Vortex-induced Vibration Fatigue 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.

Coupling Effects of a Top-Hinged Buoyancy Can on the Vortex-Induced Vibration of a Riser Model in Currents and Waves

In order to investigate the effects of the top-end dynamic boundary of risers caused by floater motions on their vortex-induced vibration (VIV) characteristics, a combined model comprising a buoyancy can with a relatively simple structural form and a riser is taken as the research object in the present study. The aspect ratios of the buoyancy can and the riser model are 5.37 and 250, respectively. A set of experimental devices is designed to support the VIV test of the riser with a dynamic boundary stimulating the vortex-induced motion (VIM) of the buoyancy can under different uniform flow and regular wave conditions. Several data processing methods are applied in the model test, i.e., mode superposition, Euler angle conversion, band pass filter, fast Fourier transform, and wavelet transform. Based on the testing results, the effect of low-frequency VIM on the high-frequency VIV of the riser is discussed in relation to a single current, a single wave, and a combined wave and current. It is found that the coupling effect of VIM on the riser VIV presents certain orthogonal features at low current velocities. The effect of the cross-flow VIM component on VIV is far more prominent than that of its counterpart, the in-line VIM, with increasing flow velocity. The VIM in the combined wave–current condition significantly enhances the modulation of vibration amplitude and frequency, resulting in larger fluctuation peaks of vibration response and further increasing the risk of VIV fatigue.

Vortex-Induced Vibration and Fatigue Damage Assessment for a Submarine Pipeline on a Sand Wave Seabed

Sand waves are commonly formed on the sandy seabed of the continental shelf and characterized by their regular wave-like shape. When a submarine pipeline is laid on this type of seabed, it often experiences free spans due to the unevenness of the seabed. These free spans are particularly vulnerable to vortex-induced vibration (VIV) and the resulting fatigue damage, which have been identified as the primary causes of pipeline failures in offshore oil and gas exploration. This study examines the VIV and fatigue damage of free spans in a submarine pipeline in the Lufeng oilfield, which is located in a large area of sand waves. The assessment conditions encompass the as-laid empty state, the flooded state, and the operational state. Additionally, both the minimum and maximum lay tension are taken into account during the evaluation of VIV and fatigue. The VIV onset screening conducted revealed a considerable number of pipeline free spans exceeding the VIV onset span lengths under both temporary and operating conditions for the non-trench seabed. Furthermore, the analyses indicate that the pipeline does not meet the criteria for VIV fatigue on a non-trenched seabed. Consequently, a proposed solution of implementing a 1 m trench rectification measure for the seabed is recommended. The results demonstrate that this measure effectively mitigates the occurrence of VIV and subsequently reduces fatigue damage across all conditions.

Benchmarking of VIV numerical analysis with prototype response for fatigue assessment of inverse catenary coldwater pipelines

High density polyethylene pipe has been used to draw coldwater for low temperature thermal desalination plants since the year 2006 at Lakshadweep Islands, India. The pipeline endured in-line oscillations due to shear current in one of the desalination plants, and this gave an opportunity to observe the response parameters of the prototype pipeline. The recorded oscillation parameters are utilized as a benchmark to verify the results of the vortex induced vibration (VIV) analysis in frequency domain and time domain, and the results show good concurrence. The in-line fatigue damage assessment is carried out for a range of current profiles showing the need for mitigation of pipeline VIV. The helical strakes on a partial length of the pipeline is modeled numerically showing significant suppression of VIV response as well as substantial increase in fatigue life of the pipeline. The benchmarking of the VIV response analysis with prototype measurements enhances the reliability of using numerical VIV fatigue analysis and mitigation measures.

