Session: 13-01: OTEC, Applications and Devices

Submission Number: 181375

Study on Vortex-Induced Vibration of a Free-Hanging Riser With Helical Strakes Under Platform Motion 

In recent years, free-hanging risers have been widely applied in offshore engineering, particularly in deep-sea mining and ocean thermal energy conversion. Unlike traditional risers supported at both ends, the top end of the free-hanging riser is attached to a floating platform or a vessel, while its bottom end is either free or fitted with a clump weight. This configuration is characterized by a movable upper boundary and a weakly constrained lower end. In addition to the effects of complex ocean currents, platform motion also has a significant influence on the response of the riser. Previous studies have shown that oscillatory flow induced by platform motion can excite vortex-induced vibrations (VIV) of the riser, leading to severe fatigue damage. Currently, the suppression efficiency of helical strakes has been extensively validated for risers supported at both ends in uniform flow. However, research on free-hanging risers with helical strakes subjected to platform motion remains limited, and their dynamic response characteristics are not yet well understood.To further improve the understanding of the VIV response of straked risers under platform motion, this study conducted VIV experiments on a free-hanging riser equipped with helical strakes in a towing tank. The top end of the riser was forced to oscillate with prescribed amplitudes and periods to simulate platform motion. The Keulegan–Carpenter (KC) number ranged from 23 to 113, and the maximum oscillation velocity at the top varied from 0.1 m/s to 0.4 m/s. Fiber Bragg grating (FBG) strain sensors were used to measure the VIV strain response of the riser. Modal analysis and wavelet transform methods were applied to extract displacement and frequency characteristics from the experimental data. The results systematically reveal the VIV response characteristics of the free-hanging riser under various KC numbers and maximum oscillation velocities, and evaluate the suppression efficiency of the helical strakes. The findings can provide valuable experimental evidence and theoretical reference for the design of free-hanging risers and the optimization of suppression devices.

Presenting Author: Zhibo Niu State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University

Presenting Author Biography: Zhibo Niu is a Ph.D. candidate in Naval Architecture and Ocean Engineering at Shanghai Jiao Tong University. His research focuses on the analysis and suppression of vortex-induced vibration of marine slender flexible risers, and the protection and monitoring of submarine cables for offshore wind turbines.

Authors:

Zhibo Niu State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University
Shixiao Fu State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University
Mengmeng Zhang State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University
Yuqi Shen Marine Design and Research Institute of China
Jiawei Shen State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University
Yuwang Xu State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University
Leijian Song State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong Universit
Bin Song State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University