Case study

What can be done when common FIV screening methods fail?

We look to understand the cause of unusual flow induced vibration.

The challenge 

Vibrations caused by the unsteady flow of production fluids are hard to predict (Flow Induced Vibration, or FIV), and can create unacceptable levels of fatigue. If FIV problems occur, then the situation must be rapidly examined to establish how production can be maintained at an acceptable level.

In this project our client measured vibration levels that were neither predicted nor explained by conventional engineering studies. We were asked to make a more detailed study that would explain the phenomenon and hopefully provide a justification for continued high production rates.


Why choose Frazer-Nash?

Frazer-Nash’s expertise in detailed fluid dynamics, fatigue analysis and subsea pipelines means that we are ideal for multi-discipline investigations of this type. The complexity of subsea systems required detailed analysis of specific features and components coupled with a parallel assessment of the global system dynamics.

Our approach

Our investigation followed three lines of study:

  • An analysis of the most credible sources, using a combination of scoping calculations and the measured data.
  • Detailed transient analysis using Large Eddy Simulation (LES), an advanced computational fluids technique that provides detailed, time-varying analysis of turbulent flows, and is considerably more accurate than conventional CFD methods.
  • Expert guidance for an onshore testing programme, including advice on scaling, set-up, instrumentation, and the analysis of results.


The outcome

We discovered that a small set of components was responding to a previously unsuspected source of vibration. We were able to predict the flow rate bands at which the phenomena occurred, allowing production to return to an optimum level. Furthermore, the fundamental insight we gained has been incorporated into our client's design guidelines, ensuring that future facilities will avoid the problem.


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Figure 1: Transient CFD modelling of 90° T-piece section, showing unsteady mixing in the downstream section.
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Figure 2: Transient CFD modelling of flow within a mitred elbow.

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