vibration induced pipework failure.pdf

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Vibration induced fatigue of process piping systems is an important and fundamental failure

mechanism. Data published by the UKs Health & Safety Executive (HSE) for the offshore industry has

shown that in the UK Sector of the North Sea, fatigue/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 pipe work failures are

caused by vibration induced fatigue.

Leading industrial consultants AV Technology Ltd (AVT) are already carrying out pipe-work vibration

assessment tests for a number of major UK companies in line with the requirements of the latest updated

guidelines,published by theEnergy Institute in 2008 (ISBN 978 0 85293 453 1). AVT engineers are fully

conversant with the necessary test procedures and all-important analysis and presentation of data.

Understanding vibration induced pipework failure


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Understanding vibration induced pipework failure

Vibration induced fatigue of process piping systems is an important and fundamental failure mechanism. Data

published by the UKs Health & Safety Executive (HSE) for the offshore industry has shown that in the UK Sector of the

North Sea, fatigue/vibration failures account for 21 % of all

hydrocarbon releases. Although overall statistics are not available

for onshore facilities, available data for individual plants indicatesthat in Western Europe, between 10 % and 15 % of pipe work

failures are caused by vibration induced fatigue.

The latest guidelines, published by the Energy Institute in 2008

(ISBN 978 0 85293 453 1), are aimed at minimising the risk of

incurring loss of containment from vibration induced fatigue

failures of process plant pipe-work. The document is an enhanced

and expanded version of the former MTD Guidelines (Marine

Technology Directorate) first issued in 1999, which has been a key element in maintaining integrity in the design and

maintenance of process pipe-work within the oil, gas and petro-chemical industries.

Key benefits of following good practice include:

Compliance with statutory duty

Improved safety and reliability

Reduced liabilities from product

leakage

Minimised plant down time

The Energy Institute guidelines are broken down into two main scenarios:

Proactive Assessments, which can be used to routinely assess all pipework (existing or proposed) on a site

to ensure that good practice has been adopted and to try to identify possible areas of concern

Reactive assessments, used to further investigate and assess known vibration issues or to trouble shoot

actual failures. Some of the sections within the guidelines are further divided into the two main categories of

Main Linesand Small Bore Connections (SBC)

AV Technology Ltd are already carrying out pipe-work vibration assessment tests for a number of major UK companies

in line with the updated requirements. Their engineers are fully conversant with the necessary test procedures and all-important analysis and presentation of data.

The Energy Institute guidelines propose very logical phased approaches and include various Technical Modules,

detailing procedural flowcharts and good/bad practice guidelines for each phase. Key phases include:

Qualitative Assessment

Visual Assessment

Basic Vibration Monitoring

Specialist Measurement Techniques

Specialist Predictive Techniques Corrective Actions


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The Qualitative Assessment phase is possibly the most difficult to implement and involves various calculations for

assessing the likelihoodof having a vibration induced fatigue

problem on existing or proposed new plant. This assessment

takes into account many relevant factors, including: fluid

energy, flow velocities, process machinery, types of valves,

chances of flashing or cavitation, construction quality andcyclic operation. The assessment includes a calculation process

for scoring likely excitation factors and conditional and

operational factors, which are then combined to predict the

Likelihood of Failure (LOF) for each pipe branch.

The Visual Inspection phase is a quick yet powerful way to

spot many obvious potential areas of concern. It is a fact that

many pipework vibration problems are simply the result of

operators not following recommended good practice. These

include installing effective pipe supports, bracing SBCs

properly, avoiding fretting, repairing poor condition or damaged supports, avoiding poor geometry and allowing for

thermal expansion of tubing, The Energy Institute guidelines give many good quality photographic examples of good

and bad practice.

The Basic Piping Vibration Measurement phase provides a simple method of identifying areas of concern based

solely upon measured values of pipework vibration. This is an area which falls within AVTs mains areas of expertise

and involves experienced engineers visiting

sites with suitable instrumentation to

measure and analyse vibration data. Initial

vibration levels are typically recorded using

a single axis accelerometer connected to a

portable data collector, with required

frequency measurements ranging from 1

Hz up to 300 Hz. The results are presented

as vibration amplitude (in units of RMS

velocity) versus frequency. By comparing

these measurements against vibration

assessment criteria presented in the

guidelines, the vibration is classified into

one of three categories:Acceptable, Concern or Problem.

For further assessment of pipework with vibration levels in the Concern or Problem classifications or for pipework with

higher frequency vibration (> 300 Hz), the guidelines recommend the implementation of further Specialist

Measurement Techniques. These include:

Dynamic Strain Measurement and Fatigue Analysis

Experimental Modal Analysis

Operating Deflection Shape Analysis

Dynamic Pressure (pulsation) Measurement


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In addition, Specialist Predictive Techniques can be deployed, including:

Finite Element Analysis,

Computational Fluid Dynamics (CFD)

Pulsation and Surge Analysis.

