Vortex-induced Vibration Fatigue Damage Research Articles

Effects of vertical and lateral riser-soil interactions on vortex-induced vibration of a steel catenary riser

A numerical model for computing the vortex-induced vibration (VIV) and fatigue damage of steel catenary risers (SCRs) was developed. The structural dynamics were accurately simulated using an absolute nodal coordinate formulation (ANCF). The Van der Pol wake oscillator is applied to generate the fluctuating lift, which is further transformed into the cross-flow direction by considering the structural deformation. The Randolph-Quiggin (RQ) model and the Coulomb friction ‘bilinear’ model are employed to simulate the vertical and lateral riser-soil interactions, respectively. After case validations, the effects of riser-soil interaction on the VIV amplitude, frequency, mode, and fatigue of the SCR at different current angles are investigated, and a sensitivity study of different seabed model parameters is discussed. The bands of significant VIV frequencies were broadened by riser-soil interactions, accompanied by more frequency components of disturbance and more abundant vibration modes. Severe fatigue damage cannot be captured by the truncated model, and seabed models that require improvement are ignored. It is suggested that vertical and lateral riser-soil interactions should be considered in the evaluation of VIV fatigue damage for SCRs.

Experimental investigation on vortex-induced vibration of a flexible pipe in combined uniform and oscillatory flow

Vortex-induced vibrations (VIV) induced by ocean currents can cause severe fatigue damage to marine risers. The background current and oscillatory flow caused by the motion of the floating platform form a combined flow field, which complicates the VIV of risers. In this paper, a series of model tests were conducted in a towing tank to investigate the VIV characteristics of a flexible pipe in combined uniform and oscillatory flow. Effects of oscillating amplitude, oscillating period and uniform flow velocity on VIV of the pipe were investigated and discussed based on the experimental results. VIV fatigue damage of the pipe was also calculated. Intermittent vibration, hysteresis and mode transition were observed in combined flow. Large oscillating amplitude and short oscillating period tend to enlarge the fatigue damage. But when the uniform flow velocity is too high, small oscillating amplitude and long oscillating period tend to enlarge the fatigue damage. Critical short period and combined ratio threshold are discovered in the VIV of the pipe in combined flow. VIV characteristics in different types of flows are also compared. VIV in combined flow may cause severe fatigue damage, which should be given enough attention.

Experimental investigation on the effectiveness of serrated helical strakes in suppressing vortex-induced vibrations for a flexible riser

The effectiveness of an unconventional configuration of serrated helical strakes (SSs) in alleviating the vortex-induced vibrations (VIVs) of flexible risers was evaluated experimentally in the Reynolds number range of 2600–10,400. Five SSs with varying serration densities and lengths were designed and compared with conventional strakes (CSs) to highlight the unparalleled features of this new geometry. The VIV parameters of the bare and straked risers, including the displacement, frequency, fatigue damage, and suppression efficiency, were systematically investigated. The comprehensive evaluation of the helical strakes was also made considering the factors of amplitude suppression, frequency inhibition, drag force, and cost control. The experimental results confirmed that all the helical strakes were capable of suppressing the VIV responses of flexible riser models. With identical strake areas, the SSs were more effective in inhibiting the VIV amplitude and fatigue damage than the CSs; whereas, the serrated geometry had few effects in improving the reduction of VIV frequency. Moreover, serration density is a critical factor in determining the suppression performance of SSs under the same strake heights and areas. An increase in the strake area enhanced VIV suppression performance; however, excess strakes deteriorated the comprehensive evaluation index due to the increased cost.

A Numerical Investigation of Vortex-Induced Vibration Response and Fatigue Damage for Flexible Cylinders Under Combined Uniform and Oscillatory Flow

Vortex-induced vibration (VIV) for flexible cylinders under combined uniform and oscillatory flow is a challenging and practical issue in ocean engineering. In this paper, a time domain numerical model is adopted to investigate the characteristics of cross-flow VIV response and fatigue damage under different combined flow cases. Firstly, the adopted VIV model and fatigue analysis procedure are validated well against the published experimental results of a 4-m cylinder model under pure oscillatory flows. Then, forty-five combined flow cases of the same cylinder model are designed to reveal the VIV response characteristics with different non-dimensional oscillation period T* and combined ratio r. The combined flow cases are classified into three categories to investigate the effect of r on cylinder’s dynamic response, and the effect of T* is described under long and short period cases. Finally, fatigue analysis is carried out to investigate how the structural fatigue damage varies with the variations of r and T*. The captured characteristics of structural response and fatigue damage are explained through the VIV mechanism analysis.

