High Mode Number Vortex-Induced Vibration Field Experiments

The method of pulling a steel catenary riser (SCR) through a steel tube (termed as a pull tube) is common practice for deepwater riser tie back applications. Vortex-induced vibration (VIV) of such a system is complex. VIV analysis programs, such as Shear7 [1], are suitable only for a single, chain-like structure. The application of such a software tool in VIV design of the SCR-pull tube system requires careful consideration of a number of structural and hydrodynamic factors. This paper presents a methodology for VIV analysis of the combined structural system of the SCR with the pull tube. Firstly, the entire SCR-pull tube system is modeled with the finite element program Flexcom [2]. The modes are then calculated for the entire structure with program Modes [3]. Afterwards, the structural nodes are rearranged for VIV analysis with Shear7. The pull tube is secured on the platform through a number of guides on the truss structure of the hull. The diameter of the pull tube is much larger than that of the SCR, and the pull tube is much stiffer in bending than the SCR is. If the entire structure is analyzed with Shear7, the mode for the pull tube (a mode involving a large motion of the pull tube section), which is very high in order, would be embedded in the analysis. It makes sense to single out the pull tube mode for study as if it is the first mode. A computer program, named as V-Span [4] for subsea span VIV analysis, is used to analyze both in-line and cross-flow VIV of the pull tube. A numerical example is presented to demonstrate this methodology. This is a deepwater SCR, which has a diameter of 9 inches. The water depth is 6,300 ft. The pull tube is 640 feet long and 20 inch in diameter. Both the loop-eddy and background currents are analyzed. The fatigue damage resulted from both in-line and cross-flow VIV is estimated.

Current tank tests have been performed on two equal-diameter, flexible cylinders in tandem with various end spacings or separation distances. While the cylinders experienced transverse VIV in only the first mode, the test Reynolds numbers were well into the transition range, and the drag crisis was clearly observed on the upstream cylinder. Both acceleration and drag were measured to provide some interesting correlations between vibration, drag, and end spacing for each of the two cylinders. Introduction During design of floating production platforms in deepwater, it has been recognized that there is the possibility of interference between adjacent production or export risers, or possibly between other riser/tendon combinations such as a drilling riser and a production riser. The two interference consequences of most concern are: a) possible increases in fatigue damage during vortex-induced vibration (VIV); and b) possible contact between adjacent risers. This research project has attempted to further study the vibration behavior of two production risers in tandem, with the hopes of characterizing the changes in drag coefficient with vibration at Reynolds numbers as close to the prototype as possible. This project is part of a research program in riser interference that has been in progress since 1989. Previous published results for current tank tests performed at low Reynolds numbers, but with high-mode numbers, include: tests with dual equal diameter cylinders in tandem [1]; and tests on dual cylinders in a staggered (offset from tandem) arrangement [2]. In addition to the published test results, low Reynolds number, high-mode number tests have also been performed with: dual unequal diameter cylinders in tandem; dual tandem cylinders with helical strakes; and three equal diameter tests in tandem. High Reynolds number tests have also been performed as part of this research program, and some of the single cylinder results have been published [3-6]. This paper presents results for tandem bare cylinders experiencing (primarily) first-mode VIV at critical high Reynolds numbers. Similar tests have also been performed on tandem cylinders with different surface roughness, helical strakes, fairings, and perforated shrouds. Tests have also been conducted on longer pipe models experiencing critical Reynolds numbers and higher mode VIV. These tests have also been complimented by various tow tests used to determine sectional drag coefficients for cylinders experiencing forced VIV. This paper presents current tank experimental results for two equal diameter ABS tubes in tandem that were spaced at several different end spacings (separation distances). The cylinders were flexible and experienced VIV in various bending modes, with the first transverse bending mode being the focus of the experiments. Simultaneous drag force and vortex-induced vibration (acceleration) measurements were made. The following sections present a summary of the test results, a description of the tests, and a detailed discussion of the test results. Test Description Current Tank. All of the experiments were performed in the Shell Westhollow Technology Center current tank facility. A ship's propeller driven by a hydraulic power unit circulates the water through the tank.

