scholarly journals The Effect of Shifting Natural Frequency on the Reduction of Vortex-Induced Vibrations of Marine Risers

2017 ◽  
Vol 1 (1) ◽  
pp. 9-16 ◽  
Author(s):  
Younes Komachi ◽  
Said Mazaheri ◽  
Mohammadreza Tabeshpour ◽  
◽  
◽  
...  
Keyword(s):  
Natural Frequency ◽  
Marine Risers ◽  
Vortex Induced Vibrations

Probabilistic Assessment of the Clashing Between Flexible Marine Risers

2010 ◽  
Author(s):  
Jian Wen He ◽  
Ying Min Low
Keyword(s):  
Time History ◽  
Value Theory ◽  
Early Developmental Stage ◽  
Random Wave ◽  
Wave Loads ◽  
Marine Risers ◽  
Important Concern ◽  
Vortex Induced Vibrations ◽  
Nodal Displacements ◽  
External Forces

Flexible marine risers are compliant to external forces from waves, current and platform motions, and clashing between risers is an important concern. In deepwater developments where the number of connected risers is large, it is not economical to space them too far apart. In this regard, it is necessary to establish the probability of riser clashing throughout the service life; however, at present there appears to be no systematic procedure for assessing this risk. This paper presents a novel procedure for estimating the probability of riser clashing based on post-processing results obtained from time domain simulations of flexible risers subjected to random wave loads. First, an efficient technique is employed to sieve out critical pairs among riser elements. From these element pairs, the time history of a normalized clearance parameter is derived from the nodal displacements of the elements. Subsequently, the mean up-crossing rate of this parameter is extracted and extrapolated to the threshold of clashing using extreme value theory. As this method is still in its early developmental stage, critical effects such as vortex-induced vibrations and wake interference will not be considered in the present work.

Mitigation of Vortex-Induced Vibrations of a Pivoted Circular Cylinder Using an Adaptive Pendulum Tuned-Mass Damper

2013 ◽  
Vol 135 (11) ◽  
Author(s):  
Sina Kheirkhah ◽  
Richard Lourenco ◽  
Serhiy Yarusevych ◽  
Sriram Narasimhan
Keyword(s):  
Frequency Response ◽  
Natural Frequency ◽  
Damping Ratio ◽  
Tuned Mass Damper ◽  
Single Frequency ◽  
Fundamental Frequencies ◽  
Transverse Vibrations ◽  
Synchronization Region ◽  
Vortex Induced Vibrations ◽  
Mass Damper

A novel adaptive pendulum tuned-mass damper (TMD) was integrated with a two degree-of-freedom (DOF) cylindrical structure in order to control vortex-induced vibrations of the structure. The natural frequency of the TMD was adjusted autonomously in order to control the vortex-induced vibrations. The experiments were performed at a constant Reynolds number of 2100 and for four reduced velocities, 4.18, 5.44, 6.00, and 6.48. Two TMD damping ratios, 0 and 0.24, were investigated for a constant TMD mass ratio of 0.087. The results demonstrate that tuning the natural frequency of the TMD to the natural frequency of the structure decreases the amplitudes of transverse and streamwise vibrations of the structure significantly. Specifically, the transverse amplitudes of vibrations are decreased by a factor of ten and streamwise amplitudes of vibrations are decreased by a factor of three. Depending on the value of the TMD damping ratio, the frequency of transverse vibrations is either characterized by the natural frequency of the structure or by two other fundamental frequencies, one higher and the other lower than the natural frequency of the structure. The results demonstrate that, independent of the TMD damping and tuning frequency ratios, the frequency of streamwise vibrations matches that of the transverse vibrations in the synchronization region, and the cylinder traces elliptic trajectories. A mathematical model is proposed to gain insight into the frequency response of the structure and fluid-structure interactions. The model shows that, for low TMD damping ratios, the frequency response of the structure equipped with the TMD is characterized by two fundamental frequencies; whereas, for relatively high TMD damping ratios, the frequency response of the structure is characterized by a single frequency, i.e., the natural frequency. In both cases, the fluid forcing within the synchronization region is linked to the fundamental frequency/frequencies of the structure. Thus, the classical definition of synchronization applies to multiple DOF structures undergoing vortex-induced vibrations.

