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June 2000 - Issue 7

Our Lips Are Sealed

Analysis By Firth have recently performed a series of detailed analyses for Hydro Agri on a new design of lip seal for a fluid impeller. This included the determination of the thermal gradient through the pump (as the fluid is hot), and then using the thermal expansion and the internal pressure of the pump to provide stresses and deflections for the pump and the seals around the shaft. The model used slide-line elements to allow the seals to slip along the surface of the shaft and yet still represent their pressure on the shaft.

 example of the technical capabilities of Nastran for Windows
Hydro Agri were particularly interested in results presentation and we provided an enhanced package of detailed stress plots and animation files as a part of our reporting. This was a good example of not only the technical capabilities of Nastran for Windows but also the high standard of presentation material it can produce.

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Clamping down on stresses

Fatigue cracking of a principal support between some pipework and the main flowline on an offshore platform resulted from excessive vibration of the pipework. Had the supports failed, the well would have closed for repairs, resulting in a significant loss of production.

JMDynamics Ltd. (JMD) were contracted to perform testing and analysis of the flowline and pipework. This analysis showed the principal deflection patterns and the dynamic loading of the cracked support. A solution to the problem was recommended based on the analysis, but would not be available for several months. To ensure further damage was not sustained in the interim, a temporary support was devised and fabricated by JMD in conjunction with their sister company, Vibration Solutions Ltd (VSL).

The clamps were based on a standard range manufactured by VSL which are patented due to several novel features of their design. The clamps are sent offshore in kit form for installation using basic hand tools. These clamps are used to prevent vibration in small bore fittings so to ensure they could withstand the relative movement between the flowline and the pipework, finite element analysis was required.

 Graph
Graph
Using geometric information supplied by JMD, Analysis by Firth created a detailed finite element model of the clamp. A number of loading conditions were applied to the finite element model, the model run and the results checked for validity prior to being passed back to JMD for interpretation.

The analysis showed that the clamp would comfortably support the worst static and dynamic load components without failure and would protect the existing support from the effects of the vibration until the final solution could be installed. The clamps are still fitted to the pipework and the installation is producing from the affected wells.

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