Shixiao Fu

Experimental investigation of the response performance of VIV on a flexible riser with helical strakes

Experimental investigations were conducted on a flexible riser with and without helical strakes. A uniform current was obtained by towing a riser model in a tank, and the vortex-induced vibration (VIV) suppression of strakes with different heights and pitches was studied. The experimental results indicate that the response characteristics of a bare riser can be quite distinct from those of a riser with helical strakes, and the suppression performance depends on the geometry of the helical strakes. The VIV responses in the cross-flow (CF) and in-line (IL) directions can be coupled via variations in the tensile force. The fatigue damage in the CF direction is of the same order as that in the IL direction for the bare riser. However, for the riser fitted with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction. The experimental results also confirmed that the strake height has a greater influence on the VIV response than the strake pitch, and the drag exerted on the riser increases with the strake pitch and height.

A method to estimate the hydroelastic behaviour of VLFS based on multi-rigid-body dynamics and beam bending

ABSTRACT This paper introduces a new method which is based on multi-body hydrodynamics and Euler–Bernoulli beam assumption to study hydroelastic behaviours of very large floating structures (VLFSs). A continuous VLFS is divided into several modules, being multi-module floating structures. The section between two adjacent modules’ centre is seen as a beam element. Based on the above assumption, the six-degree-of-freedom motion of a modules centre is both affected by the hydrodynamic interaction with its adjacent module and restricted by the deformation condition of the equivalent beam between two modules. Then the motion equation of the equivalent multi-module floating structures can be established utilising the potential flow theory and Euler–Bernoulli beam hypothesis. The results calculated by the present method are compared with experimental results and numerically calculated data by three-dimensional hydroelastic theory, which shows rather good agreement.

Experimental Investigation on VIV of the Flexible Model Under Full Scale Re Number

In this paper, VIV of a flexible model within full scale Re number regime are studied. The experiments were carried out in the towing tank by towing the flexible model with a constant forward speed. The bending stiffness of the flexible model was specially designed and controlled by a steel core pipe, whose diameter and wall thickness were selected according to the structural naturally frequency, vortex-shedding frequency and strength requirements under VIV conditions. The cylindrical rubber was bundled onto the core pipe to increase the diameter of the pipe to the full scale riser’s diameter, so that the model has the full scale hydrodynamic diameter, and therefore the full scale Re number. The strain information in both IL and CF directions of the model was measured by the optical fiber strain gages. The data was analyzed by the modal super position method. The VIV characters at both IL and CF directions under full scale Re number were studied.Copyright

Experimental Investigation on Vortex-Induced Vibration of Risers With Staggered Buoyancy

In this paper, model tests of a riser with an aspect ratio of 263 were performed in a towing tank by towing the model with constant speeds. The riser was tested with various configurations of buoyancy, including 0% coverage, 25% coverage, 50% coverage and 100% coverage of the buoyancy modules. Furthermore, for the 25% and 50% coverage conditions, the portion without buoyancy modules of the model was covered with helical strakes of 17.5D/0.25D (pitch/height). In the experiments, the strain information in cross-flow (CF) direction was measured using Fiber Bragg Grating (FBG) sensors. The VIV characters of flexible risers with different buoyancy configurations were studied and compared with each other in detail.Copyright

Vortex-Induced Vibration of Steel Catenary Riser Under Vessel Motion

A truncated steel catenary riser (SCR) model was experimentally tested in the ocean basin by oscillating the top end of the model to simulate the heave and surge vessel motion in order to investigate the vortex-induced vibration (VIV) features. Out-of-plane VIV responses were generally analyzed revealing that although the root mean square (RMS) strain distributed rather broadband, the displacement was quite consistent within a narrowband from 0.2D to 0.3D, and the touch-down point (TDP) area was found not to be the place suffering the maximum out-of-plane VIV response due to near wall effects. What’s more, strong wave propagations were firstly reported and summarized as a distinguished feature for VIV of a SCR under vessel motions, and further results reveal that wave propagation during the ‘lift up’ phase was quite different from that during ‘push down’ in terms of both wave speed and ‘power-in’ region location which is assumed to be caused by the tension variation along the model.Copyright

