Simplified Model for Evaluation of Fatigue From Vortex Induced Vibrations of Marine Risers

2004 ◽  
Author(s):  
Gro Sagli Baarholm ◽  
Carl M. Larsen ◽  
Halvor Lie ◽  
Kim Mo̸rk ◽  
Trond Stokka Meling
Keyword(s):  
Fatigue Damage ◽  
Cross Flow ◽  
Mode Shapes ◽  
Design Stage ◽  
Response Frequency ◽  
Lower Mode ◽  
Marine Risers ◽  
Vortex Induced Vibrations ◽  
Well Yield ◽  
The Cross

This paper presents a novel approach for approximate calculation of the fatigue damage from vortex-induced vibrations (VIV) of marine risers. The method is based on experience from a large number of laboratory tests with models of full-length risers, large-scale tests and also full-scale measurements. The method is intended to provide a conservative result and be used for screening purposes at the early design stage. The model is in particular aimed at predicting fatigue for risers that respond at very high mode orders (above 10), but may as well yield valid results for lower mode numbers. The model will, however, not be adequate for free span pipelines or other structures that normally will respond at first and second mode. The riser will be defined in terms of some key parameters like length, weight, tension, hydrodynamic diameter and stress diameter. A current profile perpendicular to the riser in one plane must be known. The program will apply a simple model for calculation of eigenfrequencies and mode shapes, and these are sorted into in-line (IL) and cross-flow (CF) bins. An effective current velocity and excitation length can be defined from the profile and will be applied to identify the dominating cross-flow response frequency and the total displacement rms value. The dominating in-line response frequency is taken as twice the cross-flow frequency, and inline response rms is taken as a given portion of the cross-flow rms value. A set of contributing modes is defined from an assumed frequency bandwidth that reflects observed bandwidths, but also modal composition for cases with discrete frequency response. A simple mode superposition technique is then used to find the set of modes that gives the identified rms values. Bending stresses will be found directly from the curvature of the mode shapes. Fatigue damage will be found from stress rms values, user defined stress concentration factor and given SN curves. The model has been implemented in a simple computer program and verified by comparing results to measurements. The ambition has not been to obtain an exact match between computed results and observations, but to verify that the model gives reasonable but conservative results in almost all cases. However, an unrealistic over prediction of the fatigue damage is not desired. The results are promising, but the need for more information from measurements and response analyses with programs like VIVANA and SHEAR7 is still obvious.

Comparison of Calculated In-Line Vortex Induced Vibrations to Model Tests

2012 ◽  
Author(s):  
Elizabeth Passano ◽  
Carl M. Larsen ◽  
Halvor Lie
Keyword(s):  
Cross Flow ◽  
Added Mass ◽  
Model Tests ◽  
The Finite Element Method ◽  
Response Frequency ◽  
Flow Response ◽  
Vortex Induced Vibrations ◽  
Flow Directions ◽  
The Cross ◽  
Semi Empirical

The purpose of the present paper is to compare vortex-induced vibrations (VIV) in both in-line and cross-flow directions calculated by a semi-empirical computer program to experimental data. The experiments used are the Bearman and Chaplin experiments in which a model of a tensioned riser is partly exposed to current and partly in still water. The VIVANA program is a semi-empirical frequency domain program based on the finite element method. The program was developed by MARINTEK and the Norwegian University of Science and Technology (NTNU) to predict cross-flow response due to VIV. The fluid-structure interaction in VIVANA is described using added mass, excitation and damping coefficients. Later, curves for excitation, added mass and damping for pure in-line VIV response were added. These curves are valid for low current levels, before the onset of cross-flow VIV response. Recently, calculation of response from simultaneous cross-flow and in-line excitation has been included in VIVANA. The in-line response frequency is fixed at twice the cross-flow response frequency and the in-line added mass is adjusted so that this frequency becomes an eigenfrequency. A set of curves based on forces measured during combined cross-flow and inline motions are used. At present, the in-line excitation curves are not dependent on the cross-flow response amplitude. In the paper, in-line and cross-flow response predicted by VIVANA will be compared to the Bearman and Chaplin model tests. The choice of added mass and excitation coefficients will be discussed.

