Simulation of Unstable Two-phase Flows in Long Risers
The principles of the object oriented slug tracking schemes at EPT (Department of Energy and Process Engineering, NTNU) have been developed and discussed in some detail.
Simple bench-mark testing revealed that the LASSI code suffers form a pipe inclination-dependant lack of mass conservation, the cause of which is presently unidentified.
Comparing simplified and non-simplified SLUGGIT simulations with experimental data published by Taitel <i>et al.</i> mostly indicate a reasonable correspondence, though the precision is somewhat imprecise. In particular, obtaining stable riser flow (free of significant pressure oscillations) at low liquid flow rates was not managed without excessive gas rates.
This is possibly a consequence of the method's intrinsic slug flow approximation to dispersed regimes, but further investigation showed that the methods stability response altered with recent code versions in which alterations to management procedures was identified as the main differences, indicating that the SLUGGIT method's riser stability properties are quite sensitive to intuition-based section management routines. Further developing the models to better accommodate vertical flow regimes is advised.
Supplementary testing was afforded through the development of a steady-state unit-cell type model for phase fractions in the riser. Excellent accordance with simulation data was found, confirming that the SLUGGIT model is capable of reproducing stable, expanding bubble flow. Coarse resolution served to disturb this process as bubbles becomes longer and are affected by riser entrance and exit effects. It was also found that pressure oscillations caused by such entrance and exit effects display the typical characteristics of terrain slugging and may be mistaken as such.
With basis in the P50 Girassol pipeline, a systematic investigation into operational instability phenomena has been carried out using the available boundary conditions. Instabilities rooted in gas accumulation in jumpers, possibly also influenced by the U-bend, were found when studying the fixed pressure open inlet condition. The character of these instabilities were of a frequency and intermittency uncongenial to the field data. Most instability phenomena captured in these simulations were sensitive to changes in geometry, inlet condition and management parameters.
Severe slugging was initially found to dominate the flow picture with a fixed flow closed inlet condition. Also this type of operational instability had too high a frequency to be a match with the field data. Nor does the well-know `shark fin' pressure profile of the terrain slugging liquid build-up and blow-out precesses match the sinusoidal character of the field data.
It was recently found that the supplied field data needed adjustment for phase transition at the inlet state, amounting to a considerable reduction in gas flow. This produced predictions of more stable production compatible with those generated by external participant, though significant pressure fluctuations were still observed. These fluctuations were found to originate from slugging in the U-bend and entrance effects as large Taylor bubbles formed through coalescence and penetrated into the riser.
This latter cause is believed to be a feature of the limited, non-dispersed flow objects available in the EPT models.
A production index type boundary condition was implemented to better accommodate the well production response. Even so, the conditions under which the Girassol field instability data was recorded could not be recreated satisfactorily
without gas lift and PVT support implemented.
Simulations including the productivity index inlet indicated, for the most part, that without the presence of a gas lift system, the pipeline is likely to come to a complete stand-still; unless the liquid in riser and well are strongly aerated at all times, the well head will not be sufficient to overcome the total system liquid column weight.
Place, publisher, year, edition, pages
Institutt for energi- og prosessteknikk , 2012. , 110 p.
ntnudaim:7877, MTENERG energi og miljø, Varme- og energiprosesser
IdentifiersURN: urn:nbn:no:ntnu:diva-18861Local ID: ntnudaim:7877OAI: oai:DiVA.org:ntnu-18861DiVA: diva2:566328
Nydal, Ole Jørgen, ProfessorKjeldby, Tor Brox