scholarly journals Effect of Top Tension on Vortex-Induced Vibration of Deep-Sea Risers

2020 ◽  
Vol 8 (2) ◽  
pp. 121
Author(s):  
Jie Zhang ◽  
He Guo ◽  
Yougang Tang ◽  
Yulong Li
Keyword(s):  
Deep Sea ◽  
Natural Vibration ◽  
Dominant Frequency ◽  
Bending Stress ◽  
Van Der Pol ◽  
Vibration Displacement ◽  
Vortex Induced Vibration ◽  
Towing Tank ◽  
Wake Oscillator ◽  
Shear Current

With the increase of water depth, the design and use of the top-tensioned risers (TTR) are facing more and more challenges. This research presents the effect of top tension on dynamic behavior of deep-sea risers by means of numerical simulations and experiments. First, the governing equation of vortex-induced vibration (VIV) of TTR based on Euler-Bernoulli theory and Van der Pol wake-oscillator model was established, and the effect of top tension on natural vibration of TTR was discussed. Then, the dynamic response of TTR in shear current was calculated numerically by finite difference method. The displacement, bending stress and vibration frequency of TTR with the variation of top tension were investigated. Finally, a VIV experiment of a 5 m long flexible top-tensioned model was carried out at the towing tank of Tianjin University. The results show that the vibration displacement of TTR increases and the bending stress decreases as the top tension increases. The dominant frequency of VIV of TTR is controlled by the current velocity and is barely influenced by the top tension. With the increase of top tension, the natural frequency of TTR increases, the lower order modes are excited in the same current.

A Phenomenological Model for Vortex-Induced Motions of the Monocolumn Platform and Comparison With Experiments

2009 ◽  
Author(s):  
Guilherme F. Rosetti ◽  
Rodolfo T. Gonc¸alves ◽  
Andre´ L. C. Fujarra ◽  
Kazuo Nishimoto ◽  
Marcos D. Ferreira
Keyword(s):  
Time Domain ◽  
Phenomenological Model ◽  
Structural Model ◽  
Design Phase ◽  
Van Der Pol ◽  
Floating Structures ◽  
Towing Tank ◽  
Wake Oscillator ◽  
Two Degree Of Freedom ◽  
Comparison With Experiments

Vortex-Induced Motions (VIM) of floating structures is a very relevant subject for the design of mooring and riser systems. In the design phase, Spar VIM behavior as well as Semi Submersible and Tension Leg Platform (TLP) flow-induced motions are studied and evaluated. This paper discusses flow-induced behavior on the Monocolumn concept by presenting a phenomenological model and comparing its results with a set of experiments that took place in the IPT Towing Tank - Brazil (September 2008). The experimental results have shown some fundamental differences from previous VIM tests on other units such as Spars. This numerical model attempts to identify these disparities in order to better understand the mechanics of this phenomenon. The model is based on a time-domain, two degree-of-freedom structural model coupled with a van der Pol type wake oscillator. The comparison was performed in order to calibrate the model, to study and better understand the tests results, and finally to identify important aspects to investigate in further experiments.

scholarly journals Numerical and Experimental Research on the Effect of Platform Heave Motion on Vortex-Induced Vibration of Deep Sea Top-Tensioned Riser

2021 ◽  
Vol 2021 ◽  
pp. 1-13
Author(s):  
Jie Zhang ◽  
Ying Zeng ◽  
Yougang Tang ◽  
Wenyun Guo ◽  
Zhenkui Wang
Keyword(s):  
Parametric Excitation ◽  
Deep Sea ◽  
Frequency Component ◽  
Vortex Induced Vibration ◽  
Wake Oscillator ◽  
Validation Experiment ◽  
Simulation And Experiment ◽  
Prediction And Control ◽  
Heave Motion ◽  
And Control

The prediction and control of vortex-induced vibration (VIV) is one of the key problems for riser design. The effect of platform heave motion on VIV of deep sea top-tensioned riser (TTR) is presented by means of numerical simulation and experiment in this research. First, the heave motion was modeled as a parametric excitation, and the governing equation of VIV of riser considering the parametric excitation was established. Then, the dynamic response of TTR was calculated numerically by the finite difference method based on the Van der Pol wake-oscillator model. Finally, a validation experiment was carried out at the towing tank of Tianjin university. The results show that the VIV response at the bottom of riser is significantly increased due to the platform heave motion, especially in the situation of low current velocity. The larger amplitude and the higher frequency of the platform heave motion with the greater influence are generated on VIV of TTR. In particular, the value of 0.5 times, 1 time, or other multiples of the platform heave frequency will be included in the vibration frequency component of TTR when the platform heave amplitude is large and the frequency is high.

scholarly journals Prediction of Vortex-Induced Vibration Response of Deep Sea Top-Tensioned Riser in Sheared Flow Considering Parametric Excitations

2020 ◽  
Vol 27 (2) ◽  
pp. 48-57
Author(s):  
Guanghai Gao ◽  
Yunjing Cui ◽  
Xingqi Qiu
Keyword(s):  
Prediction Model ◽  
Deep Sea ◽  
Nonlinear Partial Differential Equations ◽  
Solution Process ◽  
Cross Flow ◽  
Vortex Induced Vibration ◽  
Sheared Flow ◽  
Wake Oscillator ◽  
Real Size ◽  
Heave Motion

