In fluid dynamics, vortex-induced vibrations (VIV) are motions induced on bodies interacting with an external fluid flow, produced by – or the motion producing – periodical irregularities on this flow.
A classical example is the VIV of an underwater cylinder. You can see how this happens by putting a cylinder into the water (a swimming-pool or even a bucket) and moving it through the water in the direction perpendicular to its axis. Since real fluids always present some viscosity, the flow around the cylinder will be slowed down while in contact with its surface, forming the so-called boundary layer. At some point, however, this boundary layer can separate from the body because of its excessive curvature. Vortices are then formed changing the pressure distribution along the surface. When the vortices are not formed symmetrically around the body (with respect to its midplane), different lift forces develop on each side of the body, thus leading to motion transverse to the flow. This motion changes the nature of the vortex formation in such a way as to lead to a limited motion amplitude (differently, than, from what would be expected in a typical case of resonance).
VIV manifests itself on many different branches of engineering, from cables to heat exchanger tube arrays. It is also a major consideration in the design of ocean structures. Thus study of VIV is a part of a number of disciplines, incorporating fluid mechanics, structural mechanics, vibrations, computational fluid dynamics (CFD), acoustics, statistics, and smart materials.
Pipelines from offshore petroleum fields must frequently pass over areas with uneven seafloor. In such cases the pipeline may have free spans when crossing depressions. Hence, if dynamic loads can occur, the free span may oscillate and time varying stresses may give unacceptable fatigue damage. A major source for dynamic stresses in free span pipelines is vortex induced vibrations (VIV) caused by steady current. This effect is in fact dominating on deep water pipelines since wave induced velocities and accelerations will decay with increasing water depth. The challenge for the industry is then to verify that such spans can sustain the influence from the environment throughout the lifetime of the pipeline. The aim of the present project is to improve the understanding of vortex induced vibrations (VIV) of free span pipelines, and thereby improve methods, existing computer programs and guidelines needed for design verification. This will result in more cost effective and reliable offshore pipelines when laid on a very rugged seafloor.
A free span pipeline will necessarily oscillate close to the seabed. The presence of the seabed will therefore have some influences on the ambient flow profile and also on the flow pattern around the cylinder during oscillation. Hydrodynamic parameters may therefore vary when the pipe is close to the seabed. In the present work, the influence from spatial varying current profiles is investigated for both single and multiple span pipeline. It is shown that the difference between using uniform and spatial varying current profiles is significant for some current speeds. It is also shown that use of spatial varying current profiles can be even more important for multiple span pipeline.
The comparison of VIVANA analysis results with MARINTEK test results has been given. It shows VIVANA predicts the cross-flow response generally much higher than the test measurements, especially for the higher mode responses. To improve understanding of this phenomena, the VIVANA model was tuned to the test model and results are compared in different cases. Attempts were made to obtain a better agreement by adjusting some of the input parameters to VIAVANA.
VIV for multiple span pipeline is investigated and the dynamical interaction between adjacent spans has been shown. The interaction may lead to increased or decreased response of each spans depending on the current speed and the properties for the two spans. The extension of the contact zone between the spans and seafloor parameters will of course also be important for the interaction effect. The influence from temperature variation on vortex induced vibrations has been demonstrated. The response frequency is influenced through changes in pipe tension and sag. Both increase and decrease of the response frequency may be experienced. Moreover, it is shown that the influence from snaking of the pipe on the temperature effect is small, at least for large diameter pipes.
The reference point is tuned by changing various hydrodynamic properties, i.e. CL, St and added mass. The response frequencies are also tuned in order to have a better agreement on the results. It is been concluded that the method used here by VIVANA is not able to describe VIV for free spanning pipelines adequately. It is not possible to find a set of parameter in a rational way that will give reasonably correct results. The discrepancy between the analysis and test results are highlighted which confirms the interaction between the in-line and cross-flow vibrations. Discussions are given and addressed on different reasons which may cause this phenomena.
An improved strategy for non-linear analysis of free span pipeline is outlined. Time domain analysis for free span pipeline has been performed. The difference between time and frequency domain analysis has also been investigated by varying boundary conditions, pipe properties and axial tension. A significant difference is shown between results from time and frequency domain analysis at each end of the span where the pipe is started to interact with the seafloor. Due to high fatigue at this point, the importance of using non-linear time domain analysis is therefor obvious and highly recommended.
Source:
- https://en.wikipedia.org/wiki/Vortex-induced_vibration
- Koushan, Kamran. 2009. Vortex Induced Vibrations of Free Span Pipelines. Norway: NTNTU. (http://www.diva-portal.org/smash/get/diva2:217873/FULLTEXT01.pdf)
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