The influence of the yaw angle on the VIV fatigue damage of a yawed cylinder with or without helical strakes

The structural fatigue damage caused by vortex-induced vibration (VIV) is one of the most important failure reasons for flexible cylinders in offshore engineering applications. Helical strakes are often employed to reduce the VIV of slender marine structures. Based on a previous experimental investigation, this paper further investigates the effects of the yaw angle on the VIV fatigue damage of a yawed flexible cylinder with or without helical strakes in a uniform flow. Four yaw angles (α= 0°, 15°, 30° and 45°) are considered. The rain-flow counting (RFC) procedure and the Miner-Palmgren hypothesis are adopted to calculate the fatigue damage of the flexible cylinder. The influence of the yaw angle on the strain response, fatigue damage distribution and maximum fatigue damage of the flexible cylinder are investigated and discussed. The fatigue damage levels in the cross-flow (CF) and in-line (IL) directions of the flexible cylinder are obviously reduced by helical strakes. The VIV fatigue damage of the smooth cylinder is nearly independent of the yaw angle, while that of the straked cylinder is extremely sensitive to the yaw angle. As the yaw angle increases from 0° to 45°, the fatigue damage of the straked cylinder caused by VIV increases several orders of magnitude.

Vortex-induced vibration of flexible pipe fitted with helical strakes in oscillatory flow

Helical strakes are widely used to suppress vortex-induced vibration (VIV) in offshore engineering. However, the VIV response and suppression efficiency of the straked pipe remain unclear in an oscillatory flow. In this paper, an experimental study of VIV for a flexible pipe fitted with helical strakes was conducted in an oscillatory flow with the Keulegan-Carpenter (KC) number varying from 21 to 165 and maximum reduced velocities ranging from 4 to 12. The effects of the helical strakes on the VIV response, Strouhal number, suppression efficiency and fatigue damage in an oscillatory flow were investigated. The results show that the suppression efficiency and fatigue damage reduction ratio are not as ideal in oscillatory flow as those in steady flow. Moreover, the VIV dominant frequency is obviously magnified and a large Strouhal number is observed, reaching 0.439 under a lower reduced velocity of 4. As the reduced velocity further increases to 6, 8, 12, two branches of Strouhal numbers are clearly seen. The dominant frequency decreases at the first branch with a small KC number and increases at the second branch with a large KC number.

A numerical study on vortex-induced vibrations and the effect of slurry density variations on fatigue of ocean mining risers

Ocean mining emerges as a way of meeting future demands for valuable minerals. Risers conveying ore from the seabed to the surface must withstand large wave and current loads. In addition, vortex-induced vibrations (VIV) are an important source of fatigue. However, the VIV of mining risers may be affected by particles flowing inside. In this paper, a novel framework for coupled analysis of risers with internal flow is utilized to study VIV of a mining riser. Focus is on the effect of time-varying internal slurry flows, and the associated density waves traveling along the riser. Simulated VIV response and fatigue damage are reported in detail, to illuminate the effect of the period/wavelength of the internal flow. It is found that the internal flow produces significant changes to the VIV fatigue damage, if the wavelength of the internal density wave is close to λcr=L/n, where L is the riser length, and n is the dominating VIV mode. The results suggest that careful analysis is necessary when designing a mining riser where internal density waves are expected to occur with a wavelength near or above the critical value. In such cases, the numerical method presented here is a useful tool.

Prediction of Combined Inline and Crossflow Vortex-Induced Vibrations Response of Deepwater Risers