Experimental Investigation of Vortex-Induced Vibrations of a Flexibly Mounted Circular Cylinder in Combined Current and Wave Flows

2021 ◽  
Author(s):  
Pierre-Adrien Opinel ◽  
Narakorn Srinil
Keyword(s):  
Experimental Investigation ◽  
Circular Cylinder ◽  
Cross Flow ◽  
Wave Flow ◽  
Regular Wave ◽  
Regular Waves ◽  
Vortex Induced Vibrations ◽  
Flow Frequency ◽  
The Cross ◽  
Wave Flows

Abstract This paper presents the experimental investigation of vortex-induced vibrations (VIV) of a flexibly mounted circular cylinder in combined current and wave flows. The same experimental setup has previously been used in our previous study (OMAE2020-18161) on VIV in regular waves. The system comprises a pendulum-type vertical cylinder mounted on two-dimensional springs with equal stiffness in in-line and cross-flow directions. The mass ratio of the system is close to 3, the aspect ratio of the tested cylinder based on its submerged length is close to 27, and the damping in still water is around 3.4%. Three current velocities are considered in this study, namely 0.21 m/s, 0.29 m/s and 0.37 m/s, in combination with the generated regular waves. The cylinder motion is recorded using targets and two Qualisys cameras, and the water elevation is measured utilizing a wave probe. The covered ranges of Keulegan-Carpenter number KC are [9.6–35.4], [12.8–40.9] and [16.3–47.8], and the corresponding ranges of reduced velocity Vr are [8–16.3], [10.6–18.4] and [14–20.5] for the cases with current velocity of 0.21 m/s, 0.29 m/s and 0.37 m/s, respectively. The cylinder response amplitudes, trajectories and vibration frequencies are extracted from the recorded motion signals. In all cases the cylinder oscillates primarily at the flow frequency in the in-line direction, and the in-line VIV component additionally appears for the intermediate (0.29 m/s) and high (0.37 m/s) current velocities. The cross-flow oscillation frequency is principally at two or three times the flow frequency in the low current case, similar to what is observed in pure regular waves. For higher current velocities, the cross-flow frequency tends to lock-in with the system natural frequency, as in the steady flow case. The inline and cross-flow cylinder response amplitudes of the combined current and regular wave flow cases are eventually compared with the amplitudes from the pure current and pure regular wave flow cases.

Fatigue Analysis of Free Spanning Pipelines Subjected to Vortex Induced Vibrations

2013 ◽  
Author(s):  
F. Van den Abeele ◽  
F. Boël ◽  
M. Hill
Keyword(s):  
Fatigue Damage ◽  
Oil And Gas ◽  
Flow Direction ◽  
Cross Flow ◽  
Fatigue Analysis ◽  
Sensitivity Analyses ◽  
Vortex Induced Vibration ◽  
Vortex Induced Vibrations ◽  
Offshore Industry ◽  
Girth Welds

Vortex induced vibration is a major cause of fatigue failure in submarine oil and gas pipelines and steel catenary risers. Even moderate currents can induce vortex shedding, alternately at the top and bottom of the pipeline, at a rate determined by the flow velocity. Each time a vortex sheds, a force is generated in both the in-line and cross-flow direction, causing an oscillatory multi-mode vibration. This vortex induced vibration can give rise to fatigue damage of submarine pipeline spans, especially in the vicinity of the girth welds. In this paper, an integrated numerical framework is presented to predict and identify free spans that may be vulnerable to fatigue damage caused by vortex induced vibrations (VIV). An elegant and efficient algorithm is introduced to simulate offshore pipeline installation on an uneven seabed. Once the laydown simulation has been completed, the free spans can be automatically detected. When the fatigue screening for both inline and cross-flow VIV indicates that a particular span may be prone to vortex induced vibrations, a detailed fatigue analysis is required. Amplitude response models are constructed to predict the maximum steady state VIV amplitudes for a given pipeline configuration (mechanical properties) and sea state (hydrodynamic parameters). The vibration amplitudes are translated into corresponding stress ranges, which then provide an input for the fatigue analysis. A case study from the offshore industry is presented, and sensitivity analyses are performed to study the influence of the seabed conditions, where special emphasis is devoted on the selection of pipe soil interaction parameters.