Vortex-induced vibration fatigue damage prediction method for flexible cylinders based on RBF neural network

Vortex-induced vibration (VIV) is an important factor resulting in fatigue failure of cylindrical structures in offshore engineering applications (such as risers and cables). The traditional empirical model for VIV prediction often yields unsatisfactory results due to its dependence on the selection of empirical coefficients. Thus, establishing a general approach to VIV damage prediction based on artificial neural networks and engineering experience is of great significance to engineering guidance. In this paper, a VIV damage prediction model of a flexible cylinder was established using a radial basis function neural network (RBFNN) and model test data. Compared with back propagation neural network (BPNN), genetic algorithm-back propagation neural network (GA-BPNN), and generalized regression neural network (GRNN), the RBFNN exhibited excellent performance with high accuracy and the shortest training time in predicting the VIV damage of flexible cylinders. In addition, the VIV fatigue of flexible cylinders fitted with passive control devices (control rods and helical strakes) was predicted. Using three and four control rods, the VIV reduction effect was observed to be better when the attack angle θ was in the range of 30°–45° and 15°–30°, respectively. Further, regarding helical strakes, the damage values of the cylinder were significantly reduced; however, the VIV suppression effect was significantly affected by the yaw angles. The VIV damage prediction model of this paper is expected to serve as important engineering guidance for the preliminary design, installation arrangement, operation, and maintenance of flexible cylindrical structures in offshore engineering applications.

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.

An investigation into the fatigue damage of a long flexible cylinder with multiple control rods in crossflow

In the field of offshore engineering, the control rod is a common passive device to suppress the vortex-induced vibration (VIV) of cylindrical structures. Some key parameters of control rods (diameter ratio, spacing distance, coverage rate and attack angle) have a great influence on the VIV fatigue life of the cylinder. This paper aims to investigate the fatigue damage of the main flexible cylinder with multiple control rods caused by VIV based on rainflow counting and a standard S–N curve, with the final purpose of studying the effectiveness of multiple control rods, in different configurations around the cylinder in improving the structural fatigue life. The results show that control rods can suppress the VIV response of the cylindrical structure and reduce its fatigue damage effectively. The reduction effect of the control rods is very sensitive to the spatial arrangement. The reduction effect of the 3 control rods is the best at the attack angle of 40°, while that of the 4 control rods is the best at the attack angle of 30°. The fatigue damage of the flexible cylinder can be reduced to a lower level with 4 control rods. In addition, “mode transition”, a specific feature of VIV in uniform flow, is found in the experiment and has a strong influence on fatigue life.

An optimization method for the suppression device configuration of risers

Vortex induced vibration (VIV) can cause fatigue damage of risers. Many devices, such as fairings, helical strakes are developed to suppress riser VIV. However, risers are long in a range from hundreds of meters to thousands of meters. Where to install a limited number of suppression devices, or in other words, how to design the suppression device configuration is still a challenge for riser engineering. Evolution theory is introduced to solve the optimization problem of the suppression device configuration of risers in this paper. Several optimization methods, including GA, EA and mGA are proposed for the optimization of the suppression device configuration of risers. Each riser element is firstly represented as a gene with two states, with suppression devices or not. Then the suppression device configuration of risers will evolve in the direction of decreasing riser VIV fatigue until reaching the optimal configuration. The proposed method is demonstrated by its application to an engineering case. It turns out that the proposed method can perform the optimization design of the suppression device configuration accurately and efficiently. The riser VIV fatigue can be reduced obviously through the optimization design.

Evaluation of Vortex Induced Vibration Effective Parameters on Free-Span Subsea Pipelines

Subsea pipelines due to the reduction of transfer costs and expedite the offshore operations is one of the all-purpose structures in marine industries. Subsea pipelines are exposed to a variety of hazards, including corrosion and fatigue Etc. Free span exacerbates the fatigue required parameters due to a phenomenon called the Vortex Induced Vibration (VIV). In this research, the influence of the span's length on the free span subsea pipeline has been reviewed with ABAQUS standard code. In this study the previous result has been expanded. The results of the VIV fatigue life are extensible to all of the depth. Achieved Results indicate that the fatigue life of the pipeline even in the worst condition is much higher than the required amount that it represents the upstream design of DNV-RP-F105. In this study the backrest pipeline has been investigated and result show that the pipeline under the different conditions in the backrest, by creating more vibration and displacement on one side of the pipeline reduces the fatigue life of 113 percent compared to snap. The VIV fatigue life has undergone a lot of changes due to span length changes, maximum changes occur between cable and behavioral which the amount of these changes is reduced by 75%. The free span length is another factor in VIV fatigue. VIV fatigue life will be increased by reducing the span length. As well as increasing the flow velocity that is the main factor in creating the VIV is increased fatigue. Therefore, in terms of the accuracy in the choice of the existing conditions of very high importance for the pipeline. Comparison between effect parameters in VIV fatigue life was shown that span length is the most effective parameter.