Deepwater risers are susceptible to vortex-induced vibrations (VIV) when subjected to currents. When responding at high modes, fatigue damage in the inline (IL) direction may become equally important as the crossflow (CF) components. Accurate calculation of both IL and CF responses is therefore needed. Empirical VIV prediction programs, such as VIVANA “Passano et al. (2016, “VIVANA—Theory Manual Version 4.8,” Trondheim, Norway),” SHEAR7 “(Vandiver, J. K., and Li, L., 2007, “Shear7 v4.5 Program Theoretical Manual,” Department of Ocean Engineering, Massachusetts Institute of Technology, Cambridge, MA),” and VIVA “Triantafyllou et al. (1999, “Pragmatic Riser VIV Analysis,” Offshore Technology Conference, Houston, TX, May 3–6, Paper No. OTC-10931-MS.)” are the most common tools used by the offshore industry. Progress has been seen in the prediction of CF responses. Efforts have also been made to include an IL load model in VIVANA. A set of excitation coefficient parameters were obtained from rigid cylinder test and adjusted using measured responses of one of the flexible cylinder VIV tests. This set of excitation coefficient parameters is still considered preliminary and further validation is required. Without an accurate IL response prediction, a conservative approach in VIV analysis has to be followed, i.e., all current profiles have to be assumed to be unidirectional or acting in the same direction. The purpose of this paper is to provide a reliable combined IL and CF load model for the empirical VIV prediction programs. VIV prediction using the existing combined IL and CF load model in VIVANA is validated against selected flexible cylinder test data. A case study of a deepwater top tension riser (TTR) has been carried out. The results indicate that VIV fatigue damage using two-dimensional directional current profiles is less conservative compared to the traditional way of using unidirectional current profiles.

A simplified method to predict fatigue damage of TTR subjected to short-term VIV using artificial neural network

Marine riser is the critical component transporting hydrocarbon and fluid from well to the platform and vice versa. Riser experiences vortex-induced vibration (VIV) caused by current, leading to fatigue damage. Estimation of VIV fatigue damage is essential in designing feasible and operable riser. A simplified approach for predicting fatigue damage is required to reduce the computation time to analyse the fatigue damage. This study aims to propose a simplified approach to predict VIV fatigue damage of top tensioned riser (TTR) using artificial neural network (ANN). A total of 21,532 riser model was generated with different combination of six main input parameters: riser outer diameter, wall thickness, top tension, water depth, surface and bottom current velocity. The modal analysis was performed using OrcaFlex and VIV fatigue damage of the riser was computed using SHEAR7. The six input parameters and corresponding fatigue damage results made up the database for training a 2-layer neural network. Weight and bias values acquired from the training of ANN were used to develop the VIV fatigue damage prediction model of the riser. The hyperparameters of the ANN model were tuned to optimize performance of the model. The results showed the final ANN model predict fatigue damage well with shorter time compared to conventional semi-empirical method. Hence, the proposed approach is suitable to be used for prediction of VIV fatigue damage of TTR at early design stage of TTR.

Development of ANN Model for the Prediction of VIV Fatigue Damage of Top-tensioned Riser

Marine riser experiences vortex-induced vibration (VIV) caused by current, leading to fatigue damage if VIV is not considered in design of riser. Estimation of VIV fatigue damage is essential in designing feasible and operable riser. A simplified approach for predicting fatigue damage is required to reduce the computation time to analyze the fatigue damage. This study aims to explore the applicability of artificial neural network (ANN) approach in developing top-tensioned riser fatigue damage prediction model. A total of 2100 riser model is generated with different combination of four main input parameters: riser outer diameter, wall thickness, top tension and uniform current velocity. The modal analysis is performed using OrcaFlex and VIV fatigue damage of the riser is computed using SHEAR7. The four input parameters and corresponding fatigue damage results make up the database for training a 2-layer neural network. Weight and bias values acquired from the training of ANN are used to develop the VIV fatigue damage prediction model of the riser. The results show ANN approach is suitable for prediction of the riser fatigue damage due to VIV. The proposed approach requires further refinements and extension to more input features to improve the accuracy and usefulness of the developed prediction model.

VIV response of a long flexible riser fitted with different helical strake coverages in uniform and linearly sheared currents

ABSTRACTAn experimental investigation was conducted on a flexible riser with and without various helical strake coverages, with the aim of further improving the understanding of the response performance of the vortex-induced vibration (VIV) of a riser with helical strakes. Two current profiles, including uniform and linearly sheared currents, were simulated. The VIV displacements, VIV strains and hydrodynamic forces were directly measured from the test. Based on the fatigue damage method, the fatigue damage was obtained from the measured strain. Strakes with different coverages were analysed, and response parameters such as the displacement response and fatigue damage were studied. The results of the straked model test indicate that the response characteristics of a bare riser model can be quite distinct from those of a riser with helical strakes, and the response performance closely depends on the coverage of the helical strakes. The results also show that the flexible riser with 75% and 50% strake coverages are the best configurations to suppress VIV fatigue damage in uniform and linearly sheared currents, respectively.

