Multi-modal Vortex-Induced Vibrations of a vertical riser pipe subject to a uniform current profile (original) (raw)
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Dynamic behavior of pipelines and risers due to vortex-induced vibration in time domain
Marine Systems & Ocean Technology, 2011
Vortex-Induced Vibration (VIV) plays an important role in the design of oil and gas production systems in offshore pre-salt fields in Santos Basin, Brazil. The importance is represented by the large depth ofpetroleum reservoirs bellow the subsalt layer, the ultra-deep waterdepth and the far distance ofthe oilfield from the coast. Very aggressive fluid components like C02 and S02 present in the produced petroleum make more complex the necessities in terms of material properties for pipelines and risers, which will increase problems due to the fatigue. The VIV in otfshore structures is complex and still not been completely understood, particularly for current with high Reynolds number (Re). This paper describes a time domain simulation ofthe dynamics ofpipeline and risers subjected to VIV. A semi-empirical model is adopted to calculate the cross-flow force due to VIV based on the lift coefficient and Strouhal number. This force is evaluated by a Morison-type formulation. The flow is assumed two-dimensional and the shedding frequency is not locked on to the structure vibrating frequency. The proposed simulation model is validated through comparisons ofits results with experimental data.
Vortex-Induced Vibrations of Risers: Theoretical, Numerical and Experimental Investigation
Oil & Gas Science and Technology, 2002
Vibrations d'un riser soumis à un écoulement : étude théorique, numérique et expérimentale -Les vibrations engendrées par le relâcher tourbillonnaire dans le sillage d'un cylindre soumis à un courant peuvent créer une fatigue importante dans les tubes utilisés par l'industrie offshore pour amener le pétrole ou le gaz du fond de la mer jusqu'à la plate-forme ou le navire de stockage. Ce sujet fait l'objet de très nombreuses études et, à l'Institut français du pétrole, plusieurs modèles sont développés pour prédire la durée de vie de ces tubes. Les méthodes vont d'une simple analyse modale de la réponse de la structure jusqu'à un calcul couplé fluide-structure avec résolution des équations de Navier-Stokes. Au travers du projet Hydlines, des campagnes d'essais sont menées pour valider les différentes approches.
Vortex modes and vortex-induced vibration of a long, flexible riser
Ocean Engineering, 2009
Investigations of the velocity and vorticity fields in the wake of a flexible riser with a length to diameter ratio of 181 were conducted in a towing tank at moderate Reynolds numbers in the range of 9400-47,000. Wake velocity measurements were made with the riser freely vibrating in both in-line and cross-flow directions. The motion and wake field of the riser, undergoing free vibration, were simultaneously measured by accelerometers installed inside the riser and by using a digital particle image velocimetry (DPIV) system. The vortex-induced vibration (VIV) results show that the riser freely oscillated at multiple vibration frequencies and amplitudes at each Reynolds number. Mixed vortex modes, '2S', '2P' and 'P+S', were observed in the near wake of the riser at different instants of time. The occurrence of these vortex modes depended on the Reynolds number, dominant frequency and mean amplitude. At lower Reynolds number, the single stable mode '2S' dominated the wake. With the increase of Reynolds number, the percentage of the '2S' modes decreased while the percentage of '2P' modes increased steadily except at Reynolds numbers of 14,100 and 47,000. The 'P+S' modes occurred mostly at a Reynolds number of 14,100 accompanied by more '2P' modes and less '2S' modes. At this Reynolds number, the frequency of the VIV was very close to the natural frequency of 0.72 Hz, which was obtained from a riser decay test in steady water and the average amplitude to diameter ratio reached 0.95, the highest found in these tests.
Concomitant Vortex-Induced Vibrations (VIV) and axial motion prescribed to the top of a vertical and flexible cylinder were experimentally investigated in a towing tank facility. The technological motivation for such experimental campaign is the risers dynamics scenario. Vertical motions are applied to the top of such structures, causing geometric stiffness modulations. Such modulation cause modulations in the natural frequencies which, in turn, may affect VIV phenomenon. Both statistical and modal analyses were carried out, showing that the f t : f N,1 = 2 : 1 top-motion ex-citation enhances the crosswise vibrations for all free-stream velocities tested. Focus was put on the analysis of the modal-amplitude time-histories corresponding to the first-vibration mode. It was also observed that for large reduced velocities, the Fourier spectra obtained from these time-histories are broadbanded. Still considering the latter range of free-stream velocity, the presence of top-motion enhances the characteristic oscillation amplitude if compared to the non top-motion excited case.