A mini review of recent progress on vortex-induced vibrations of marine risers

2020 ◽  
Vol 195 ◽  
pp. 106704 ◽  
Author(s):  
Guijie Liu ◽  
Haiyang Li ◽  
Zhaozun Qiu ◽  
Dingxin Leng ◽  
Zhixiong Li ◽  
...  
Keyword(s):  
Recent Progress ◽  
Marine Risers ◽  
Vortex Induced Vibrations

Numerical Analysis of Combined VIV and Slug Flow in Time Domain

2016 ◽  
Author(s):  
Tor Huse Knudsen ◽  
Svein Sævik ◽  
Mats Jørgen Thorsen
Keyword(s):  
Multiphase Flow ◽  
Time Domain ◽  
Slug Flow ◽  
Structural Model ◽  
Numerical Study ◽  
Two Phase ◽  
Marine Risers ◽  
Numerical Studies ◽  
Vortex Induced Vibrations ◽  
Internal Velocity

Vortex induced vibrations (VIV) and slug flow are two important aspects for marine risers conveying a multiphase flow, and should be carefully examined due to the influence on the fatigue life of the structure. This article examines a truncated riser exposed to VIV with an internal two-phase slug flow. The main focus of the article was to examine the effect of internal slug flow on the VIV of a riser. The VIV were simulated in time domain with a linear structural model with constant pretension. Approximately 150 vortex shedding periods were simulated after the response reached steady state. An internal two-fluid flow was introduced, with constant internal velocity, pressure and uniform slug lengths. From the numerical study it was apparent that the slug velocity and slug length had an influence on the response pattern, amplitude and frequency. An analytical model that predicts additional response frequencies due to slug flow was also compared to the numerical studies. The analytical study produced similar additional response frequencies as the numerical study. The slug length and internal velocity can influence the response of the riser, and should be considered for marine risers conveying multiphase flow.

Vortex-Induced Vibrations of Long Marine Risers in Sheared Flows: DNS Studies

2003 ◽  
Author(s):  
Didier Lucor ◽  
X. Ma ◽  
M. S. Triantafyllou ◽  
G. E. Karniadakis
Keyword(s):  
Fatigue Life ◽  
Floating Structures ◽  
Marine Risers ◽  
Sheared Flows ◽  
Vortex Induced Vibrations ◽  
Offshore Industry ◽  
Water Depths

The offshore industry is moving to ever-increasing water depths, producing presently oil in depths of up to 2,500m, requiring detailed fatigue calculations for the risers and tendons used in the floating structures, see [1, 2]. Currents in the ocean are invariably highly sheared, hence the modes that can potentially be excited are many, see [3]. The calculation of how many and which modes are excited, can affect fatigue life very significantly, but there are no guidelines presently available for conducting this calculation.

Simplified Model for Evaluation of Fatigue From Vortex Induced Vibrations of Marine Risers

2004 ◽  
Author(s):  
Gro Sagli Baarholm ◽  
Carl M. Larsen ◽  
Halvor Lie ◽  
Kim Mo̸rk ◽  
Trond Stokka Meling
Keyword(s):  
Fatigue Damage ◽  
Cross Flow ◽  
Mode Shapes ◽  
Design Stage ◽  
Response Frequency ◽  
Lower Mode ◽  
Marine Risers ◽  
Vortex Induced Vibrations ◽  
Well Yield ◽  
The Cross

This paper presents a novel approach for approximate calculation of the fatigue damage from vortex-induced vibrations (VIV) of marine risers. The method is based on experience from a large number of laboratory tests with models of full-length risers, large-scale tests and also full-scale measurements. The method is intended to provide a conservative result and be used for screening purposes at the early design stage. The model is in particular aimed at predicting fatigue for risers that respond at very high mode orders (above 10), but may as well yield valid results for lower mode numbers. The model will, however, not be adequate for free span pipelines or other structures that normally will respond at first and second mode. The riser will be defined in terms of some key parameters like length, weight, tension, hydrodynamic diameter and stress diameter. A current profile perpendicular to the riser in one plane must be known. The program will apply a simple model for calculation of eigenfrequencies and mode shapes, and these are sorted into in-line (IL) and cross-flow (CF) bins. An effective current velocity and excitation length can be defined from the profile and will be applied to identify the dominating cross-flow response frequency and the total displacement rms value. The dominating in-line response frequency is taken as twice the cross-flow frequency, and inline response rms is taken as a given portion of the cross-flow rms value. A set of contributing modes is defined from an assumed frequency bandwidth that reflects observed bandwidths, but also modal composition for cases with discrete frequency response. A simple mode superposition technique is then used to find the set of modes that gives the identified rms values. Bending stresses will be found directly from the curvature of the mode shapes. Fatigue damage will be found from stress rms values, user defined stress concentration factor and given SN curves. The model has been implemented in a simple computer program and verified by comparing results to measurements. The ambition has not been to obtain an exact match between computed results and observations, but to verify that the model gives reasonable but conservative results in almost all cases. However, an unrealistic over prediction of the fatigue damage is not desired. The results are promising, but the need for more information from measurements and response analyses with programs like VIVANA and SHEAR7 is still obvious.