Fatigue damage from time domain simulation of combined in-line and cross-flow vortex-induced vibrations

A semi-empirical method for time domain simulation of vortex-induced vibrations (VIV) is used to calculate the in-line and cross-flow fatigue damage of a tensioned riser in uniform and sheared flow. Simulations are run for flow velocities ranging from 0.3 m/s to 2.4 m/s, and a detailed comparison with experimental observations is performed. Results are reported in terms of dominating frequency, mode of vibration and mean of r.m.s. of displacement, as well as fatigue damage distribution along the length of the structure and maximum fatigue damage rates for each case. Fatigue damage is calculated by rainflow counting of the strain time series together with an idealized S–N curve with slope m = 3. The results show that the model reproduces the measured fatigue damage with a satisfactory level of realism, using a consistent set of parameters. This indicates that the model is usable for calculation of riser VIV fatigue damage in various current conditions, assuming the Reynolds number is in the subcritical range.

Fatigue damage of a steel catenary riser from vortex-induced vibration caused by vessel motions

A large-scale model test of a truncated steel catenary riser (SCR) was performed in an ocean basin to investigate the vortex-induced vibration (VIV) and its fatigue damage under pure top vessel motion. The top end of the test model was forced to oscillate at given vessel motion trajectories. Fiber Bragg grating (FBG) strain sensors were used to measure both in-plane and out-of-plane responses. Four different factors have been discussed to understand the VIV responses and fatigue damage results: instantaneous shedding frequency, touch down point (TDP) variation, tension variation and traveling waves. Out-of-plane VIV associated with strong time-varying features was confirmed to have occurred under pure vessel motion. Both KC number and maximum shedding frequency were investigated and indicated that the middle part of the truncated model riser was the ‘power-in’ region for out-of-plane VIV. Meanwhile, fatigue damage caused by out-of-plane VIV was found to be strongly dependent on both top motion amplitude and period. The probability distribution of the maximum damage exhibits 3 critical locations in the test model: TDP, upper sag-bend and top of the SCR. Strong traveling waves, TDP variation and end wave reflection have been proven to cause the maximum damage locations to shift from the ‘power-in’ region to these three positions. Finally, a maximum fatigue damage diagram with top motion amplitude, period and maximum shedding frequency was constructed.

Prediction and analysis of fatigue damage due to cross-flow and in-line VIV for marine risers in non-uniform current

Fatigue damage due to vortex induced vibrations (VIV) is one of the most significant failure modes for marine risers. Previous studies of fatigue prediction and analysis for marine risers typically focus on cross-flow VIV due to its larger response amplitude than in-line vibrations. In this study, a prediction model of VIV fatigue damage for riser accounting for both cross-flow and in-line vibrations is proposed based on the energy equilibrium theory and the forced vibration experimental data of a rigid cylinder. The coupling effect of CF- and IL-VIV is taken into account and the response amplitude is corrected by a weighted factor according to the input power within the overlap excitation region for each vibration mode. The experimental models of flexible riser under stepped and sheared current are simulated to validate the proposed approach and the results show reasonable agreement. Moreover, based on the prediction model, the top velocity of sheared current, top tension, internal fluid density and the material of structures are invested to discuss their effects on the fatigue damage. The study may be worthwhile for fatigue assessment and broaden the understanding of VIV fatigue damage characteristics for deepwater risers.

Monitoring VIV fatigue damage on marine risers

Long flexible cylinders (e.g., risers, tendons and mooring lines) exposed to the marine environment encounter ocean currents leading to vortex-induced vibration (VIV). These oscillations, often driven at high frequencies over extended periods of time, may result in structural failure of the member due to fatigue damage accumulation. Recent developments in instrumentation and installation of data acquisition systems on board marine risers have made accurate measurement of riser responses possible. This paper aims at using the data from these data acquisition devices (typically strain gages and accelerometers) in order to understand the evolution of the riser VIV, with the final aim of estimating the fatigue damage. For this purpose we employ systematic techniques to reconstruct riser VIV response using the data from the available sensors. The reconstructed riser response allows estimation of the dynamic axial stresses due to bending and consequently the estimates of the fatigue damage along the entire riser. The above methods can take into account the fatigue damage arising from complicated riser motions involving the presence of traveling waves even with the use of very few sensors. An alternate approach using a Van der Pol wake oscillator model is also explored to obtain fatigue life estimates caused by riser VIV.