Understanding vibration-induced pipework failure
14 May 2014
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. Here Neil Parkinson, Technical Director at asset integrity specialist AV Technology Ltd (AVT), looks at understanding, correcting and preventing vibration-induced pipework failure
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 Fatigue 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 leakage and minimise plant downtime. Proactive assessments can be used to routinely evaluate all pipework on a site, whether existing or planned, and to identify possible areas of concern. Reactive assessments follow, and are used to investigate known vibration issues or troubleshoot actual failures within mainline pipework as well as small bore connections (SBCs).
Where SBCs are concerned, errors in bracing design can actually increase the likelihood of fatigue damage, for example if a SBC is braced to a nearby structure instead of being locally braced to the parent pipe. In addition, bracing an SBC at the wrong position, for example too close to the welded connection rather than supporting the main valve mass, will be ineffective in preventing vibration problems.
One of the most common design errors for offshore applications is the use of local welded supports made from flat bar, which can lead to the possibility of punching shear failures and introduces additional unnecessary fatigue initiation sites at the new welds. Another very common mistake is to utilise braces which are too flexible or only effective in one plane. Commonly, these are fabricated from angled sections steel of flat bar and rely on the stiffness of a cantilevered arm to restrain the SBC. The optimum design should always have multiple support arms to form a tripod-like truss arrangement which has excellent geometric stiffness.
Pipework containing multi-phase flow is a common source of vibration in offshore applications, resulting in high transient vibration. Steam pipework is another typical source of vibration in onshore applications, commonly caused by steam traps.
Due to the nature of the pipework, the additional consideration of thermal expansion must therefore be taken into account when designing a solution meaning that fixed bracing is likely to be unsuitable. In these instances visco-elastic dampers prove most appropriate, allowing for the slow movement caused by thermal expansion but still providing effective restraint.
Unsupported SBC at gas refinery
Vibration-related failures of small bore instrument tubing (SBT) arrangements can cause significant process interruption. In one example, although a system only carried instrument air, failure of the pipework resulted in automatic closure of the control valves causing sudden and unexpected gas production disruption. Here, the problem was exacerbated by the poor routing of the SBT system, which contained lengthy sections of unsupported tubes, and the fact that the SBT also carried the weight of unsupported booster relays – all of which were successfully remedied with brace installations.
There are six phases to achieving pipework vibration assessments in line with requirements of the Energy Institute guidelines:
• Qualitative assessment
• Visual assessment
• Basic vibration monitoring
• Specialist measurement techniques
• Specialist predictive techniques
• Corrective actions
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. This assessment takes into account relevant factors from fluid energy, flow velocities and cyclic operation to the construction quality of infrastructure. 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’ for each pipe branch.
Many pipework vibration problems are the result of operators not following recommended practices, 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, 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. Specialist engineers will first use a single axis accelerometer connected to a portable data collector to take initial vibration levels, ranging from 1 Hz 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 in-depth tests can be deployed, 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 computer-based modelling to provide a detailed assessment of the dynamics of specific pipelines. 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.
With the correct knowledge of what constitutes good practice, designing an effective SBC bracing system can be straightforward, even for SBCs with relatively complicated geometry.
A good design would normally satisfy the following requirements:
• Provide support as close as possible to the centre of gravity of any supported valve mass, not just the SBC tubing
• Provide support close to the position of maximum movement, rather than the position of maximum stress
Energy Institute chart
• Comprise of 2-3 arms to form a truss arrangement, each of which should have good inherent stiffness in both bending planes (flat bars should never be used)
• Avoid welds through the use of bolted clamped connections
• Include a suitable wear resistant liner to prevent fretting and galvanic corrosion
As a general rule, solutions for minimising pipework vibration to accepted levels can be categorised by the type of system. SBCs and SBTs typically benefit from local bracing back to the mainline pipe, whereas mainline solutions normally utilise supports to existing nearby steelwork. Solutions for mainline pipework must take into account static movements such as thermal expansion, meaning visco-elastic dampers are the most appropriate solutions. However, where there is no nearby structural steelwork to connect to, dynamic vibration absorbers provide an effective solution for mainline pipes.
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, FEA and Operating Deflection Shape (ODS) analysis 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 – giving a complete three-dimensional picture of the modal behaviour of a structure.
* 2nd edition 2008, current edition. ISBN 978 0 85293 453 1.
About the author:
Neil Parkinson joined AVT as a consulting engineer in 1985 and has been Technical Director of the company since 1993. He has been responsible for a variety of structural monitoring projects for the onshore and offshore industries including Sellafield Ltd, British Energy, BP Saltend and Cargill plc. He is a Fellow of the Institution of Mechanical Engineers (IMechE).