Vortex-induced Vibrations Of Flexible Riser In Frequency Domain

When the flexible riser exposed to a certain current, Vortex-Induced Vibrations (VIVs) occur once shedding vortices exert periodical forces on the flexible riser in the cross flow (CF) and in line (IL) directions. The vibration of flexible riser, in turn, changes its wake structure; this fluid-structures interaction problem is called the "VIV". The presence of VIV would lead to fatigue damage of mooring system and riser, reduce the fatigue life of the entire platform. Therefore, more efforts have been put on the research activities on VIVs during the past ten years.Based on VIV theories in frequency domain, the present work explores some basic issues of VIV. The main contents and contributions of this thesis may be summarized as follows:1, In order to explain the reason why linear axial tension causes mode shapes unlike sine function, the present work considers two scenarios, namely vertical tensioned risers without and with the bending stiffness. The vibrations of former risers were solved with both Bessel and Asymptotic solutions and the latter ones were presented with SHEAR7 theory.2, It introduces theories of both SHEAR7 and VIVANA programs in detail. To provide the basic input parameters in VIV tools, the catenary equation was firstly established. According to the equation, the SCR was defined as an equivalent vertical straight riser with linearly varying tension.3, It predicts VIV responses of SCR under different water depths, various flow profiles and top tensions with both SHEAR7 and VIVANA. An experiment was studied to achieve three objectives: (1) the effect of changed axial tension on mode shapes; (2) the effect of mode shapes on modal weight; and (3) the effect of modal analysis method on low modal weights. It is found that the variation of axial tension exerts little impact on mode shapes for the weak self-weight and flexural stiffness reasons. The comparison results of two mode shapes, sinusoidal and non-sinusoidal, show also little effect of mode shapes on modal weights. Thus, sinusoidal mode shapes can take place of real mode shapes to obtain modal weights when the tension is dominating. When they are away from the dominating mode, low modal weights become unreliable with modal analysis method. The reasons come from two aspects, one is the modal analysis method itself and the other the presence of noise.4, In order to study the interaction of adjacent elements on the added mass coefficients, the present work proposes a formula, based on FEM, for estimating the added mass coefficients along the flexible riser under known displacements time series. This formula not only wasn't limited by the experimental data of rigid circular cylinders and considered its flexible characteristics, but also overcame the flaw that added mass coefficients of flexible risers are assumed to be constant. In addition, it also takes into account the varying amplitudes of various points at the flexible risers and different characteristics of flow field at different depths.