On the Significance of the Higher-Order Stress in Riser Vortex-Induced Vibrations Responses
Journal of Offshore Mechanics and Arctic Engineering
Vortex-induced vibrations (VIV) can lead to fast accumulation of fatigue damage and increased drag loads for slender marine structures. VIV responses mainly occur at the vortex shedding frequency, while higher harmonics can also be excited. Recent VIV model tests with flexible pipes have shown that higher harmonics in the crossflow (CF) direction can contribute to the fatigue damage significantly due to its higher frequency. Rigid cylinder experiments show that the CF third-order harmonics are more pronounced when the motion orbit is close to a “figure 8” shape and the cylinder is moving against the flow at its largest CF motion. However, there is still lack of understanding of when and where higher harmonics occur for a flexible pipe. Therefore, significant uncertainty remains on how to account for fatigue damage due to higher harmonics in VIV prediction. In the present paper, representative VIV data from various riser model test campaigns are carefully studied and analyzed. The ke...
Journal of Marine Science and Application, 2017
In this paper, numerical simulations of vortex-induced vibrations in a vertical top-tension riser with a length-to-diameter ratio of 500 using our in-house code viv-FOAM-SJTU are presented. The time-dependent hydrodynamic forces on two-dimensional strips are obtained by solving the Navier-Stokes equations, which are, in turn, integrated into a finite-element structural model to obtain the riser deflections. The riser is discretized into 80 elements with its two ends set as pinned and 20 strips are located equidistant along the risers. Flow and structure are coupled by hydrodynamic forces and structural displacements. In order to study the effects of the shear rate, of the current profiles on the vortex-induced vibrations in the riser, vibrations, with varying shear rates, in both the in-line and cross-flow directions, are simulated. In addition to the time domain analysis, spectral analysis was conducted in both the temporal and spatial domains. Multi-mode vibration characteristics were observed in the riser. The relationship between dominant vibration mode number and the shear rate of current profiles is discussed. In general, the overall vibrations in the riser pipe include contributions from several modes and each mode persists over a range of shear rates. Moreover, the results suggest that with a larger shear rate the position of the maximum in-line time-averaged displacement will move closer to the end where the largest velocity is located.
Mono- and multi-frequency vortex-induced vibrations of a long tensioned beam in shear flow
Journal of Fluids and Structures, 2012
The mono-frequency as well as multi-frequency vortex-induced vibrations of a tensioned beam of aspect ratio 200, immersed in a linear shear flow at Reynolds number 330 and free to move in both the in-line and cross-flow directions, are studied by means of direct numerical simulation. The structural responses are composed of mixed standing-traveling wave patterns. We observe a switch between mono-and multi-frequency vibrations when the mass ratio changes from a value of 3 to 6, while keeping constant the non-dimensional cable and beam phase velocities. This switch is attributed to the accompanying change in the time-averaged in-line curvature of the beam, which alters the oncoming flow velocity component normal to the structure configuration. It is shown, in general, that the mono-or multi-frequency nature of the response is controlled by the form of the profile of the normal component of the oncoming flow. Mono-and multi-frequency vibrations may occur in both the in-line and cross-flow directions, with a frequency ratio close to 2. Each excited frequency is associated with a single structural wavenumber. The local synchronization between the vortex shedding and the cross-flow oscillation, i.e. the lock-in condition, occurs in the high velocity zone and covers a similar spanwise extent in both the mono-and multi-frequency cases. Counterclockwise figure-eight trajectories are very likely to occur within the lock-in region. In both the mono-and multi-frequency types of response, the flow excites the structural vibrations within the lock-in region and damps the structural motions in the non-lock-in region. The multi-frequency character of the response impacts both the lock-in phenomenon and the fluid-structure energy transfer.
Ships and Offshore Structures, 2015
The main purpose of the research described in this paper is to investigate the hydroelastic interactions that take place between oscillating flexible cylinders and fluid forces. The cylinders are subject to currents and shear flow, and the hydrodynamic forces are estimated by a discrete vortex method. The calculations are compared with results obtained using the quasi-steady theory, as proposed by Bearman et al. (Appl. Ocean Res. 6(3) (1984) 166) and employed by Ferrari (Ph.D. Thesis, University of London, UK, 1998). In addition, the calculations are compared with experiments carried out by Fujarra (M.Sc. Thesis, Universidade de S* ao Paulo, 1997) involving a cantilever flexible cylinder immersed in a current. The normalized amplitude curve obtained in the calculations is compared with the experimental results. Visualizations of the wake indicate a hybrid mode of vortex shedding along the span. Employing the terminology suggested by Williamson and Roshko (J. Fluids Struct. 2 (1988) 355), a 2S mode is found in regions of low amplitudes, changing to a 2P mode in regions of larger amplitudes. The position where transition of modes occurs varies with the reduced velocity. A practical case of a vertical marine riser is shown, and the numerical results for various current profiles are discussed. r