AVT have years of cumulative expertise in many of theseSpecialist Techniques, specialising in the provision of

expertise and resources to apply sophisticated tools and

analysis methods in order to provide a more detailed

assessment of the dynamics of specific pipelines throughout

their life cycles. A high percentage of the work carried out

by AVTs Special Projects Division is concerned with the on-

site measurements of dynamic strain and vibration data

using multi-channel instrumentation systems, together with

the analysis and assessment of the measured data,

including fatigue life analysis.

The final stage of any pipe work assessment is to recommend Corrective Actions to reduce vibration levels and the

likelihood of future vibration induced fatigue failures. Corrective actions can take many forms, including:

Improving pipe supports,

Implementing more effective SBC bracing,

Modifying process conditions to reduce fluid loadings

Installing pulsation dampeners and viscous dampers.

The design of practical and appropriate corrective actions is important to achieve cost effective yet practical solutions.

Such work is often supported by AVTs in-house FEA capability, which is particularly useful for predicting the effects of

remedial repairs. Mechanical design of supports is performed using AVTs SolidWorks 3-D CAD software.

AVTs Technical Director Neil Parkinson is clear on the role his company can play in pipe work fatigue analysis and as

he explains:

Our strengths in predictive and condition based maintenance, together with those in structural monitoring and

vibration assessment, provide the perfect knowledge base for delivering a comprehensive service for vibration induced

fatigue projects. Vibration in pipe-work can be affected by a number of

direct and indirect factors relating not just to the pipe-work itself, but

also adjacent support structures and buildings. It is therefore vitally

important to provide clients with a comprehensive overview of the

vibration patterns together with constructive improvement

recommendations.

Strain gauging and FEA are powerful tools in the analysis process.

Although these are often perceived as distinct, alternative technologies

for assessing structural integrity, AVT have long recognised the


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increased power of combining practical strain gauge work with theoretical Finite Element Analysis (FEA).to give us a

distinct competitive edge.

Whilst FEA models can produce important predictions of stress, displacement and frequency, often actual in-service

loading conditions are unknown and so results can be highly unreliable unless validated using actual in-service data

such as strain measurements.

Conversely, whilst actual strain gauge data can yield vital accurate information about stress amplitudes and fatigue,

they are normally limited to measurements at just a few positions and therefore do not give a complete 3D picture of

the modal behaviour of a structure.

Mulit-axis accelerometers assembly fixed to pipework


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Lining provides protection to line from fretting at the

U-bolt.

U-bolt is attached to the connection on a reducer

section and is not lined. It is therefore susceptible to

fretting damage.

Fretting damage to main pipe. There is no resilient

pad between support and pipe. Note also pipe clash

below.

Reinforcement plate on pipe at rest support to resist

fretting damage to pipe.

Fretting damage to pipe caused by pipework

vibrating relative to deck penetration cover

Resilient pad between support and pipe protects

against fretting damage.

Examples of good and bad practice in pipework


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Bracing insufficiently stiff; single plane only;

only protecting weld to parent pipe.

Bracing stiffness increased; diagonal brace

protects in two planes; valve now supported.

Temporary fix of mass loading to detune a

structural resonance still in place some time later.

Rope used to support cable tray!

Fretting due to loose cladding and damage

caused by knife edge contact at insulation end

cap (existing cladding has been removed

Images courtesy of the Energy Institute

Examples of good and bad practice in pipework


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AV Technology LimitedAVTECH House

Arkle AvenueStanley Green Trading Estate

Handforth, CheshireSK9 3RW

Tel: +44 (0) 161 486 3737Fax: +44 (0)161 486 3747

[email protected]

www.avtechnology.co.uk

AV Technology operates an ISO 9001: 2000 QA system and applies a strict health & safety policy.

AV Technology Ltd is a leading multi-discipled consultancy with over 30 years experience.

Our portfolio includes:

Consultancy:

Condition Monitoring

Facilities & Assets

Maintenance

Noise & Vibration

Structural Integrity

Training

Hydraulic Systems

Products:

Acoustic Emission Sensors

Condition Monitoring Systems

Lubrication & Filtration

Silo Weigh

Structural Monitoring Systems

Vision Systems

Spi-VR vibration recorder

Services:

Condition Monitoring

Lubrication Management

Maintenance Solutions

Noise & Vibration

Strain Gauging

Structural Monitoring

Visual Inspection

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