Role of Acoustic-Induced Vibration and Flow-Induced Vibration Studies in Oil & Gas Sector
Mechartes has been successfully executing AIV & FIV studies for the Vibration-induced fatigue failures in piping systems for the oil and gas industrial projects across the globe by providing special studies engineering consultancy services and solutions for the various problems that occur at the site and operation fields.
This article discusses more about the Acoustic-Induced Vibration (AIV) and Flow-Induced Vibration (FIV) which are the common phenomena that can lead to Vibration failures for the piping, associated equipment and structures.
The reader will gain an increased understanding of the importance of AIV & FIV in maintaining integrity of their facilities and be provided with tools and knowledge to mitigate any risks that may be encountered.
So, let’s dive right in!
1. INTRODUCTION
Piping systems having high-capacity or compressor recycle systems are typically exposed to piping vibrations and vibratory stresses in the piping system.
If the piping system is not properly designed to minimize the effect of such acoustic excitation, excessive vibration may cause undesired fatigue failures of the piping system. Acoustic induced vibration can cause piping failure at pressure reducing valves, safety valves or other pressure drop areas in a piping system.
Flow-induced vibration is due to high flow velocities and High mass flow rates. With certain flow conditions, piping systems will develop high levels of noise and vibration that can damage the pipes and related systems.
High-frequency piping vibrations are called acoustic-induced vibration (AIV), because the flow is mostly gas and the vibrations typically produce sound waves within the range of human hearing.
Low-frequency vibrations are called flow-induced vibration (FIV), as the flow typically contains liquid and less sound waves. Both frequencies cause fatigue failures – the weakening of a material caused by repetitive motion that results in cracks.
There are several factors which increases incidence of vibration related fatigue failures in piping systems both on offshore installations and on petrochemical plants. The most significant factors have been:
Ø Increased flow rates as a result of debottlenecking and the relaxation of erosion velocity limits, resulting in higher flow velocities with a correspondingly greater level of turbulent energy in process systems.
Ø For new designs of offshore plant the greater use of thin walled pipework which results in more flexible pipework and higher stress concentrations particularly at small bore connections.
2. OBJECTIVE
The objective of the AIV and FIV studies is to identifying potential risks in their piping systems due to vibrations.
3. DESCRIPTION OF ACTIVITIES
3.1 ACOUSTIC INDUCED VIBRATION (AIV)
AIV is generally applicable to lines in gas or two-phase piping system. High levels of high frequency acoustic energy can be generated by a pressure reducing device such as a valves, OR plates, PRV etc. The amplitude of this energy is governed primarily by the Flow rate & Pressure drop. Excitation due to this can lead to fatigue failure of welded downstream connections. Piping downstream of Pressure reducing devices as below is prone to Acoustic Induced Vibration AIV. Pressure reducing devices can generate high acoustic energy that excites the pipe shell vibration modes. This acoustic induced vibration (AIV) leads to fatigue failure in the process piping or nearby small-bore connections and generates broadband sound radiation in the range of 500 Hz to 2000 Hz.
The AIV study is typically performed for:
3.1.1 IMPACT of AIV
Acoustic Induced Vibration can lead to Fatigue failure at Small Bore branches. The broadband excitation causes the locations to be resonant, resulting in cracking and fatigue failure.
AIV fatigue failure may occur in a very short period of time (minutes to hours).
3.1.2 MITIGATION of AIV
Considerations for AIV mitigation must be done during the design stage of the project as failures in AIV can happen within minutes of operation. There are various options for AIV mitigation. However, the following options are the most common
3.2 FLOW-INDUCED VIBRATION (FIV)
Flow-induced vibration or vortex shedding is due to high flow velocities caused in the piping system. Turbulent flows are known to sometimes generate significant levels of excitations and consequently, vibrations of the piping and associated structures. Piping systems, valves, some devices and inlines can generate strong vibrations, transient pulses, cavitation effects and various types of flow instabilities. There have been many different forms of such problems including turbulence-induced vibrations, fluid hammer, different types of vortex shedding, cavitation-induced vibrations and others.
Flow-Induced Vibration or FIV is a large amplitude, low frequency (generally <100 Hz) vibration that can occur in piping systems carrying high-velocity turbulent fluids.
Rapid changes in flow conditions or fluid properties caused by opening valves, cavitation or large pressure variations leading to changes in fluid conditions. A well-known example is those produced by pressure-reducing devices.
Failures of piping systems due to flow-induced vibration or other types of flow-related vibration have been one of the major causes of unscheduled shutdown, downtime, fires and explosions.
3.2.1 IMPACT of FIV
FIV displaces the piping system in the longitudinal and transverse direction and in some cases leads to damage to the pipe supports.
Considering the momentum flux (density X velocity2), High-density Liquids are more prone to Flow-Induced Vibration as compared to gases.
3.2.2 MITIGATION of FIV
The most common mitigation is to add supports or restraints. By adding the appropriate guide and line stop supports, i.e by increasing system rigidity, the damaging effect of FIV can be reduced a lot. These added supports will minimize the shaking tendency of the pipe, thus reducing the tendency of pipe failure. Other mitigations against Flow-Induced Vibration in Piping Systems could be.,
4. ENERGY INSTITUTE GUIDELINES
In January 2008 the Energy Institute (EI) published the ‘Guidelines for the Avoidance of Vibration Induced Fatigue in Process Pipework’.
The EI Guidelines were published to help minimise the risk of vibration induced fatigue of process piping, intended to be used by engineers with no prerequisite vibration knowledge.
The EI method generates a Likelihood of Failure (LOF) score. The LOF is a form of scoring to be used for screening purposes. It should be noted the LOF is not an absolute probability of failure nor an absolute measure of failure. Higher LOF scores suggest greater susceptibility of piping system to risk of vibration failure.