This study presents an advanced numerical prediction model based on nonlinear wake oscillators for the investigation of multidirectional vortex-induced vibration (VIV) responses of a long catenary riser depending on the structural curved configuration versus the incoming flow direction. By considering a uniform free-stream flow aligned with the initial curvature plane of the catenary riser in a convex or concave shape, the normal flow velocity component is nonlinearly sheared owing to the spanwise variation of inclination angles. This is different from the perpendicular flow case in which the normal flow velocity is spatially constant. To capture the influence of flow direction on the three-dimensionally coupled cross-flow and in-line VIV, distributed wake oscillators are introduced and applied to simulate the fluctuating hydrodynamic lift and drag forces that account for the important effects of cylinder inclination, variable vortex excitation frequencies, global–local​ displacement relationships, and relative flow-cylinder velocities. The amplified mean drag force and the axial force depending on the tangential flow velocity are also considered. These empirical hydrodynamic effects are nonlinearly coupled with the equations of riser three-dimensional motion, and the overall governing equations are numerically solved to assess the multimode, multi-frequency and multi-degree-of-freedom responses. Validation of the model is carried out for VIV of flexible straight and curved cylinders based on experimental results in the literature. Parametric investigations are performed by varying the incoming flow velocities in perpendicular, convex and concave configurations for a long catenary riser with a low mass-damping ratio. In-plane (horizontal and vertical) and out-of-plane VIV features are presented and discussed in terms of space–time varying amplitudes, drag-amplified shape reconfigurations, oscillation frequencies, lock-in occurrences, dual resonances, orbital motion trajectories, fluid–structure energy transfer, and multimodal contributions. Overall prediction results highlight the influence of the cylinder geometric configuration versus the incoming flow orientation on multidirectional VIV characteristics that should be recognised and incorporated into practical analysis tools.

As the search for oil and gas takes place on increasing water depths, the traditional production systems based on fixed or floating platforms becomes increasingly costly. A good alternative to the traditional production systems are subsea to shore solutions, where pipelines transport the product to a land terminal for processing. One of the main challenges of documenting pipeline integrity in subsea to shore solutions is related to fatigue induced by vortex induced vibrations (VIV). Rough and uneven seabed introduce free spans that can be short or long, have small or large clearance and can have a large variety of boundary conditions at the shoulders. Hence, there is a need for methods that can give reliable estimate for fatigue damage under all realistic conditions. This paper investigates the measured VIV for a free spanning pipeline model. The model has L/D ratios from 72 to 350 and includes both single and multi-spans. It is exposed to uniform current and is free to vibrate in both cross-flow (CF) and in-line (IL) direction. Bending strains are measured in CF and IL direction at ten locations along the pipe. The observed cross-sectional trajectories are analyzed, with particular focus on the complex interaction between CF and IL VIV, and the significance of higher order response components. The purpose of this investigation is to understand the behaviour of the free spanning pipelines with respect to response amplitudes, frequency and modal composition, and also to identify characteristic cross section trajectories for further investigation. This work is a continuation of the earlier published results by Nielsen et al. [1] and Soreide et al. [2].

To avoid or mitigate global warming, several ocean carbon capture and storage concepts have been proposed. One of the recent approaches is to dispose carbon dioxide via a fixed vertical cantilevered pipe onto the seabed in deep waters. Due to a high aspect ratio and flexibility of such long pipe conveying fluid with fixed-free end conditions and external hydrodynamic loading caused by currents, the pipe may experience large-amplitude 3-D vibrations leading to structural failure. Hence, it is essential to understand and investigate the pipe nonlinear dynamic behaviors subject to combined flow-induced vibration (FIV) and vortex-induced vibration (VIV). In this study, the 3-D nonlinear equations of a cantilevered pipe discharging fluid in the sea are analyzed using a Galerkin-based multi modal approach combined with a finite difference Houbolt’s integration scheme. Particular attention is paid to the combined effects of FIV and VIV on the dynamic response of the cantilevered pipe in water. To model the fluctuating lift and drag forces associated with VIV, the two dimensional wake oscillators distributed along the pipe are adopted. Numerical simulations in the FIV case of a pipe discharging fluid in the air are first validated with experimental results in the literature to justify the mathematical models and numerical approaches. Modal convergence analysis is also performed. Results in the combined FIV and VIV cases are then highlighted in order to show the effects of cross-flow and in-line VIV when compared with the pure FIV case. The effects of geometric nonlinearities, the coupling/interaction of multi modes and the space-time modifications of pipe responses and trajectories are highlighted. It is hoped that the numerical observations and findings obtained from this study could be verified by experimental studies which are presently lacking in the literature.

