This paper presents data from the Midgard gas condensate field in the North sea. The production is tied back to Åsgard B. Field tests were conducted to measure the accumulation of water/MEG and condensate volumes in two 20" flowlines at different production rates. Here, data from one of these flowlines are presented.
During reduced rates when liquid starts to accumulate in the flowlines, long liquid surge waves are experienced topside. The onset of the surges is well inside the friction dominated pressure drop domain. The surges are separated into two parts, a condensate surge followed by a water/MEG surge. The duration and frequency of the surges varies. Up to 2 hour long surges are observed. Likewise, the time period between each surge varies considerably. Only quite small pressure fluctuations (~1 bar) are recorded at the well heads when the surges propagate through the ~300 m high riser. Only a minor effect is seen on the pressure before the topside chokes. The gas flow rate is reduced somewhat during the arrival of the surges. To the authors knowledge the mechanism behind the three phase surge waves is poorly understood.
The surge wave regime causes problems for the operation topside. It is especially the handling of the water/MEG phase which reaches its limit when the surges become too large. This defines the minimum turn down rate for the flowlines. One has also experienced problems with hydrate formation in the periods between each surge. During the liquid accumulation period no liquid water/MEG phase arrives topside. The likely scenario is that hydrate particles are formed as water condenses out when the gas propagate through the riser and experiences Joule Thompson cooling due to the pressure drop. The hydrate particles can then clog small bore piping topside.