The Åsgard field was started up in 1999 and comprises the Smørbukk oil and Midgard gas assets. Midgard is routed through two 20" pipe lines, approximately 45 km of length, to the Åsgard field centre. Additionally, the gas field Mikkel located 42 km further away is tied back to Midgard and further on to the field centre.
As the pressure in the Midgard and Mikkel reservoirs decreases and the corresponding flow rate of gas also decreases, liquid surge waves start to form in the production lines. To avoid reaching the minimum flow rate in the production flowlines and extend the life time of the Åsgard Field, the world's first subsea compression station was started up in September 2015 to boost the well stream close to the Midgard wells. It will secure production of 306 MMboe from the Åsgard field up to 2029. The compression station includes two individual and identical compression trains, each consisting of an inlet cooler, a scrubber where liquid is drained out and boosted through a liquid pump whereas the gas is routed through a compressor before it is cooled down and mixed with the liquid.
There are numerous challenges when designing a subsea compression plant such as ÅSC from a flow assurance perspective. Safe hydrate design and the ability to handle liquid transients are essential for reliable operation and must be built into all parts of the process. The inlet cooler is designed to handle and distribute surge waves and provide an efficient cooling of the well stream. The scrubber is acting both as a combined separator, scrubber, and slug catcher. The compact integrated gas compressor and electrical motor is developed to also be able to handle liquid, i.e. well stream.
The station has since start-up operated very well, and the production availability has been excellent. Experiences from operation are given for start-up situations with varying liquid accumulated upstream the compression station as well as an incident where a hydrate slurry was generated at Mikkel.