On the Coupling Between In-Line and Cross-Flow VIV Response

2002 ◽  
Author(s):  
Martin So̸reide
Keyword(s):  
Limit State ◽  
Preliminary Investigation ◽  
Cross Flow ◽  
Design Criterion ◽  
Wave Loads ◽  
Flow Response ◽  
Large Current ◽  
Vortex Induced Vibrations ◽  
The Cross ◽  
Fatigue Limit State

As offshore installations are moving into deeper water, engineers have to face new challenges in design of structures. Risers and free-span pipelines, subjected to heavy wave loads and large current velocities, are important components of these installations. Vortex induced vibrations (VIV) is a well known subject for most offshore engineers. VIV can cause large stresses and fatigue damage of slender marine structures. Hence, large safety factors are applied to the fatigue limit state design criterion (FLS), due to uncertainties regarding VIV. The present paper describes the preliminary investigation into the coupling between in-line and cross-flow VIV response. Most experimental data so far has been concentrated on predicting the cross-flow response. However, in-line displacements can make a valuable contribution. In fact, it has been proved that in-line responses may decrease the cross-flow response significantly when allowing the pipe to oscillate in both directions. The paper is based on a master of science thesis at the Norwegian University of Science and Technology (NTNU).

Effect of the Boundary Conditions on the Dynamic Behaviours of Subsea Free-Spanning Pipelines

2018 ◽  
Vol Vol 160 (A2) ◽  
Author(s):  
T T Li ◽  
C An ◽  
M L Duan ◽  
H Huang ◽  
W Liang
Keyword(s):  
Boundary Conditions ◽  
Structural Equation ◽  
Eigenvalue Problems ◽  
Integral Transform ◽  
Cross Flow ◽  
Interaction Model ◽  
Accurate Solution ◽  
Mode Shapes ◽  
Different Boundary Conditions ◽  
The Cross

This paper establishes a fast and accurate solution of the dynamic behaviours of subsea free-spanning pipelines under four different boundary conditions, based on GITT - the generalised integral transform technique. The fluid-structure interaction model is proposed by combining a linear structural equation and a non-linear distributed wake oscillator model, which simulates the effect of external current acting on the pipeline. The eigenvalue problems for the cross-flow vibration of the free-spanning submarine pipeline conveying internal fluid for four different boundary conditions are examined. The solution method of the natural frequency based on GITT is proposed. The explicit analytical formulae for the cross-flow displacement of the pipeline free span are derived, and the mode shapes and dynamic behaviours of the pipeline free span are discussed with different boundary conditions. The methodology and results in this paper can also expand to solving even more complicated boundary-value problems.

Fatigue From Vortex-Induced Vibrations of Free Span Pipelines Using Statistics of Current Speed and Direction

2003 ◽  
Author(s):  
Rune Yttervik ◽  
Carl M. Larsen ◽  
Gunnar K. Furnes
Keyword(s):  
Fatigue Damage ◽  
Current Velocity ◽  
Current Flow ◽  
Flow Direction ◽  
Cross Flow ◽  
Longitudinal Axis ◽  
Current Speed ◽  
Parameter Study ◽  
Span Length ◽  
Vortex Induced Vibrations

