Mechanism Research On Suppression Of Vortex-induced Vibration Of Deep-water Top Tension Riser By Helical Strakes

Vortex-Induced Vibration (VIV) acts as the dominant fatigue damage to risers systems widely used in offshore engineering. Although there are many research works have been done concerning model tests and numerical simulation, many problems are still unsolved. For the sake of actualizing the numerical application of CFD method to VIV study, a parametric study of the efficiency of Helical strakes to suppress VIV has been carried out, including the interpretation between vortex shedding pattern and riser oscillation. This paper is a systematic report about suppressing VIV. It's the combination of model tests and numerical simulation, which using the FEM software system ANSYS-CFX.In actual practice, the model tests and numerical simulation task works as supplementary to each other. On one hand, the former provided the orientation and instrument for the setup of the latter, and acted as reliable criteria for numerical simulation result; on the other hand, the well-designed post-test numerical simulation offered the corresponding vortex shedding visualization behind the wake of riser and the corresponding instantaneous displacement information. Taking advantage of the combination between Helical strakes oscillation from model tests and vortex shedding patterns from numerical simulation, some coupled phenomena about fluid-structure interaction have been well comprehended.In the model tests, a series of riser models with F5 H0.10 Helical strakes and bare riser configurations have been tested. As the current speeds vary, the results of the bare riser and Helical strakes show that the drag coefficient and lift coefficient change with Re and have been compared to the related tests.A series of pre-test numerical simulation tasks aiming at solving more complicated problems have been accomplished, including 2D and 3D of bare riser and Helical strakes on the Re=200 and Re=1500. In the 2D condition, a set of different heights of strakes have been simulation and four different sections of the strakes have been taken into account. The results which show the drag coefficient and lift coefficient change with Re have been compared to Morison formula and the related tests. In the 3D condition, both the pitch and the height of the strakes have been systematically changed. Bare riser and four different strakes have been simulated; they are P5_H0.10, P5_H0.14, P5_H0.25 and P16_H0.25. A comparison with results of the two conditions with strake and bare riser is given. The effects on suppressing VIV fatigue of long risers vary considerably according to the geometrical shapes of Helical strakes. The pre-test simulation work accomplished in this paper will be an available orientation and reference to the further model tests and even to the practical engineering application.The innovations in this thesis can be concluded as follows:the model tests and numerical simulation have provided a powerful tool to study the Helical strakes; many important conclusions are obtained from model tests and numerical simulation; taking advantage of the combination of these two methods, some problems related to the vibration mode, the amplitude and the vortex shedding mode have been well explained.