AbstractIt is widely accepted that vortex-induced vibration (VIV) is a major concern in the design of deep sea top-tensioned risers, especially when the riser is subjected to axial parametric excitations. An improved time domain prediction model was proposed in this paper. The prediction model was based on classical van der Pol wake oscillator models, and the impacts of the riser in-line vibration and vessel heave motion were considered. The finite element, Newmark-β and Newton‒Raphson methods were adopted to solve the coupled nonlinear partial differential equations. The entire numerical solution process was realised by a self-developed program based on MATLAB. Comparisons between the numerical calculation and the published experimental test were conducted in this paper. The in-line and cross-flow VIV responses of a real size top-tensioned riser in linear sheared flow were analysed. The effects of the vessel heave amplitude and frequency on the riser VIV were also studied. The results show that the vibration displacements of the riser are larger than the case without vessel heave motion. The vibration modes and frequencies of the riser are also changed due to the vessel heave motion

A Double Birkhoff Wake Oscillator for the Modeling of Vortex-Induced Vibration

2011 ◽  
Author(s):  
R. H. M. Ogink
Keyword(s):  
Free Vibration ◽  
Added Mass ◽  
Mass Force ◽  
Near Wake ◽  
Vibration Measurements ◽  
Vortex Induced Vibration ◽  
Fluid Forces ◽  
Wake Oscillator ◽  
Model Equations ◽  
Far Wake

A double Birkhoff wake oscillator for the modeling of vortex-induced vibration is presented in which the oscillating variables are assumed to be associated with the boundary layer/near wake and the far wake. The fluid forces are assumed to consist of a potential added mass force and a force due to vortex shedding. In the limit of vanishing incoming flow velocity, the model equations reduce to a form similar to the Morison equation. The results of the double wake oscillator have been compared with forced vibration measurements and free vibration measurements over a range of mass and damping ratios. The model is capable of describing the most important trends in both the forced and free vibration experiments. Specifically, the double wake oscillator is able to model both the upper and lower branch of free vibration.

Fatigue Calculation of Risers Using a Van Der Pol Wake Oscillator Model With Random Parameters

2008 ◽  
Author(s):  
Yahya Modarres-Sadeghi ◽  
Franz S. Hover ◽  
Michael S. Triantafyllou
Keyword(s):  
Fatigue Life ◽  
Standing Wave ◽  
Oscillator Model ◽  
Random Parameters ◽  
Van Der Pol ◽  
Sheared Flow ◽  
Wake Oscillator Model ◽  
Model Scale ◽  
Wake Oscillator ◽  
Vortex Induced Vibrations

Vortex induced vibrations of long distributed structures (risers and mooring cables) is an inherently complicated phenomenon in which due to the riser multi-mode excitations, various combinations of traveling and standing wave patterns along the length is observed. These observations are made based on a series of model scale experiments conducted on a riser for both uniform and linearly sheared flow cases. In these model scale experiments, strain and acceleration measurements are conducted at selected points along the riser. The contour plots of amplitudes of oscillations in these experiments show a mainly traveling wave behavior for linearly sheared flow cases and a mainly standing wave one for the uniform flow cases. In order to model the vortex induced vibrations of the riser used in these experiments, a wake oscillator model is used. In this model, the riser is assumed to be a tensioned string and the wake dynamics is represented by a Van der Pol oscillator whose driving force is in parallel with the riser acceleration. Randomness in the current, added mass and lift coefficients is taken into account by considering random parameters for the wake oscillator model. By using the proper parameters in this wake oscillator model, its results can be compared with the experimental ones. The comparison is made in terms of dynamical behavior (traveling waves versus standing waves, amplitudes and frequencies of oscillations) as well as the fatigue life calculations. The statistics of fatigue life calculations based on the experimental reconstruction compares well with those of the model results showing that the theoretical model can predict fatigue damage of the riser fairly well.

scholarly journals VIV Fracture Investigation into 3D Marine Riser with a Circumferential Outside Surface Crack

2021 ◽  
Vol 2021 ◽  
pp. 1-13
Author(s):  
Jun Liu ◽  
Zhigang Du ◽  
Xiaoqiang Guo ◽  
Liming Dai ◽  
Liang Huang ◽  
...  
Keyword(s):  
Vibration Amplitude ◽  
Lateral Displacement ◽  
Axial Stress ◽  
Assessment Model ◽  
Surface Cracks ◽  
Bending Stress ◽  
Marine Riser ◽  
Vortex Induced Vibration ◽  
Marine Risers ◽  
Failure Assessment

Vortex-induced vibration (VIV) is one of the most common dynamic mechanisms that cause damage to marine risers. Hamilton’s variational principle is used to establish a vortex-induced vibration (VIV) model of a flexible riser in which the wake oscillator model is used to simulate cross-flow (CF) and inline flow (IL) vortex-induced forces and their coupling, taking into account the effect of the top tension and internal flow in the riser. The VIV model is solved by combining the Newmark-β and Runge–Kutta methods and verified with experimental data from the literature. Combining Option 1 and Option 2 failure assessment diagrams (FADs) in the BS7910 standard, a fracture failure assessment model for a marine riser with circumferential semielliptical outside surface cracks is established. Using the VIV model and FAD failure assessment chart, the effects of riser length, inside/outside flows, and top tension on the VIV response and safety assessment of marine risers with outside surface cracks are investigated. It is shown that increasing the top tension can inhibit the lateral displacement amplitude and bending stress in a riser, but excessive top tension can increase the axial stress in the riser, which counteracts the decrease in the bending stress, so that the effect of top tension on crack safety is not significant. The increasing outside flow velocity significantly increases the lateral vibration amplitude and bending stress in the riser and reduces the crack safety. When other parameters remain unchanged, increasing riser length has no significant effect on the vibration amplitude of the lower part of the riser.

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