6/19/2016 HazardEx ­ Understanding vibration­induced pipework failure

http://www.hazardexonthenet.net/article/77015/Understanding­vibration­induced­pipework­failure.aspx 1/3

14 MAY 2014

Understanding vibration­induced pipework failure

Data published by the UK’s Health & Safety Executive (HSE) for the offshoreindustry has shown that in the UK sector of the North Sea, fatigue and vibrationfailures account for 21% of all hydrocarbon releases. Here Neil Parkinson,Technical Director at asset integrity specialist AV Technology Ltd (AVT), looks atunderstanding, correcting and preventing vibration­induced pipework failure

Although overall statistics are not available foronshore facilities, available data for individual plantsindicates that in Western Europe, between 10 and15% of pipework failures are caused by vibrationinduced fatigue.

Based on the Energy Institutepublication ‘Guidelines for the Avoidance of VibrationInduced Fatigue Failure in ProcessPipework’*, current best practice is aimed atminimising the risk of incurring loss of containmentfrom vibration­induced failures. An enhanced andexpanded version of the former Marine TechnologyDirectorate Guidelines (1999), the document plays a key role in maintaining integrity in the design and maintenanceof process pipework within the oil, gas and petrochemical industries.

The Energy Institute guidelines break down into two main scenarios – proactive and reactive assessments – and aimto ensure compliance with statutory duty, improve safety and reliability, reduce liability from leakage and minimiseplant downtime. Proactive assessments can be used to routinely evaluate all pipework on a site, whether existing orplanned, and to identify possible areas of concern. Reactive assessments follow, and are used to investigate knownvibration 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, forexample 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 themain 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 flatbar, which can lead to the possibility of punching shear failures and introduces additional unnecessary fatigueinitiation sites at the new welds. Another very common mistake is to utilise braces which are too flexible or onlyeffective in one plane. Commonly, these are fabricated from angled sections steel of flat bar and rely on the stiffnessof a cantilevered arm to restrain the SBC. The optimum design should always have multiple support arms to form atripod­like truss arrangement which has excellent geometric stiffness.

Pipework containing multi­phase flow is a common source of vibration in offshore applications, resulting in hightransient vibration. Steam pipework is another typical source of vibration in onshore applications, commonly causedby steam traps.

Due to the nature of the pipework, the additional consideration of thermal expansion must therefore be taken intoaccount when designing a solution meaning that fixed bracing is likely to be unsuitable. In these instances visco­elastic dampers prove most appropriate, allowing for the slow movement caused by thermal expansion but stillproviding effective restraint.

Vibration­related failures of small bore instrument tubing (SBT) arrangements can cause significant processinterruption. In one example, although a system only carried instrument air, failure of the pipework resulted in

6/19/2016 HazardEx ­ Understanding vibration­induced pipework failure

http://www.hazardexonthenet.net/article/77015/Understanding­vibration­induced­pipework­failure.aspx 2/3

Unsupported SBC at gas refinery

Visco­elastic damper

automatic closure of the control valves causingsudden and unexpected gas production disruption.Here, the problem was exacerbated by the poorrouting of the SBT system, which contained lengthysections of unsupported tubes, and the fact that theSBT also carried the weight of unsupported boosterrelays – all of which were successfully remedied withbrace installations.

There are six phases to achieving pipeworkvibration assessments in line with requirements ofthe 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 calculationsfor assessing the likelihood of encountering a vibration­induced 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 theconstruction quality of infrastructure. It also assesses the chance of flashing or cavitation, and includes a calculationprocess for scoring likely excitation factors – which are combined with conditional and operational factors to predictthe ‘likelihood of failure’ for each pipe branch.

Many pipework vibration problems are the result of operators not following recommended practices, and visualinspection by skilled assessors can quickly flag up areas for improvement relating to pipe infrastructure. This mayinclude installing more effective pipe supports, proper bracing of SBCs, avoiding fretting and poor geometry, andallowing for thermal expansion of tubing.

The basic piping vibration measurement phaseidentifies areas of concern based on measuredvalues of pipework vibration. Specialist engineerswill first use a single axis accelerometer connectedto a portable data collector to take initial vibrationlevels, ranging from 1 Hz to 300 Hz. Thesemeasurements are presented as vibration amplitudeversus frequency and enable the vibration to beclassified as acceptable, concern or problem, basedon comparison with assessment criteria in theEnergy Institute guidelines.

If vibration is assessed as being at a concern orproblem level, or for pipework with a higherfrequency vibration of more than 300 Hz, the next phase used by vibration engineers is based on specialistmeasurement techniques. Here, a variety of in­depth tests can be deployed, including: dynamic strain measurementand fatigue analysis; experimental modal analysis; operating deflection shape analysis; and dynamic pressure(pulsation) measurement. In addition, engineers can implement specialist predictive techniques, applyingsophisticated tools and computer­based modelling to provide a detailed assessment of the dynamics of specificpipelines. Specialist predictive techniques include finite element analysis (FEA), computational fluid dynamics and

6/19/2016 HazardEx ­ Understanding vibration­induced pipework failure

http://www.hazardexonthenet.net/article/77015/Understanding­vibration­induced­pipework­failure.aspx 3/3

Energy Institute chart

pulsation and surge analysis.

The final stage of any pipework assessment is to recommend corrective actions to reduce vibration levels and thelikelihood of future vibration­induced fatigue failures. These actions vary from improving the support infrastructurearound pipework including bracing and dampening, or modifying the process conditions themselves to reduce fluidloadings.

The design of practical and appropriate corrective actions is important in achieving cost effective yet thoroughsolutions, and often utilises FEA techniques to predict the effect of remedial repairs, alongside CAD software formechanical design of supports and bracing systems.

With the correct knowledge of what constitutes good practice, designing an effective SBC bracing system can bestraightforward, 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

• Comprise of 2­3 arms to form a trussarrangement, each of which should have goodinherent stiffness in both bending planes (flat barsshould never be used)

• Avoid welds through the use of bolted clampedconnections

• Include a suitable wear resistant liner to preventfretting and galvanic corrosion

As a general rule, solutions for minimising pipeworkvibration to accepted levels can be categorised bythe type of system. SBCs and SBTs typically benefit

from local bracing back to the mainline pipe, whereas mainline solutions normally utilise supports to existing nearbysteelwork. Solutions for mainline pipework must take into account static movements such as thermal expansion,meaning visco­elastic dampers are the most appropriate solutions. However, where there is no nearby structuralsteelwork 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 butalso including the adjacent support structures and buildings. It is therefore vital to develop a comprehensive overviewof vibration patterns in order to recommend constructive improvements. Strain gauging, FEA and OperatingDeflection Shape (ODS) analysis are powerful tools in this analysis process and although these are often perceivedas being distinct and alternative assessment technologies, AVT has long recognised the power of combining practicalstrain gauge work with theoretical FEA – giving a complete three­dimensional picture of the modal behaviour of astructure.

* 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 industriesincluding Sellafield Ltd, British Energy, BP Saltend and Cargill plc. He is a Fellow of the Institution of MechanicalEngineers (IMechE).