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Norton Straw completes complex flow-induced vibration analysis of brine cavern pipe - 16 August 2017

Norton Straw completes complex flow-induced vibration analysis of brine cavern pipe

Norton Straw Consultants, the engineering and technology consultancy, recently completed a complex and uncommon assessment relating to Flow-Induced Vibration (FIV) for ABB Limited, on a brine cavern restart project in Cheshire, UK.

Flow-induced vibrations are caused by turbulence generated by the flow within a pipe that can cause failure through fatigue, and frequently need to be assessed in the offshore and process industries.  In this case the requirement was to assess potential vibration of a brine pipe for restarting operations in a brine cavern.  Brine caverns are created to exploit underground salt deposits by injecting water into the deposit and dissolving the salt.  The brine is then pumped out of the cavern and the salt recovered, with the cavern growing over time as more salt is extracted.

Salt production at the site had been halted and the cavern converted for gas storage – a common application for disused caverns.  However, the operator wished to re-brine the cavern, which is a rarely performed procedure.  This was to enable the site to re-complete the wells. During the operation of the cavern, rock falls from the roof of the cavern had dented the pipe and deflected it from the vertical.  There was a concern that turbulence generated as flow passed through the dented, bent section of pipe during refilling could lead to vibration and subsequent fatigue failure.

Flow-induced vibration assessments typically begin with a screening assessment using guidelines such as those formulated by the Energy Institute, to assess the likelihood of failure.  However, such an approach is not applicable in the scenario where a pipe has been damaged, and therefore engineering simulation was needed to assess the system.   It was necessary to build a model to predict the natural frequencies of the system, and determine whether the flow-induced turbulence would excite the structure into a vibrational response.  Norton Straw's fluid and structural dynamics experts used specialist Computational Fluid Dynamics (CFD) software to predict the turbulent pressure spectrum within the pipe, with a Large Eddy Simulation (LES) approach to resolve time-dependent variations.  The pressure fluctuations were then mapped onto a structural model, using Finite Element (FE) software, to assess the structural response and stresses in the pipe and subsequently to assess its fatigue life.  Further CFD analysis was used to assess the presence of any unsteady jet effects from the free end of the pipe.  Ultimately, the simulation work demonstrated that the pipe passed technical scrutiny and the refilling operation has now commenced.

Mr Benjamin Bradley, Process Engineer, from ABB, commented that "We approached Norton Straw to provide us with the technical insight and detailed supporting evidence we required to make our engineering and operational decisions confidently and ensure that vibration of the brine pipe was not an issue for re-brining the salt cavern. Whilst de-brining salt caverns is a well understood process, with years of operational experience, re-brining caverns is rare and less understood process that has only been conducted a few times in the UK".

Dr Simon Rees, Projects Director of Norton Straw, stated "While we assess vibration of process piping regularly at Norton Straw, this is actually a rarely studied scenario relating to a damaged pipe section.  We had to think hard about potential failure mechanisms and conduct the analysis necessary to rule each one out.  Working with ABB was an absolute pleasure and we look forward to supporting them again in the future".

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