The severe slugging flow instability in flowline-riser systems is an upstream compressibility problem, where trapped gas upstream the riser base can be compressed and eventually cause blow out of the accumulated liquid in the riser. For the case of a very long riser, flow instabilities can be induced by gas expansion in the riser itself. Even small amounts of trapped gas can cause flow instabilities, as the accumulated gas is flushed through the bend at the riser base and into the riser. Small scale air-water experiments have been carried out to illustrate this phenomenon in an 8m long vertical riser with an internal diameter of 16mm. Constant inlet pressure is obtained by an overflow arrangement for the liquid. The gas is injected in a small undulation upstream the riser base. Unstable production has been observed both for continuous and non-continuous liquid production at the outlet, and the experiments have successfully been simulated by a Lagrangian slug tracking code.

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