Probabilistic modelling of free spanning pipelines considering multiple failure modes

Free Spanning occurs due to unevenness of seabed, artificial supports and etc. Based on DNV-OS-F101, free spanning may cause Vortex-Induced Vibration (VIV), local buckling and pipeline bursting. The purpose of this study is to present a methodology for assessing free spanning pipelines in terms of Probability of Failure (POF). In order to assess VIV fatigue of subsea pipelines, a Finite-Element Reliability based method with aim of Artificial Neural Network (ANN) is proposed. For training ANN, 560 different ABAQUS FE scenarios with different pipeline configurations are defined, and their results are transferred to MATLAB. According to the recommended criteria, ANN is optimized. For determining reliability level of VIV fatigue, Monte-Carlo Sampling (MCS) is used to optimize ANN, and POF is determined consequently. In addition, using recommended method by DNV-RP-F105, Fatigue Life Capacity (FLC) of pipeline is determined for different conditions. For assessment of pipeline bursting and local buckling, recommended models by DNV-OS-F101 is adopted to determine their relevant POF using First-Order Reliability Method (FORM) and MCS. Furthermore, the effect of different operating pressure on reliability level of pipeline is investigated. Finally, in order to determine the effect of each parameter on POF of pipeline bursting and local buckling sensitivity analysis is carried out, using alpha index.

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 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.

Simulation of Vortex-Induced Vibration of Long-Span Bridges: A Nonlinear Normal Mode Approach

Due to the lack of analytic technique for simulating the vortex-induced vibration (VIV) of long-span bridges, a combination of the VIV semi-empirical model with the structural equation of motion is widely employed to calculate the responses of bridge structures. However, the applicability of this method has seldom been investigated before. In this study, the theoretical defects of the conventional combination strategy (i.e. the finite element procedure or the linear normal mode procedure, LNM) are first discussed, a more theoretically reliable approach (the nonlinear norm mode approach, NNM) is then proposed, and the closed-form expression for the NNM of the VIV system is derived. The accuracy of the proposed method is further illustrated by two case studies. This new approach offers a theoretically reliable tool for analyzing the VIV of long-span bridges. It can also be applied to the process of VIV fatigue analysis or control strategy optimization.

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.

VIV fatigue reliability analysis of marine risers with uncertainties in the wake oscillator model

Uncertainties are rife in the fatigue life prediction of marine risers subjected to vortex-induced vibration (VIV). Industry deals with this issue by imposing large factors of safety that may not be properly justified, resulting in over-conservative riser designs in general. One important source of uncertainty arises from the VIV prediction models. This paper focusses on identifying the uncertainties of a wake oscillator model which approximates the fluctuating hydrodynamic force coupled with the riser equation of motion for nonlinear fluid–structure interaction analysis. This van der Pol-type oscillator relies on two wake coefficients which are described deterministically by empirical equations obtained via curve-fitting. However, the underlying data exhibit wide scatter; thus, it is proposed to model the two key coefficients as random variables. Based on experimental data, the joint probability density function of the variables is approximated. A new fast reliability approach is proposed for the VIV fatigue reliability analysis, while Monte Carlo simulations are performed for comparisons. Case studies of a vertical riser in a uniform flow show that the proposed method compares favorably with Monte Carlo in terms of predicting the failure probability as well as safety factors conforming to prescribed reliability levels. Moreover, this study reveals that the randomness of wake coefficients leads to large variability in the riser fatigue damage. The correlation between the coefficients should be properly incorporated as it affects the fatigue reliability of risers experiencing VIV.

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.

