Prediction model for multidirectional vortex-induced vibrations of catenary riser in convex/concave and perpendicular flows

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 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.

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.

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

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.

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].

This paper presents the initial results from VIV (Vortex-Induced Vibration) field testing of a long, flexible, model riser at high mode number. The experiments were designed to better understand the dynamic behavior of a long riser in uniform flow responding at mode numbers from 10 to 25 in cross-flow vibration. The 1.31 inch, (0.0333 m), diameter riser model was made from fiberglass line pipe, manufactured by Fiberspar Corp. Two model configurations with length of 201 feet (61.26m) and 401 feet (122.23m) were towed behind a vessel in a deep lake in upstate New York. Motion was recorded with twenty four evenly spaced internal tri-axial accelerometers. Observed reduced velocity and RMS displacement response levels are reported. Mean drag coefficients and hydrodynamic damping derived from measured data are compared to calculated values from formulas commonly used in engineering design of offshore systems. Introduction The Experiments at Lake Seneca, New York took place in the summer of 2004. They are part of a larger testing program developed by DEEPSTAR (A joint industry technology development project) aimed at improving the ability to model and mitigate VIV. While these particular tests were focused on investigating uniform flow conditions, a second set of tests designed for shear flow conditions was subsequently performed in November 2004 and will be reported on in the future. The initial motivation for this uniform flow VIV field experiment was to improve the understanding of VIV at high mode number. Most model testing has been conducted at low mode numbers (<10). As a consequence, low mode number VIV is quite well understood. There have been numerous tests conducted on very long cables as well, Reference [1]. In such experiments the dynamic behavior can be represented by cables of infinite length. Traveling waves dominate the response and standing wave modal behavior is not observed. Current drilling and production in 1000 to 3000 meters depths has brought on the need to understand VIV behavior at higher mode number, but still short of infinite length behavior. The principal objectives, of the MIT towed pipe experiments were:To gather vortex-induced vibration response data on a densely instrumented circular pipe at high mode number,To investigate mean drag coefficient (CD) at high mode number,To validate or improve drag coefficient prediction formulas,To test the efficacy of helical strakes at high mode numbers,To obtain statistics on the distribution of single-mode vs. multi-mode response of a pipe at high mode number,To determine the relative contribution to damage rate from the in-line and cross-flow VIV,To improve knowledge of damping factors. In this paper, RMS response, reduced velocities, drag coefficients and damping factors are presented. Experiment Description The Lake Seneca test facility was selected as the testing site, because it is a fully equipped field test station moored in calm, deep water. It was ideal for conducting a controlled test on a long circular pipe in uniform flow. This was accomplished by towing a vertical, composite pipe with a suspended bottom weight to produce the desired tension.

Fatigue life calculations have been made for suspended spans of offshore pipelines exposed to steady flow and undergoing vortex-induced vibrations. The effect of the proximity of the sea bottom was taken into account by utilizing the results of related model tests. It was found that the effect of the sea-bottom proximity is to increase the fatigue life of a suspended span of given length or equivalently to increase the safe length of a suspended span for a given fatigue life. Furthermore, the effect of the pipe-to-wall gap to pipe diameter ratio on the fatigue life of such spans has been determined. INTRODUCTION A steady flow about a bluff body, such as a pipeline with a circular cross section, will separate and form a wake. Because of the instability and mutual interaction of the separated shear layers, vortices form and periodically shed from alternate sides of the body. The result is steady and unsteady drag forces in line with the flow and unsteady lift forces? in the transverse, or cross flow direction. If the pipeline is flexible and/or flexibly supported, it begins to vibrate under the action of these periodic forces. The vibrations of the pipeline modify the flow and lead to a nonlinear interaction between the elastic and the fluid systems. The maximum amplitude of inline vortex-induced vibrations is much smaller than the maximum amplitude of transverse vortex induced vibrations, so their contribution to the fatigue life of a pipeline is small. Consequently, attention is focused on the transverse vibrations which will be referred to as "vortex induced vibrations" hereafter. In the last ten years a number of comprehensive overviews of this important class of fluid-structure interaction problems have appeared [1-4]. Experience with unburied offshore pipelines laid in strong current areas has shown that such pipelines may develop unsupported spans due to sea bottom being scoured out from under the pipe due to current action. In turn such unsupported spans exposed to currents may undergo vortex induced vibrations which may affect the fatigue life of the pipeline. Due to lack of data for the vortex-induced vibrations of a flexible cylinder in close proximity to a plane boundary, fatigue life calculations of suspended pipeline spans that determine the maximum safe length of such spans have been based, out of necessity, on data for the vortex-induced vibrations of an isolated flexible cylinder, i.e., no wall effect. In a recent experimental study, Tsahalis and Jones [5] determined the effect of the proximity of a plane boundary on the vortex-induced vibrations of a flexible pipe. They found that the proximity of the plane boundary has the following effects on the response of the pipe as compared to the response of the isolated pipe, i.e., no wall effect (see also Figure 1). First, the increase of the response frequency with the flow velocity is less steep. Second, the first perceptible vibrations take place at a higher flow velocity. Fourth, the maximum amplitude is achieved at a higher flow velocity. Fifth, the maximum amplitude is reduced. Sixth, once the maximum amplitude is achieved it appears that it remains constant for higher flow velocities.

First Report on Elevated Transcranial Doppler Ultrasound Velocities As First Stroke Risk and Their Association with Hemoglobin F Expression in Saudi Arabian Pediatric Sickle Cell Patients

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.

