Case study

Prolonging the design life of a gas turbine exhaust

The challenge

Offshore gas Turbine exhaust systems must support heat recovery and also be light enough to be easily installed or removed, whilst remaining unaffected by cyclic thermal loads and dynamic pressure fluctuations. Failures are common due to the extremes of gas temperature and velocity that occur. Gas turbine downtime can be extremely costly and remedial measures can involve long lead times and may not address the root cause of past failures. The gas turbine exhaust systems on the Bruce platform have suffered repeated failures during their operating lifetime. BP came to Frazer-Nash to see whether we could determine the shortcomings of the existing design. We then suggested and evaluated design solutions to prevent future failures.

Our solution

We diagnosed the cause of the problems in a three stage process:

  • A Computational Fluid Dynamics (CFD) model incorporating advanced turbulence modelling techniques to capture in detail the pressure fluctuations inside the exhaust system.
  • These fluctuations were applied to a structural model of the system to determine local stresses and pinpoint regions of likely failure. A thermal model of the system then determined the stresses generated by thermal cycling during start-up and shut down.
  • New design concepts were developed and assessed based on the results of the analyses. This included additional analyses to cover installation and lifting loads.

The benefits

We undertook detailed analysis of the failure mechanisms of the existing exhaust systems; provided concept designs for improved replacements; supported the engineering, procurement and construction; undertook overall project management and assessed the suitability of the installation and lifting systems. As a result, BP can have greater confidence that the new design will improve the overall reliability of the system.

Screenshot 2020 05 03 At 16.28.02
Images of CFD modelling of a GT exhaust duct. On the left is the varying pressure of the gas on the duct walls and on the right show the stream lines and velocities on moving particles within the duct.

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