Vibration Induced Pipework Fatigue Failure Analysis

Vibration Induced Pipework Fatigue Failure Analysis follows a Risk Based Inspection (RBI) methodology. The Energy Institute document ‘Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework’ is the industry standard for evaluating vibration integrity risks. 21% of hydrocarbon releases are due to vibration-induced fatigue failures (UK Health & Safety Executive), Around 80% of vibration-induced failures are associated with small-bore connections (SBCs). The remaining 20% are generally associated with parent pipe girth weld failure.

Energy Institute publication ‘Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework’ provides current best practice, which is aimed at minimising the risk of incurring loss of containment from vibration-induced failures. The guidelines play a key role in maintaining integrity in the design and maintenance of process pipework within the oil, gas and petrochemical industries. The Energy Institutes guidelines are an enhanced and expanded version of the former Marine Technology Directorate Guidelines (1999).

The Energy Institute guidelines break down into two main scenarios. Proactive and Reactive assessments. They 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 trouble-shoot actual failures within mainline pipework and 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 a 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.

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.

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. This failure could have been successfully remedied with brace installations.

There are six phases to achieving pipework vibration assessments in line with requirements of the Energy Institute guidelines:

1. Qualitative assessment
2. Visual assessment

• Basic vibration monitoring

1. Specialist measurement techniques
2. Specialist predictive techniques
3. 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 concerning or problem level, or for pipework with vibration above 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. These advanced tests include dynamic strain measurement and fatigue analysis, experimental modal analysis, operating deflection shape analysis (ODS), and dynamic pressure (pulsation) measurement.

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 damping, or modifying the process conditions themselves to reduce fluid loadings.