Dual resonance in vortex-induced vibrations at subcritical and supercritical Reynolds numbers

2010 ◽  
Vol 643 ◽  
pp. 395-424 ◽  
Author(s):  
J. M. DAHL ◽  
F. S. HOVER ◽  
M. S. TRIANTAFYLLOU ◽  
O. H. OAKLEY
Keyword(s):  
Reynolds Number ◽  
Natural Frequency ◽  
Frequency Ratio ◽  
Reynolds Numbers ◽  
Large Reynolds Number ◽  
Resonant Response ◽  
Third Harmonic ◽  
Vortex Induced Vibrations ◽  
Dual Resonance ◽  
Harmonic Components

An experimental study is performed on the vortex induced vibrations of a rigid flexibly mounted circular cylinder placed in a crossflow. The cylinder is allowed to oscillate in combined crossflow and in-line motions, and the ratio of the nominal in-line and transverse natural frequencies is varied systematically. Experiments were conducted on a smooth cylinder at subcritical Reynolds numbers between 15 000 and 60 000 and on a roughened cylinder at supercritical Reynolds numbers between 320 000 and 710 000, with a surface roughness equal to 0.23 % of the cylinder diameter. Strong qualitative and quantitative similarities between the subcritical and supercritical experiments are found, especially when the in-line natural frequency is close to twice the value of the crossflow natural frequency. In both Reynolds number regimes, the test cylinder may exhibit a ‘dual-resonant’ response, resulting in resonant crossflow motion at a frequency fv, near the Strouhal frequency, and resonant in-line motion at 2 fv. This dual resonance is shown to occur over a relatively wide frequency region around the Strouhal frequency, accompanied by stable, highly repeatable figure-eight cylinder orbits, as well as large third-harmonic components of the lift force. Under dual-resonance conditions, both the subcritical and the supercritical response is shown to collapse into a narrow parametric region in which the effective natural-frequency ratio is near the value 2, regardless of the nominal natural-frequency ratio. Some differences are noted in the magnitudes of forces and the cylinder response between the two different Reynolds number regimes, but the dual-resonant response and the resulting force trends are preserved despite the large Reynolds number difference.

scholarly journals PLUCK TESTS ON OPERATING DEEPWATER PIPELINE SPANS

2020 ◽  
Author(s):  
Ralf Peek ◽  
Chiara A. Bernardo ◽  
Hui Min Hong ◽  
Conleth D. O’Loughlin ◽  
David White ◽  
...  
Keyword(s):  
Natural Frequency ◽  
Axial Force ◽  
Weak Link ◽  
Free Vibrations ◽  
Spring Stiffness ◽  
Subsea Pipeline ◽  
Vortex Induced Vibrations ◽  
Soil Damping ◽  
To Come

Key uncertainties in the assessment of subsea pipeline spans for fatigue due to vortex-induced vibrations (VIV) are the effective axial force, the soil spring stiffness, and the soil damping. To reduce these uncertainties, pluck tests have been carried out, to determine the natural frequency and damping of single and multiple spans. These are carried out by pulling the span laterally at midspan with the ROV, until a 6mm or 8mm PP rope that serves as a weak link in the connection from the pipeline and the ROV breaks. The free vibrations resulting from this pluck are measured with accelerometers attached to the pipeline. The paper presents selected results from these tests and their interpretation in terms modal frequencies and damping ratios. Already at the achieved amplitudes of vibration of up to about 0.01D, the results already show considerable nonlinearity and inelasticity that is thought to come from the soil supporting the pipe at the shoulders of the span, and can be captured in FE models by making the soil springs nonlinear and inelastic.

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