1 ABSTRACT

Three phase flow system instabilities have recently been investigated in a 3" flowline-riser medium scale model system at the Equinor Porsgrunn test facilities in Norway. The test setup is well instrumented and comprises an 87 [m] long horizontal flowline followed by a 13 [m] high C-shaped riser. At riser top the riser is connected to both 1st and 2nd stage separators, thus reflecting a setup similar to that typically found in a real offshore flowline-riser process plant system. The model fluids used in the current work are air, Exxsol D60 and a viscous water/MEG phase. The system pressure is 4 [bara]. In this paper selected experimental results and observations will be presented, supporting increased understanding of the underlying physical phenomena and the coupled system instability resulting from interaction between the different process elements. Selected OLGA simulations are also presented.

2 INTRODUCTION

Three phase surge wave instabilities have been observed in the Mikkel/Midgard (1, 2013) and Snøhvit (2, 2009) gas condensate fields at turn down rates and during late life production. This phenomenon can have large operational effects on a receiving facility. For the Mikkel/Midgard fields, which are tied back to the Åsgard B platform, the instabilities enforce minimum flow rates in the flowlines due to liquid handling of the surges topside. Åsgard B has also experienced hydrate problems related to no liquid MEG inhibitor arriving topside between each liquid surge. In a recent study (3, 2017) it was demonstrated that a 1D transient flow model can reproduce the observed surge waves at Åsgard B. The model, which was tuned against field data from Åsgard B, predicts that the surges originate in the lazy-S riser at Åsgard B due to water/MEG accumulation with no water/MEG arriving topside between each surge. The field data and model show that condensate arrives topside also in the accumulation period. The model predicts that along with the release of the accumulated water/MEG from the riser a condensate surge is initiated in the flowline close to the riser.

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