Based on LOF score, the following actions are recommended
LOF greater than equal to 1 (LOF ≥ )
LOF greater than equal to 0.5 and less than 1 (1.0 > LOF ≥ 0.5 )
LOF greater than equal to 0.3 and less than 0.5 (0.5 > LOF ≥ 0.3 )
LOF less than 0.3 (LOF ≥ 0.3 )
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5. SCREENING METHOD AND CALCULATION STEPS
Mechartes executed AIV / FIV studies for the Brownfield Modifications of existing Clusters, Adequacy Checks and Modification Works, Water Supply & Disposal Network including tie-in to associated wells, new Pipelines & Crossings, Tie-in to existing Pipeline System and Overhead Transmission Line (OHL).
The main objective of the AIV/FIV Study is to calculate the LOF (Likelihood of failure) for all new and modified piping networks. The AIV/FIV calculations are performed as per Energy Institute Guidelines. Vibration fatigue induced by acoustic phenomena and fluid flows are studied in all new and modified piping networks. In this approach, a quantitative assessment is then undertaken to determine the likelihood of a pipe vibration. Small bore connection assessment is performed if LOF of main line exceeds 0.29.
For the main lines, for each excitation mechanism that has been identified as potentially at risk, a quantitative assessment is carried out to determine the likelihood of vibration-induced pipe failure.
The potential excitation mechanisms that are considered here:
Ø Flow induced turbulence
Ø Mechanical excitation
Ø Pulsation in Reciprocating/Positive displacement pumps and compressors
Ø Flow induced excitation
Ø High frequency acoustic excitation
Ø Surge/Momentum change due to valve operation
Ø Cavitation and Flashing
5.1 FLOW INDUCED TURBULENCE
In this assessment, kinetic head is calculated for mainlines and branch lines. Kinetic head of all lines are categorized based on Low, medium and high likelihood of classifications. If kinetic head comes under the medium or high category, then LOF calculation for flow induced turbulence need to be performed as per flow chart below.
5.2 FLOW INDUCED EXCITATION
Flow-induced vibration is generated due to high flow velocities and high mass flow rates in the main lines. With certain flow conditions, piping systems will develop high levels of noise and vibration that can damage the pipes and related systems in flow streams. This mechanism considered in gaseous system where flows past a branch with a closed end (a dead leg branch off the main line).
5.3 High Frequency Acoustic Excitation
The response caused by high frequency acoustic excitation affects the pipework downstream of the source to the first major vessel, e.g. separator, KO drum.
The assessment generates a main line LOF value at each welded discontinuity, e.g. SBC, Welded Tee, Welded support. It is at the discontinuities with an LOF equal to one where corrective actions are required. The sources of high frequency acoustic excitation are pressure reducing devices such as control / relief valves, restriction orifices, or branch connections.
5.4 SURGE/MOMENTUM CHANGES DUE TO VALVE OPERATION
In this assessment, calculation needs to be performed to calculate the likelihood of failure. Calculations are performed for maximum pressure surge and maximum force exerted on the pipework by the pressure surge. If maximum force is less than 1kN than LOF is 0 else further steps are followed as shown in flow chart.
Typical automatic valves that need to be considered in the assessment include:
The assessment of excitation due to surge and momentum changes can be split into the three following operational cases:
5.5 SBC LOF ASSESSMENT
In this assessment, geometric LOF and location LOF are calculated based on methodology given in module T3 of energy institute of guidelines. Details of the Location LOF assessment are performed as per procedure in Flowchart.
5.6 Thermowell LOF assessment
In this assessment, fundamental structural natural frequency is predicted based on thermowell type. A comparison of structural natural frequency and parent pipe wall thickness modifier is made with the vortex excitation frequency to determine the LOF.
6. CONCLUSION
The acoustic induced vibration and flow induced vibration calculations will be performed for all liquid fluid systems as per energy institute guidelines.
7. SPECIAL STUDIES FOR VIBRATIONS
Drawing on the team’s expertise in developing and validating methods for FIV and AIV characterization, Computational Fluid Dynamics (CFD) will be used to develop loading spectra and then applied to frequency domain Finite Element Analysis (FEA) to establish stress spectra. Using frequency domain cycle counting methods it was then possible to calculate fatigue lives for critical elements.
Mechartes offers a range of vibration analysis services to determine and manage the risk of vibration-induced fatigue failure due to piping vibration, with specialist skills and knowledge in the following areas:
8. BENEFITS
Our service enables successful and cost-effective management of pipework vibration risks, and is supported by a wealth of experience in the oil and gas industry across the company, with benefits including:
9. References
Energy Institute (EI) Guidelines: Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipe Work.
ASME B31.3: Process Piping
10. Contact
For more details, please contact.
Chandramohan P
Business Development, Project Manager - Oil & Gas Division
Mechartes International Fzc, UAE
M : +91 829 625 9911 (India)
T : +971 4 381 2060 (UAE)
Skype : pcm.mech@gmail (Chandra Mohan)
CFD Analysis | FEM Analysis | Piping Analysis | Engineering Services |
PRINCIPAL ENGINEER-MECHANICAL at WORLEY OMAN
9moHi Chandramohan....this is nice piece of information. Could you please tell me whether we can find the frequency of the vibration of the liquid under FIV through CFD analysis?.....If yes, how?
Project Manager @ Hydroserv | Expertise in Hydraulic Solutions| Energy | Engineering | Construction | Completion | Operations | Commissioning and Start-up Project Engineer | EPC | PMI
10moChandramohan, thanks for sharing!