Flow induced riser vibration and its control

Ji Lu

    Research output: ThesisDoctoral Thesis

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    Abstract

    In this thesis, the vibration of a model riser induced by a rotating flow field is investigated. The first task was to develop a rotating flow field in a water tank and to qualify the flow field has been made experimentally and numerically. Qualification of the unsteady rotating flow field by analytical solution of Navier-Stokes equations for this flow field at the decay stage, and computational fluid dynamic (CFD) simulation for high Reynolds number at the spin-up stage and decay stage. Experimental verification was provided for the analytical solution of flow at the decay stage. Based on the experimental and numerical investigation of different kinds of model risers, including straight, curved and Steel Catenary riser (SCR) models, the effects of the boundary condition and curvature of the risers on the flow-structure interaction of the risers, have been studied. The results for SCR model testing exhibited a strong beating motion in response over the high curvature, which corresponds to a wider upper branch in the lock-in (synchronization of structural vibration frequency and vortex shedding frequency). The increasing modal damping at higher modes could excite large modal coupling effect on in-plane and out-of-plane response under excitation particularly at outof- plane direction. Both modal coupling and beating motion are significantly enhanced by reducing curvature on either forced vibration case or the Vortex-induced Vibration (VIV). Different boundary conditions by experimental and numerical investigation were studies for straight and curved model risers, indicating that the 1st mode of simple supported straight rod acts in the water qualitatively to the 2nd mode of the cantilever rod, and similarly, the 2nd mode of the former is similar to the 3rd mode of the later. The excitation region to the structure response was identified, which was less significant under the constraint boundary at the bottom than in cantilever case. Based on the experimental investigation on shifting of natural frequencies, the numerical model for response in cross-flow direction predicted that beating motion appeared in both the forced vibration case, and the free vibration VIV case. The model indicated that the beating motion was significantly enhanced by reducing curvature on either forced vibration case or the VIV situation, which echoed the experimental investigations in Chapter 3. This thesis work investigated passive control of riser vibration using a splitter plate and the boundary control of riser vibration. The effect of splitter plate with various lengths on the response of an oscillating cylinder was investigated experimentally at cross-flow direction with a set of reduced velocities. The CFD model was also simulated in ANSYS CFX to investigate the relationship between the detached vortices and the splitter plate length. Based on the energy approach, the equations of motion of a SCR model subject to the time-varying hydrodynamic disturbance have been derived. The Lyapunov direct method was applied to design the boundary controller at the top end of the model based on the nonlinear partial differential equations. The proof of the closed-looped stability under external disturbance, and proof of existence and uniqueness of the solutions of the closedloop system were given. The numerical simulation results verified the proposed boundary control.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2013

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