustrated in the simulations.
Vortex-Induced Vibration(VIV) of marine risers has been receiving many attentions for decades. The riser is the weakest part in the production system connecting between the platform at the surface and the well at the seabed. VIV is a typical problem of fluid-structure interaction. Vortices generates and sheds alternately from both sides of the riser leading to a periodic vortex-induced pressure around the riser. The riser vibrates under the effect of the pressure and the vibration also affects the flow field around. When the vibration frequency is close to the natural frequency of the riser, "lock-in" phenomenon is usually observed with violent vibration. VIV has become the main source of fatigue damage of the riser. Therefore, it is important to predict the VIV response accurately.
For the VIV problem, many previous works have been done in this field. Some reviews have summarized the research progress in the past decades including computational model, vibration characteristics and mechanisms of the vibration (Wan and Duan, 2017; Williamson and Govardhan, 2004; Wu et al, 2011). Many factors have influences on the VIV response, such as current profiles, aspect ratio and top tensions. Besides, effect of platform motions cannot be ignored, especially for floating structures.
Traditionally, a fixed platform has only small amplitude of motions while a floating structure may have relatively large motions over a long period limited by mooring systems. Effects of the platform motions can be divided into two aspects: motions horizontally and motions vertically. It should be noted that effects of heave motions of the platform are different for different kinds of risers. For a steel catenary riser(SCR), most parts are declining and are not vertical to the surface. Heave motions of the platform generates relatively oscillatory flow between the riser and water. For a top-tensioned riser, it is nearly vertical to the horizontal plane from the surface to the seabed, as shown in Figure 1. The platform relates to the riser by a heave-compensator acting as a spring. The stiffness of the spring is much smaller than the axial stiffness of the riser and the top displacement of the riser is small and can be ignored at a model scale. Therefore, influences of a heaving platform to a top-tensioned riser can be simplified as the influences of the varying axial tensions.