Our customer elected to post-install suppression fairings for vortex-induced vibration (VIV) on the suspended gas export pipeline in Green Canyon Block 113 due to recent data acquired on the depth and magnitude of loop/eddy and cold core current profiles in the Gulf of Mexico. VIV suppression fairings minimize pipeline excitation during high current events, thereby preventing accelerated fatigue failure of pipeline components and minimizing current-related disruption on producing operations. VIV suppression hardware installed on the span was designed and fabricated by Shell Global Solutions (US) Inc. (Shell GSUS) and was in the form of short (6-foot) fairing sections. For hardware installation, our customer teamed with Shell GSUS to develop a method and associated tooling to allow the fairing assemblies to be installed. The method of installation included a deepwater construction vessel using a work-class ROV and heave compensation techniques. This method and equipment can be utilized without affecting flow line operations. This paper reviews the planning, procedures and results of a deepwater, horizontal pipeline span VIV suppression installation project. This paper will be presented by the author at Offshore Technology Conference 2004 in Houston, Texas. Case for Action Span Description. During an as-built pipeline survey of a deepwater offshore gas pipeline, a span of over 300 feet was discovered. The span is in the Green Canyon area of the Gulf of Mexico (Figure 1). The depth of the span at location is approximately -2300 fsw. The span length is over 300 feet long and at times over 10 feet above the sea floor. The Gulf of Mexico has recently verified currents that can potentially harm suspended pipelines in deepwater. Fig. 1 - Approximate Span Geographic Location(Available in full paper) To limit vortex-induced vibration (VIV) during deepwater current events, suppression hardware is required. Shell GSUS was tasked with installing the VIV suppression devices. Fairings were the specified device due to their low drag properties and low cost. VIV suppression fairings have in the past been installed on vertical pipeline steel catenary risers (SCR) and top tensioned risers (TTR) by Shell GSUS. This was our first attempt at a horizontal span retrofit. Installation Method Development VIV Suppression Device Decision. Previously Shell GSUS developed and patented VIV suppression fairings. Fairings were found to be superior to commonly used helical strakes due to their light-weight, low-cost and low-drag properties. VIV suppression fairings developed by Shell GSUS (Figure 2) weathervane to keep the vortex-interrupting fin on the downcurrent side of the tubular that they are attached to. Unlike helical strakes, VIV suppression fairings do not increase the diameter of a pipeline substantially. Most deepwater facilities utilize VIV suppression in some fashion. Shell Exploration and Production Company (SEPCo) chose VIV suppression fairings for their tension leg platform (TLP) production risers in the high-current Mississippi Canyon region of the Gulf of Mexico. The Ursa TLP production risers were installed with VIV suppression fairings attached. Mars TLP production risers were also installed with VIV suppression fairings as well as strakes in the splash zone, and additional VIV suppression devices have been installed since arriving on location, due to a deeper understanding of the current profile.

Data-driven prediction and study of vortex induced vibrations by leveraging hydrodynamic coefficient databases learned from sparse sensors

Vortex-induced vibration response characteristics of catenary riser conveying two-phase internal flow

Vortex-induced vibrations (VIV) establish key design parameters for offshore and subsea structures subject to current flows. Understanding and predicting VIV phenomena have been improved in recent years. Further, there is a need to determine how to effectively and economically mitigate VIV effects. In this study, linear and nonlinear velocity feedback controllers are applied to actively suppress the combined cross-flow and in-line VIV of an elastically-mounted rigid circular cylinder. The strongly coupled fluid-structure interactions are numerically modelled and investigated using a calibrated reduced-order wake oscillator derived from the vortex strength concept. The importance of structural geometrical nonlinearities is studied which highlights the model ability in matching experimental results. The effectiveness of linear vs nonlinear controllers are analysed with regard to the control direction, gain and power. Parametric studies are carried out which allow us to choose the linear vs nonlinear control, depending on the target controlled amplitudes and associated power requirements.

Modelling and prediction of vortex-induced vibrations (VIV) of marine risers is a challenging task due to the associated multi degrees of freedom in both cross-flow/in-line directions and the multi-mode fluid-structure interactions. In addition, the axial motion and its geometrically nonlinear coupling with lateral responses can be significant, especially at higher-order modes. Nevertheless, several papers in the literature dealing with VIV predictions have often overlooked such aspects. Therefore, this study aims to investigate and understand the effect of axial or longitudinal motion through a theoretical model and numerical approach in time domain. Attention is paid to VIV of vertical risers subjected to linearly sheared currents. To capture a three-dimensional aspect of the flexible cylinder experiencing VIV, a semi-empirical model is developed consisting of nonlinear equations of cross-flow, in-line and axial structural oscillations which are coupled with the distributed van der Pol-type wake-oscillators modelling the fluctuating fluid lift/drag forces. The mean drag force is also taken into account. These model equations are numerically solved via a space-time finite difference scheme, and the obtained numerical results highlight several aspects of VIV of elastic cylinders along with the axial motion effects. Apart from the validation of the numerical model with published experimental results, this study reveals how the effect of axial motion and its nonlinear coupling with the two lateral cross-flow/in-line motions can be very important. These depend on the flow velocity, the fluid-structure parameters, the single or multi-mode lock-in condition, and the standing-wave or travelling-wave feature. We recommend that the axial response should be accounted for in VIV analysis and prediction model.