Session: 06-03-04 Fluid-Structure, Multi-body and Wave-body Interaction

Submission Number: 157547

A Prediction Model for Coupled In-Line and Cross-Flow Vortex-Induced Vibration of a Near-Wall Cylinder Suffering Collision 

Submarine pipelines play a vital role in gathering and transporting ocean oil and gas. Generally, they are embedded originally in the seabed, but gradually be exposed to sea currents due to the natural erosion of the seabed. Periodical fluid loads, caused by the cross flow and applied on exposed pipes, can induce the vortex-induced vibration (VIV) which is the main factor of fatigue and fracture of submarine pipelines according to the failure statistic. From a practical perspective, predicting VIV is of great importance to analyze reliability of submarine pipelines. Because of the large aspect ratio, high Reynolds number (Re) and diverse underwater environment of real pipelines, empirical models are better in engineering applications than high fidelity simulations and experiments to some extent. Present paper aims to propose a model consisting of structure and wake oscillators to predict coupled in-line and cross-flow vibration of a near-wall cylinder suffering collision with the seabed. Based on VIV phenomena, mechanisms and data analysis, some hydrodynamics, namely vortex shedding frequency and time-varying/time-averaged lift/drag coefficients, are modeled by Reynolds number, thickness of boundary layer and gap (G) between cylinder and wall. Based on collision theory, collision factors are added into the model to control the motion after collision. With above improvements, the model is validated by experimental data, and model responses reflect effects of the wall and collision on the vibrating cylinder successfully. When G/D drops from 2 to 1 (D is diameter of the cylinder), amplitudes monotonously decrease, and the maximum amplitude shifts to a smaller reduced velocity (Ur). Vibration regimes and resonant tend to start/end at a larger/smaller Ur, leading to narrowing lock-in regions. Time-averaged transverse displacements increase monotonously in whole, while stream-wise displacements drop in the resonance regime. Trajectories are shapes of symmetric ‘8’, being similar to that of an isolated cylinder in free stream. There are some unique features when G/D decreases from 1 to 0.35. First peak of in-line vibration tends to disappear. Trajectories become asymmetric shape of ‘8’ or oval because the cylinder tends to vibrate in approximating frequencies in 2 directions. When gap ratio is small, collision is likely to happen. The cylinder collides the wall and then is bounced away. The angles of incidence and reflection are almost the same, therefore the trajectory has a symmetric tip in the lower part. Moreover, collisions interfere the following vibration, resulting more irregular trajectories which are not smooth oval. With further researches on effects of wall proximity and collision, improvements can be better to enhance model’s rationality and accuracy. Accurate model responses are essential to analyze the safety and reliability of submarine pipelines suffering vortex-induced vibration.

Presenting Author: Mengmeng Tao China University of Petroleum (Beijing)

Presenting Author Biography: Presenting author is a PhD Student at China University of Petroleum - Beijing, majoring in oil and gas engieering. Her interesting fields include fluid-structure interaction, vortex-induced vibration, empirical models, computational fluid dynamics and structural reliability.