Huldra is a gas-condensate field located in the Norwegian sector of the North Sea. The Huldra field is currently producing at maximum rate of approximately 11.5 MSm3/d [406 MMSCF/d], but it is expected to decline rapidly from later this year and the field will thus enter the tail-end production phase. The produced rich gas is transported as a multiphase mixture of gas, condensate and aqueous glycol through a 22" diameter and 150 km [93.2 miles] long sub sea pipeline to the Heimdal platform for final processing and export. The operational lower transport limit for the rich gas pipeline from Huldra to Heimdal is 7.5 MSm3/d [265 MMSCF/d]. However, the economical cut off limit is much lower. The technical cut off limit was in a tail-end production project estimated to be about 2.7 MSm3/d [95 MMSCF/d] provided increased cooling of the export gas before the gas scrubber. Additionally, lowering the inlet pressure at Heimdal will have a positive effect. Installing a compressor at Huldra will lower the technical cut off rate even further, all the way down to the economical cut off rate. It will additionally be possible to process significantly more of the reservoir fluid with a compressor at Huldra.
Tail-end production is becoming more and more important at the Norwegian Continental Shelf as many of the existing oil and gas fields are declining. The overall objectives are to recover as much of the remaining oil and gas as possible from such fields, and to use spare processing capacity to drain nearby fields by linking them to these facilities as satellites. This demands cost-effective operations, modified operational procedures and sometimes retrofitting of the topside facilities. The Huldra field in the North Sea came into production at the end of 2001 and is now about to enter its tail-end production phase. This field is a particular challenge as the rich gas is transported as a multiphase mixture of gas, condensate and aqueous glycol through a 22" OD and 150 km [93.2 miles] long pipeline to the distant Heimdal platform for final processing and export. Long distance multiphase transport has become rather common at the Norwegian Continental Shelf (Troll, Midgard, Mikkel, Kvitebjørn etc.) and future developments like Snøhvit and Ormen Lange will extend this technology further in terms of both distance and water-depth. The stepwise development has become feasible partly due to the extensive efforts made over more than two decades to develop and improve multiphase flow models such as Olga and PeTra. For the Huldra case, the multiphase pipeline to Heimdal will represent an increasingly difficult operational challenge during the tail-end phase as the throughput is reduced gradually because the pipeline has a very large liquid accumulation potential. Furthermore, the receiving facilities at Heimdal have very limited liquid storage capacity to cope with liquid surges. The ability to cope with this combined challenge will influence the lifetime of Huldra.