A section of a sub sea pipeline that is suspended between two points on an uneven seafloor is often referred to as ‘a free span pipeline’. Pipelines, installed on a seabed with a highly irregular topography, may have to be designed with several free spans. If a free span is exposed to a current flow, vortex-induced vibrations (VIV) of the suspended part of the pipeline may occur. These vibrations may cause unacceptable fatigue damage in the structure. Statistical distributions of current speed and direction close to a small mountain on the seabed (approximately 20 m high and 40 m wide) are established based on full-scale measurements of the current velocity in the area. Some results from recent model tests of VIV in free span pipelines, including some tests in which the flow direction was not perpendicular to the longitudinal axis of the pipe, are shown. These results indicate that it is sufficient to use the component of the current velocity vector that is normal to the pipe when using empirical models for estimating the response due to vortex shedding. An existing empirical model for analysis of VIV [1] is extended such as to include oscillations in the same plane as the current flow (in-line VIV). The effect of including the directional variability of the current when estimating the VIV fatigue damage, using the extended VIV model on a typical free span pipeline, is demonstrated, and found to be of great importance. A parameter study, in which the length of the free span is varied, is also carried out. The conclusion from this study is that a reduction of free span length affects the parameters that govern the accumulation of fatigue damage differently. Stresses are increased, but the number of current conditions capable of inducing VIV is reduced when the length of the span is reduced. It is therefore difficult to predict whether the accumulated damage will increase or decrease when the span length is reduced, and detailed analyses are required for each particular free span and current distribution. The damage from in-line VIV is generally lower than the damage from the cross flow VIV for all but the shortest span lengths.

Prediction of Combined IL and CF VIV Response of Deepwater Risers

2017 ◽  
Author(s):  
Jie Wu ◽  
Malakonda Reddy Lekkala ◽  
Muk Chen Ong ◽  
Elizabeth Passano ◽  
Per Erlend Voie
Keyword(s):  
Fatigue Damage ◽  
Cross Flow ◽  
Response Prediction ◽  
Flexible Cylinder ◽  
Load Model ◽  
Cylinder Test ◽  
Excitation Coefficient ◽  
The Cross ◽  
Offshore Industry ◽  
Deepwater Risers

Deepwater risers are susceptible to Vortex Induced Vibrations (VIV) when subjected to currents. When responding at high modes, fatigue damage the in in-line (IL) direction may become equally important as the cross-flow (CF) components. If a riser experiences directional currents, fatigue damage must be evaluated at several locations on the cross-section’s circumference. Accurate calculation of both IL and CF responses are therefore needed. Empirical VIV prediction programs, such as VIVANA, SHEAR7 and VIVA, are the most common tools used by the offshore industry to design against VIV loads. 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 assumed to be uni-directional or acting in the same direction. The purpose of the present 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 VIV fatigue damage 1 using 2D directional current profiles is less conservative compared to the traditional way of using unidirectional current profiles.

On the Occurrence of Higher Harmonics in the VIV Response

2015 ◽  
Author(s):  
Jie Wu ◽  
Decao Yin ◽  
Halvor Lie ◽  
Carl M. Larsen ◽  
Rolf J. Baarholm ◽  
...  
Keyword(s):  
Fatigue Damage ◽  
Cross Flow ◽  
Transverse Motion ◽  
Flexible Beam ◽  
Flow Profile ◽  
Third Order ◽  
Higher Harmonics ◽  
Sheared Flow ◽  
Vortex Induced Vibrations ◽  
Shedding Frequency

Vortex Induced Vibrations (VIV) can lead to fast accumulation of fatigue damage and increased drag loads on slender marine structures. A cylinder subjected to VIV can vibrate in both in-line (IL) and cross-flow (CF) directions. The CF response is dominated by the primary shedding frequency and the IL response frequency is often two times of the primary CF frequency. In addition, higher harmonics can also be present. The third order harmonics are more pronounced when the motion orbit of the cylinder is close to “figure 8″ shape and cylinder is moving against the flow at its largest transverse motion. Recent studies with flexible beam VIV tests have shown that higher harmonics can have significant contribution to the fatigue damage in addition to the loads at the primary shedding frequency. However, there is a lack of understanding of when and where higher harmonic loads occur. The fatigue damage due to the higher harmonics is not considered in the present VIV prediction tools. In the present paper, the test data of selected cases subjected to linearly sheared flow profile from two test programs, the Shell high mode VIV test[11] and the Hanøytangen test[5] have been studied. The factors that may influence the occurrence of the higher harmonics, such as the bending stiffness, reduced velocity and orbits stability, have been studied. The importance of higher harmonics in VIV fatigue has also been investigated. Finally, a method to include higher harmonics in the fatigue calculation is presented.

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