Marine risers may experience a complex vortex-induced vibration (VIV) owing to the enhanced nonlinear effects from structural curved shapes versus sheared flows. Many studies have focused on VIV of straight risers in sheared flows or curved risers in uniform flows. In this study, a numerical prediction model is advanced to investigate the multidirectional VIV of a catenary riser subjected to linearly sheared flow. The prediction model is described by equations of three-dimensional responses of the catenary riser coupled with the spatially distributed nonlinear wake oscillators calibrated against experimentally observed VIV responses of curved cylinders. Hydrodynamic lift and drag forces are mathematically modelled based on fluid-structural variables accounting for effects of space-varying riser local inclinations, vortex excitation frequencies and relative flow-cylinder velocities following the Independence Principle. The coupled VIV system is discretized in space by a second order finite difference scheme and integrated in time. To validate the model, simulations are performed based on a flexible curved cylinder under the concave sheared flow and compared with experimental results in the literature. By varying the relative directions between the riser equilibrium and the incoming flow in planar and non-planar configurations, VIV responses of a long flexible catenary riser in a uniform free stream flow versus a linearly sheared flow are compared and analysed in terms of amplitudes, resonant frequencies, lock-in conditions, and energy transfer mechanisms under perpendicular, convex and concave flows. Typical VIV features are captured alongside travelling wave responses, modulated amplitudes and frequency switching caused by sheared flows. Overall results highlight the influence of sheared flows on multidirectional VIV features.

The cross-flow vortex-induced vibration (VIV) response of an elastically mounted idealized undulatory seal whisker (USW) shape is investigated in a wide range of reduced velocity at angles of attack (AOAs) from 0° to 90° and a low Reynolds number of 300. The mass ratio is set to 1.0 to represent the real seal whisker. Dynamic mode decomposition is used to investigate the vortex shedding mode in various cases. In agreement with past studies, the VIV response of the USW is highly AOA-dependent because of the change in the underlying vortex dynamics. At zero AOA, the undulatory shape leads to a hairpin vortex mode that results in extremely low lift force oscillation with a lowered frequency. The frequency remains unaffected by VIV throughout the tested range of reduced velocity. As the AOA deviates from zero, alternating shedding of spanwise vortices becomes dominant. A mixed vortex shedding mode is observed at AOA = 15° in the transition. As the AOA deviated from zero, the VIV amplitude increases rapidly by two orders, reaching the maximum of about 3 times diameter at 90°. An infinite lock-in branch is present for AOA from 60° to 90°, where the VIV amplitude remains high regardless of the increase in reduced velocity.

Numerical study on the characteristics of vortex-induced vibrations of a small-scale subsea jumper using a wake oscillator model

Numerical simulation of vortex-induced vibration of a full-scale flexible riser at supercritical Reynolds number in uniform and linear shear flow

The STRIDE JIP has performed test initiatives into several areas considered to be crucial to the design of steel catenary risers for harsh, deepwater environments. The following investigations are described:Water-tank testing of a ¼" diameter × 100ft long catenary riser model, looking at response to production vessel motions, in particular the response in the touch-down region for rigid and elastic seabed simulations.Vortex induced vibration tests on riser models 20ft long, 6" in diameter, in simulated currents up to 16.5 ft/s (5m/s - Reynolds 6.7x105). Helical straked and bare pipe models were towed at angles of up to 45° to the flow, simulating current flow over non-vertical riser sections of simple catenary and buoyant wave risers. The requirement and effectiveness of strakes on inclined sections was investigated.Vortex induced vibration tests on riser strings 650ft long, 10.75" diameter in simulated currents up to 5.8 knots (3m/s - Reynolds 6x105). Helical straked and bare pipe models were towed at angles of up to 75° to the vertical in a Norwegian Fjord between two tugs, continuing the investigation into non-vertical riser response to high currents. Introduction Phase II of the STRIDE JIP was initiated by 2H Offshore in March 1998, and is sponsored by 15 operators and 5 engineering contractors. During Phase I of the project, 2 areas were identified as being of prime importance to the design of deepwater steel catenary risers: the touch down point (TDP) and vortex induced vibration (VIV)1. TDP - A deepwater catenary riser TDP can actually involve a wide region of possible contact between different parts of the riser and different areas of the seabed, as dictated by the environmental loading on the floating production vessel and riser system. STRIDE Phase I identified this area of a catenary riser as critical with regards to the strength and fatigue life of the whole system. Confidence in analysis techniques was considered to be of paramount importance to such developments, and this lead to the proposed test program. VIV - Lack of empirical data on VIV of inclined and curved structures required a conservative design approach in STRIDE Phase I. In particular, it had to be assumed that VIV suppression strakes would be required over most of the riser length. The increase in riser drag due to such heavy straking produces a succession of technical and commercial knock-on effects. A high drag catenary riser needs extra ballast in the vertical section to accommodate the current loading, otherwise the riser can be pushed into shapes that overstress the TDP - Figure 1. Within the analysis, ballast was assumed to be most effectively applied as increased riser pipe wall thickness, and resulted in an increase of +40% on that required for pressure considerations alone. Increased wall thickness was shown to have a major impact on installation costs, particularly the amount of temporary buoyancy required for what was found to be the leading installation method at the time, controlled depth tow out. At the kick-off meeting for STRIDE Phase II, it was agreed by all participants that VIV and TDP be top of the list for further investigations. TDP Testing Program The main objective of these tests was to benchmark the ability of industry software to predict the response of a riser pipe catenary at its TDP, particularly as it is put into compressive loading or buckling as a result of extreme motions at the top end, for instance due to extreme storm motions on the floating production vessel.

