Vibration Induced Fatigue in Piping

28 April 2017 by Martin Hamilton, Principal Consultant

Spectrum has a long history of pipework vibration studies through the 100’s of API618 studies carried out for reciprocating compressor installations over the past 28 years. More recently the Energy Institute “E I Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework” has brought together the consideration of all potential vibration mechanisms as part of a single process which can be carried out for new or existing process pipework installations.

Spectrum has expanded its portfolio of services to include studies using these E I Guidelines for clients in the North Sea and elsewhere. Recent projects for AMEC include applying the guidelines to installations on the Armada platform for changes of duty to accommodate the processing of fluids from the Varg gas export project.

The change of duty required the assessment of a number of lines for the following mechanisms of vibration:

The E I Guidelines use a scoring concept of Likelihood of Failure (LOF). The numerical scoring approach is a method which allows some identification of priority areas for consideration to be made beyond what would be undertaken in a simple qualitative assessment. They are a method of screening and identifying risk of vibration induced fatigue failure which is designed to be used in conjunction with more specialist studies to determine whether an identified risk is likely to arise in practice. Where the risk requires mitigation it is often possible to make relatively simple design changes to reduce the LOF values to acceptable levels. Alternatively more specialist recommendations may be required. Spectrum has the experience and capability to undertake many of the more specialist studies relating to vibration induced fatigue failure and so is able to provide not only the assessment but also many of the recommendations required to achieve a design which is fit for purpose.

The assessment carried out for the Armada Rev topsides was for a number of pressure safety valve and blowdown tailpipes connecting into the High Pressure header of the relief and blowdown network and required the consideration of varying process flow conditions with these systems.

In order to meet requirements for acceptable LOF values recommendations for additional pipe supports were provided but should these have proved problematic for the existing installation Spectrum were also able to carry out detailed modal analysis to verify the screening study conclusions as required offering an additional level of support to the client in achieving the most appropriate solution.

In order to satisfy the requirements from the HFAE analysis, stiffening rings were recommended for PSV tailpipes after a detailed consideration of other options. Spectrum then provided detailed design advice for the sizing and location of the rings in order to achieve the desired mitigation.