VIV of Flexible Cylinder in Oscillatory Flow

VIV in oscillatory flow is experimentally investigated in the ocean basin. The flexible test cylinder was forced to harmonically oscillate in various combinations of amplitude and period. VIV responses at cross flow direction are investigated using modal decomposition and wavelet transformation. The results show that VIV in oscillatory flow is quite different from that in steady flow; novel features such as ‘intermittent VIV’, amplitude modulation, mode transition are observed. Moreover, a VIV developing process including “Building-Up”, “Lock-In” and “Dying-Out” in oscillatory flow, is further proposed and analyzed.© 2013 ASME

Hydrodynamic Forces and Coefficients on Flexible Risers Undergoing Vortex-Induced Vibrations in Uniform Flow

AbstractIn this study, the distribution and variation of the hydrodynamic forces and hydrodynamic coefficients in both inline (IL) and cross-flow (CF) directions on flexible risers experiencing vortex-induced vibration (VIV) in a uniform flow were investigated. The hydrodynamic forces in the CF and IL directions were theoretically calculated by the Euler-Bernoulli beam vibration equation. With the least-squares method, the hydrodynamic coefficients in both CF and IL directions were further investigated. The results indicate that the hydrodynamic force and vibration displacement almost simultaneously reach their maximums. The excitation coefficients obtained in this paper do not always agree with those obtained by the forced-oscillation tests: some excitation coefficients are even negative within the usual defined exciting nondimensional frequency regime and are related with not only the nondimensional frequency and amplitude but also phase angles of the CF and IL displacements. The added-mass coefficient ...

Experimental study on response performance of vortex-induced vibration on a flexible cylinder

Abstract Laboratory tests were conducted on a flexible cylinder to improve the understanding of the response performance of vortex-induced vibration (VIV). The experiment was performed in a towing tank and the relative current was simulated by towing the flexible cylinder in one direction. Based on modal superposition, VIV displacements were obtained from the measured strain. VIV responses from different current cases were analysed, and the response performances, such as the motion trajectory, strain response, and fatigue damage, were studied. The motion trajectory for the flexible cylinder at low velocity displays the typical type-eight figure, very similar to that for a rigid cylinder. However, with increased velocity, the motion trajectory becomes chaotic, induced by the multi-mode VIV responses. Simplified fatigue damage based on the typical steady motion trajectory is usually a little larger than the measured fatigue damage based on the chaotic motion trajectory, but of the same order.

Extreme Response of Very Large Floating Structure Considering Second-Order Hydroelastic Effects in Multidirectional Irregular Waves

Hydroelasticity theory, considering the second-order fluid forces induced by the coupling of first-order wave potentials, is introduced briefly in this paper. Based on the numerical results of second-order principal coordinates induced by the difference frequency and sum-frequency fluid forces in multidirectional irregular waves, the bending moments, as well as the vertical displacements of a floating plate used as a numerical example are obtained in an efficient manner. As the phase angle components of the multidirectional waves are random variables, the principal coordinates, the vertical displacements, and the bending moments are all random variables. Extreme values of bending moments are predicted on the basis of the theory of stationary stochastic processes. The predicted linear and nonlinear results of bending moments show that the influences of nonlinear fluid forces are different not only for the different wave phase angles, but also for the different incident wave angles. In the example very large floating structure (VLFS) considered in this paper, the influence of nonlinear fluid force on the predicted extreme bending moment may be as large as 22% of the linear wave exciting forces. For an elastic body with large rigidity, the influence of nonlinear fluid force on the responses may be larger than the first-order exciting forces and should be considered in the hydroelastic analysis.

The Application of Non-Sinusoidal Mode Shape Function in the Identification of Modal Weight

The main objective of this paper is to study the effect of non-sinusoidal mode shape on the identification of modal weights. The simplified analytical solution of the transverse vibration equation of uniform tensioned long flexible riser was presented, and the corresponding non-sinusoidal but orthogonal mode shape function is deduced. Then the data from the VIV model tests of long flexible riser conducted by ExxonMobil at MARINTEK are analyzed by the mode superposition method. Modal weights calculated by two different mode shape functions, sinusoidal and non-sinusoidal function, with two methods, namely direct method and indirect method are compared. The comparison shows the good performance of the non-sinusoidal mode shape in the identification of modal weights.Copyright