Species richness and reproductive modes of chondrichthyans in relation to temperature and fishing effort in the Southwestern Atlantic Shelf (34–54° S)

As an important component transporting resources such as oil and mineral ores mixture from the seabed to the surface in ocean engineering, vortex-induced vibration (VIV) of flexible risers can be encountered when the risers are subjected to the external environmental conditions. As VIV can lead to structural fatigue for the riser system, which threatens to the facility safety during deepsea resource exploitation, it is of great significance to investigate VIV mechanism and dynamics. Therefore, VIV dynamics of a flexible fluid-conveying riser undergoing external shear current is studied based on the combination of the Euler-Bernoulli beam theory and the semi-empirical hydrodynamic model. The finite element method and Newmark-β method are adopted to discretize and solve the governing equation. The model is firstly validated by comparing with the experimental data in order to examine the accuracy of the present model. Subsequently, cross-flow (CF) VIV response of the fluid-conveying riser is mainly examined and analyzed while various internal flow velocity and fluid density are considered and changed. The results show that when the flexible riser is subjected to both internal flow and shear current, there appears multi-frequency response for CF VIV. And the CF vibrating frequency and the CF root mean square (RMS) displacement are evidently influenced by the internal flow velocity and fluid density. With the increase of the internal flow velocity and fluid density, the CF vibrating frequency decreases while the RMS displacement shows an increasing trend in CF direction. Furthermore, in addition to the variation of the CF vibrating frequency and RMS displacement, the change of internal flow densities can cause notable mode and frequency transitions.

Vortex-induced vibrations of flexible cylinders predicted by wake oscillator model with random components of mean drag coefficient and lift coefficient

It is well known that VIV (Vortex-Induced Vibration) occurs in a riser. Effects of the top tension and the surface roughness of a riser have not been clear for a behavior of a riser. In this study, the model test in which geometrical similarity models are used is carried out in order to understand the behavior of a riser including VIV phenomenon. The model risers correspond to the production riser of 120m in a real scale, and the corresponding model tests are carried out in scales of 1/50. The behaviors in uniform flows, waves and under the forced oscillation are measured three dimensionally by the real time graphic data processing system. This model test focuses on the effect of top tension and the surface roughness.The present paper shows the vertical distributions of a maximum horizontal displacement and the time histories at arbitral point of the corresponding riser model. The results show that the behavior characteristics of the risers differ against the top tension. Then VIV is remarkably influenced by the top tension. In addition, the maximum horizontal displacement is different about 20% due to the surface roughness.

The interfering characteristics of vortex-induced vibration (VIV) responses for tandem double risers under the recently discovered bidirectionally sheared flow are unclear. To reveal the interfering VIV characteristics in bidirectionally sheared flow, a rotating test rig was built to simulate bidirectionally sheared flow in the ocean basin. A series of VIV experiments for two scaled riser models were carried out, which included a single riser and tandem double risers with two different center-to-center spacing (3D and 5D). Then the modal superposition method was used to reconstruct the VIV displacements in cross-flow (CF) direction and the drag displacement. Furthermore, the drag coefficient was identified and analyzed. Results indicate that the VIV frequency of tandem double risers remains consistent at same spacing. When the reduced velocity (Vr) is in range of 5.0 to 18.0, the VIV amplitude of tandem double risers is basically same, but slightly smaller than that of the single riser. However, the VIV amplitude of riser 2 is significantly higher than that of riser 1 at Vr > 18.0. Moreover, it is found that the overall average drag coefficient in tandem double risers is lower than 1.38 of single riser, with values of 1.25 (S = 3D) and 1.33 (S = 5D) respectively.