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February 2016

Good vibrations

No-one wants to pick-up bad vibes whether it’s the inference in something said, that gut feeling something isn’t quite right or experience of a similar previous situation which turned unpleasant. As well as the metaphorical ‘bad vibes’ that can affect our daily lives, the oil and gas industry has had to contend with literal bad vibes causing significant problems to the integrity of offshore platforms, pipework and associated subsea equipment.

Vibration-induced fatigue is still a major source of loss containment on North Sea offshore assets – causing more than one in five hydrocarbon pipework releases. This often leads to significant remediation costs, associated loss of production, re-design and replacement of failed piping, investigation by regulatory authorities and potential damage to reputation. Though robust engineering solutions at the design phase and pre-commissioning phase can often eliminate its occurrence, the problem – and its potentially catastrophic effects – is still found on new assets and is being re-introduced through brownfield modifications.

Though this is not a new phenomenon, the risk of vibration-induced fatigue is not adequately covered in existing design codes (e.g. B31.3), leading to potential problems being introduced at the design and construction stages. Guidelines do exist to aid with design and the most widely used is the Energy Institute’s, ‘Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework’ (2nd edition, published 2008). However, while design contractors are using the guidelines, they may be incorrectly applied, often due to inexperience interpreting the recommendations.

The majority of pipework and structural vibration concerns first occur during plant commissioning or early operations. Site surveys, pre and post commissioning, is an important stage in the process to assess vibration issues beyond the design stage. Recent experience has shown that ‘at risk’ small bore connections (SBCs) are still being installed on assets that have undergone a full vibration assessment at design stage.

Surveys can identify high-risk locations that can either be modified prior to start-up or reviewed during operations. Many vibration issues are caused by transmission from nearby machinery or pipework and these cannot always be identified during the design phase.

Figure 1: At risk small bore connections. Transmission from nearby machinery or pipework may cause vibration issues which cannot always be identified during the design phase.
Figure 1: At risk small bore connections. Transmission from nearby machinery or pipework may cause vibration issues which cannot always be identified during the design phase.

Brownfield modifications
In the mature North Sea, field and asset life extension is re-energising E&P activity. This can involve brownfield modifications to existing, and often ageing, structures and systems. Any such modification to an existing facility requires rigorous investigation and planning to avoid shutdown and further, costly modifications. The installation of new equipment, which may not have been comprehensively assessed for vibration issues at the design stage, is putting the asset at risk of major safety, reliability and operational problems. All brownfield modifications should be treated like a new asset, even if it is like-for-like replacement, and a vibration screening assessment should be conducted, followed by a commissioning review. Though the likelihood of a failure is low, the consequences can be high, resulting in an unacceptable level of risk.

Figure 2: Vibration issues in new equipment is putting the asset at risk.
Figure 2: Vibration issues in new equipment is putting the asset at risk.

Subsea pipework
A vibration issue may take place subsea without any obvious signs topsides, this presents a number of unique challenges in the subsea environment given its remoteness and complexity to access. Less than a decade ago, no significant issues had been experienced, but that picture has now changed. As flow rates increase, and the need for flexibility and mobility in the piping to accommodate the impact of HPHT becomes a key design requirement, vibration concerns are increasing.

To counter some of these problems, Xodus, in co-operation with a wide variety of operators, equipment designers and consultants, is the technical author of a forthcoming Energy Institute document: ‘Guidelines for the avoidance of vibration induced fatigue failure in subsea systems’, which is expected to be launched in 2016.

Similar to the topside guidelines, this will determine the sensitivity of the piping response to changes to process conditions and/or structural parameters. As in-service assessments are not practical for subsea systems, more detailed simulation methods can be applied to systems identified as high risk by the screening assessment.

This combines a detailed structural model of the piping system with prediction of excitation amplitudes to provide an estimate of dynamic stresses and piping vibrational response.

Figure 3: A vibration issue may take place subsea without any obvious signs topsides.
Figure 3: A vibration issue may take place subsea without any obvious signs topsides.

Screening and assessment tools
Piping and pipeline condition assessment is crucial to ensure safety and maintain integrity. Damage caused by vibration, alongside erosion and corrosion, is responsible for 35 percent of all pipeline failures. During the design phase, knowledge of field operating conditions, together with modelling of dynamic systems to quantify the fatigue threat posed by dynamic response, is imperative. Rigorous assessment of the effect of potential processes and/or structural modifications will ultimately reduce potential vibration related problems before construction starts.

Xodus has developed HAWXEYE, a new approach to asset lifecycle screening, which allows operators to focus on investigating and preventing failures rather than locating and analysing data. The secure, cloud-based web app replaces complex and hard-to-share spreadsheets and is geared to screen assets for vibration, erosion and corrosion related issues. Complex data from monitors and piping and instrumentation diagrams is brought together into one user-friendly interface creating instantly accessible and transparent information for both internal company use and external reporting.

HAWXEYE can highlight poor design in the project and operations phases and detect the root causes of piping and pipeline failures and monitor reliability. By storing historical data throughout the asset and project lifecycles, the online tool allows concise data to be used to inform asset integrity management and continuous improvement programmes.

It will screen against industry standards and can avoid costly repair and replacement work, as well as the implications of lost production and project delays. HAWXEYE is backed up by the company’s specialists who boast more than 25 years’ experience in this area.

Figure 4: HAWXEYE combines complex data from piping and instrumentation diagram/drawings and process information into one user-friendly database.
Figure 4: HAWXEYE combines complex data from piping and instrumentation diagram/drawings and process information into one user-friendly database.

Setting new standards
For mature assets and life extension studies, engineers from Xodus adopt a risk-based approach to piping vibration, focussing on safety and production critical systems. This analysis can help identify the vibration issues of greatest concern and provide advice on the most cost-effective anomaly management. Xodus will be leading the forthcoming revision of the industry-standard Energy Institute document: ‘Guidelines for the avoidance of vibration induced fatigue failure in process pipework’, which is expected to kick-off in early 2016.

For new brownfield developments or debottlenecking studies, the company adopts a proactive approach to pipework vibration assessment, based on the Energy Institute Guidelines, but augmented by many years of carrying out such studies. This support helps to increase asset uptime, delivering substantial cost benefits.

Eliminating downtime and its financial implications, particularly in today’s cost-constrained marketplace, is vital in ensuring it's good vibes that operators are putting out for all to see.