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In the field of offshore engineering, the control rod is a common passive device to suppress the vortex-induced vibration (VIV) of cylindrical structures. Some key parameters of control rods (diameter ratio, spacing distance, coverage rate and attack angle) have a great influence on the VIV fatigue life of the cylinder. This paper aims to investigate the fatigue damage of the main flexible cylinder with multiple control rods caused by VIV based on rainflow counting and a standard S–N curve, with the final purpose of studying the effectiveness of multiple control rods, in different configurations around the cylinder in improving the structural fatigue life. The results show that control rods can suppress the VIV response of the cylindrical structure and reduce its fatigue damage effectively. The reduction effect of the control rods is very sensitive to the spatial arrangement. The reduction effect of the 3 control rods is the best at the attack angle of 40°, while that of the 4 control rods is the best at the attack angle of 30°. The fatigue damage of the flexible cylinder can be reduced to a lower level with 4 control rods. In addition, "mode transition", a specific feature of VIV in uniform flow, is found in the experiment and has a strong influence on fatigue life. • The influence of the spatial arrangement of control rods on the VIV fatigue of the flexible cylinder is studied. • The use of control rods can effectively reduce the CF fatigue damage of flexible cylinder structures. • The layout angle of the control rod has a great influence on the fatigue damage value and the damage distribution of the flexible cylinder. • The damage reduction effect of 4 control rods is obviously better than that of 3 control rods. [ABSTRACT FROM AUTHOR]

• The influence of the spatial arrangement of 3 and 4 control rods on CF VIV of the main cylinder is experimentally investigated. • 3 control rods with θ = 40° and 4 control rods with θ = 30° are the best choice for suppressing the CF VIV in the tests. • It has a better effect for passively controlling the VIV of the flexible cylinder with 4 control rods than the cases with 3 control rods. Control rod is one of the common passive control methods to suppress the vortex-induced vibration (VIV) of cylindrical structures. In this paper, the experimental study is conducted to detailed understand the performance of multiple control rods in suppressing the cross-flow (CF) VIV for a long flexible cylinder. The influence of the spatial arrangement of 3 and 4 control rods on CF VIV response of the main cylinder is investigated in a towing tank. It is observed that the attack angle θ is a very significant parameter to affect the vibration response, dominant frequency and the VIV suppression efficiency of the main cylinder. Based on the suppression efficiencies analysis of VIV response in the present experimental investigation, the spatial arrangement of 3 control rods with θ = 40° and 4 control rods with θ = 30° is the best choice for suppressing the CF VIV response of the main flexible cylinder. Overall, the use of 4 control rods could reduce VIV more effectively than the application of 3 control rods. [ABSTRACT FROM AUTHOR]

Vortex-induced vibration (VIV) is an important factor resulting in fatigue failure of cylindrical structures in offshore engineering applications (such as risers and cables). The traditional empirical model for VIV prediction often yields unsatisfactory results due to its dependence on the selection of empirical coefficients. Thus, establishing a general approach to VIV damage prediction based on artificial neural networks and engineering experience is of great significance to engineering guidance. In this paper, a VIV damage prediction model of a flexible cylinder was established using a radial basis function neural network (RBFNN) and model test data. Compared with back propagation neural network (BPNN), genetic algorithm-back propagation neural network (GA-BPNN), and generalized regression neural network (GRNN), the RBFNN exhibited excellent performance with high accuracy and the shortest training time in predicting the VIV damage of flexible cylinders. In addition, the VIV fatigue of flexible cylinders fitted with passive control devices (control rods and helical strakes) was predicted. Using three and four control rods, the VIV reduction effect was observed to be better when the attack angle θ was in the range of 30°–45° and 15°–30°, respectively. Further, regarding helical strakes, the damage values of the cylinder were significantly reduced; however, the VIV suppression effect was significantly affected by the yaw angles. The VIV damage prediction model of this paper is expected to serve as important engineering guidance for the preliminary design, installation arrangement, operation, and maintenance of flexible cylindrical structures in offshore engineering applications. • A general approach to VIV damage prediction based on artificial neural networks was proposed. • Four types of neural networks were presented: BPNN, GA-BPNN, RBFNN and GRNN. • The RBF neural network exhibited excellent performance in predicting VIV damage of flexible cylinders. [ABSTRACT FROM AUTHOR]

The structural fatigue damage caused by vortex-induced vibration (VIV) is one of the most important failure reasons for flexible cylinders in offshore engineering applications. Helical strakes are often employed to reduce the VIV of slender marine structures. Based on a previous experimental investigation, this paper further investigates the effects of the yaw angle on the VIV fatigue damage of a yawed flexible cylinder with or without helical strakes in a uniform flow. Four yaw angles (α = 0°, 15°, 30° and 45°) are considered. The rain-flow counting (RFC) procedure and the Miner-Palmgren hypothesis are adopted to calculate the fatigue damage of the flexible cylinder. The influence of the yaw angle on the strain response, fatigue damage distribution and maximum fatigue damage of the flexible cylinder are investigated and discussed. The fatigue damage levels in the cross-flow (CF) and in-line (IL) directions of the flexible cylinder are obviously reduced by helical strakes. The VIV fatigue damage of the smooth cylinder is nearly independent of the yaw angle, while that of the straked cylinder is extremely sensitive to the yaw angle. As the yaw angle increases from 0° to 45°, the fatigue damage of the straked cylinder caused by VIV increases several orders of magnitude. [ABSTRACT FROM AUTHOR]