Evaluation of Wellhead Fatigue With Structural-Monitoring Data

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 23981, ’Evaluation of Wellhead Fatigue Using In-Service Structural-Monitoring Data,’ by Phil Ward, Alex Rimmer, and Hugh Howells, 2H Offshore, prepared for the 2013 Offshore Technology Conference, Houston, 6-9 May. The paper has not been peer reviewed. Copyright 2013 Offshore Technology Conference. Reproduced by permission. There is an increasing need to understand the strength and durability of subsea wellheads and conductors. Recently, in-situ monitoring of drilling-riser and wellhead response has been used for projects in the Gulf of Mexico and the North Sea. Wellhead-fatigue-life predictions, for which minimal in-service validation has traditionally been available, have been found to be conservative by factors of 10 or greater on the basis of the field-measurement data collected. Fatigue of Subsea Equipment Fatigue is a mechanism of progressive and localized structural damage and occurs only when a material is subjected to cyclical loading. Fatigue damage can occur even when a structure experiences loads that generate stresses significantly below the elastic yield strength of the material. Because subsea wellheads are exposed to constantly varying loads of high magnitude, their fatigue response is of particular concern. The fatigue life of a component is greatly affected by the likelihood that flaws exist within it and by the complexity of its geometry. If welds are included in the manufacturing process of a component, or are used to join components, crack-like flaws will be introduced. Alternatively, the presence of sharp corners, notches, holes, or threads leads to concentrations of stress in certain areas of a component. The ratio of the maximum stress in such an area to the average stress in the component is termed the stress-concentration factor (SCF) or the stress-amplification factor (SAF). The higher the SCF or SAF, the shorter the fatigue life under a given regime of load cycling. There are two primary mechanisms by which environmental loads cause fatigue of offshore structures. These are vortex-induced-vibration (VIV) fatigue, driven by current loads, and wave-induced fatigue, caused by wave loading. VIV fatigue occurs if the frequency with which vortices are shed from the leeward side of the riser, a natural effect for a cylindrical body placed in flow, matches one or more of the natural frequencies of vibration of the riser and conductor system. These rapid oscillations generate significant stress fluctuations and can cause significant fatigue damage very quickly. By contrast, wave-induced fatigue is a constant effect, but generally results in lower fatigue-damage rates at the wellhead. This is because wave loading will excite the upper portion of the riser, creating loads and motions that are transmitted down to the wellhead. Hydrodynamic damping and inertia associated with the lower riser portion will act to reduce these motions. For specifications of both VIV and wave-induced-fatigue monitoring systems, please see the complete paper. Besides the wellhead and connector, areas of concern include the welds between the wellhead and the adjacent pipe and the seam weld in the conductor pipe. A typical SCF in one of these welds is approximately 1.3, generated primarily from the geometric misalignment and ovality of the components on either side of the weld. However, because of flaws and residual stresses induced by the welding process, the minimum fatigue life may occur in the weld despite the lower SCF. Subsea wellheads are positioned below the blowout preventer (BOP), the primary control mechanism for the well, and therefore are critical in ensuring that well control can be maintained. Furthermore, inspection of these components for flaws after installation is not feasible. These factors lead to the application of a safety factor that can cause analytical fatigue-life predictions to fall short of the target life.

Reliability-Based Factors of Safety for Vortex Induced Vibration Fatigue Using Field Measurements

The development of a vortex induced vibration (VIV) fatigue factor of safety (FoS) consistent with state-of-the-art industry design practice is cast within the coherent framework of reliability analysis. The proposed methodology consists of the following steps: (i) define the failure criteria or limit-state function (ii) setup a deterministic analysis model (iii) characterize the uncertainties involved in the problem (iv) propagate the uncertainties through the deterministic model and assess the probability of failure due to VIV fatigue and (v) calculate the FoS required to achieve a given failure probability. The proposed methodology is demonstrated by determining the FoS associated with using state-of-the-art VIV prediction models to attain varying reliability levels (probabilities of failure) in a hypothetical design scenario. Prediction uncertainty is based herein on measured flow and response data for several full-scale drilling risers working in the field. Results indicate that depending on the reliability level required of a particular design, different FoS than those that currently appear in guidance may be appropriate. Results also indicate the sensitivity of the FoS to the riser and prevailing current type, analysis program and input parameters, and accumulation of conservatism in aggregate versus single-event damage predictions.