Understanding vibration-induced pipework failure
29 April 2014
Vibration induced fatigue of process piping systems is an important and fundamental failure mechanism. Neil Parkinson, technical director at asset integrity specialist AV Technology, explains the six phases in understanding, correcting and preventing vibration-induced pipework failure.
Data published by the UK’s Health & Safety Executive (HSE) for the offshore industry has shown that in the UK sector of the North Sea, fatigue and vibration failures account for 21% of all hydrocarbon releases. Although overall statistics are not available for onshore facilities, available data for individual plants indicates that in Western Europe, between 10% and 15% of pipework failures are caused by vibration induced fatigue.
Based on the Energy Institute publication ‘Guidelines for the Avoidance of Vibration Induced Fatigure Failure in Process Pipework’, current best practice is aimed at minimising the risk of incurring loss of containment from vibration-induced failures. An enhanced and expanded version of the former Marine Technology Directorate Guidelines (1999), the document plays a key role in maintaining integrity in the design and maintenance of process pipework within the oil, gas and petrochemical industries.
The Energy Institute guidelines break down into two main scenarios – proactive and reactive assessments – and aim to ensure compliance with statutory duty, improve safety and reliability, reduce liability from leakeage, and minimise plant downtime. Proactive assessments can be used to routinely evaluate all pipework on a site, whether existing or planned, to ensure that best practice has been adopted and to identify possible areas of concern. Reactive assessments follow, and are used to further investigate known vibration issues or troubleshoot actual failures within both mainline pipework as well as small bore connections (SBCs). There are six key phases to achieving pipework vibration assessments in line with requirements of the guidelines:
•Basic vibration monitoring.
•Specialist measurement techniques.
•Specialist predictive techniques.
The qualitative assessment phase is perhaps the most challenging to implement and involves numerous calculations for assessing the likelihood of encountering a vibration-induced fatigue issue – on either an existing or planned plant. It takes into account many relevant factors, including fluid energy, flow velocities and cyclic operation as well as the construction quality of infrastructure including process machinery and types of valves. It also assesses the chance of flashing or cavitation, and includes a calculation process for scoring likely excitation factors – which are combined with conditional and operational factors to predict the ‘likelihood of failure’ (LOF) for each pipe branch.
Visual inspection is a quick and effective method for identifying potential areas for concern. Many pipework vibration problems are simply the result of operators not following recommended good practice, and visual inspection by skilled assessors can quickly flag up areas for improvement relating to pipe infrastructure. This may include installing more effective pipe supports or replacing worn or damaged supports, proper bracing of SBCs, avoiding fretting and poor geometry, and allowing for thermal expansion of tubing.
The basic piping vibration measurement phase identifies areas of concern based on measured values of pipework vibration. In this phase specialist engineers will first use a single axis accelerometer connected to a portable data collector to take initial vibration levels, ranging from 1 Hz up to 300 Hz. These measurements are presented as vibration amplitude versus frequency and enable the vibration to be classified as acceptable, concern or problem, based on comparison with assessment criteria in the Energy Institute guidelines.
If vibration is assessed as being at a concern or problem level, or for pipework with a higher frequency vibration of more than 300 Hz, the next phase used by vibration engineers is based on specialist measurement techniques. Here, a variety of more in-depth tests can be deployed depending on need, including: dynamic strain measurement and fatigue analysis; experimental modal analysis; operating deflection shape analysis; and dynamic pressure (pulsation) measurement. In addition, engineers can implement specialist predictive techniques, applying sophisticated tools and modelling to provide a more detailed assessment of the dynamics of specific pipelines throughout their lifecycles. A high percentage of the work carried out by our own special projects division is concerned with on-site measurements of dynamic strain and vibration data, using multi-channel instrumentation systems together with the previously measured data. Specialist predictive techniques include finite element analysis (FEA), computational fluid dynamics, and pulsation and surge analysis.
The final stage of any pipework assessment is to recommend corrective actions to reduce vibration levels and the likelihood of future vibration-induced fatigue failures. These actions vary from improving the support infrastructure around pipework including bracing and dampening, or modifying the process conditions themselves to reduce fluid loadings.
The design of practical and appropriate corrective actions is important in achieving cost effective yet thorough solutions, and often utilises FEA techniques to predict the effect of remedial repairs, alongside CAD software for mechanical design of supports and bracing systems.
Vibration in pipework can be affected by a number of direct and indirect factors, not limited to the pipework itself but also including the adjacent support structures and buildings. It is therefore vital to develop a comprehensive overview of vibration patterns in order to recommend constructive improvements. Strain gauging and FEA are powerful tools in this analysis process and although these are often perceived as being distinct and alternative assessment technologies, AVT has long recognised the power of combining practical strain gauge work with theoretical FEA.
While FEA models can provide important predictions of stress, displacement and frequency, results can often be unreliable unless validated with actual in-service data such as strain measurements. Conversely, while actual strain gauge data can yield accurate information on stress amplitudes and fatigue, they are generally limited to measurements at just a few positions along the line – therefore not giving a complete three-dimensional picture of the modal behaviour of a structure.
The most comprehensive offering of its kind in the UK and Ireland, AVT’s Pipework Vibration Solutions service offers everything from initial on-site surveys and vibration diagnosis, through to solution recommendation, design and installation. As well as developing its own bracing and strapping solutions for controlling vibration and enhancing structural integrity, AVT’s service helps customers significantly reduce the risk of major incidents caused by pipework vibration, as well as underpinning HSE responsibilities under the COMAH and Offshore Installations (Safety Case) Regulations 2005 (OSCR).
Further information on AVT’s Pipework Vibration Service and its wider condition-based maintenance and asset integrity offering can be found at www.avtechnology.co.uk
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