TOGI: Multiphase Flow From Troll to Oseberg

ABSTRACT The TOGI 20" pipeline has been designed to transport unprocessed well fluid 48.5 Km from a subsea installation to an offshore production platform. The paper describes how the pipeline has been designed to handle potential problems related to multiphase flow transport and gives data from measurements carried out during operation. INTRODUCTION Transport of unprocessed well streams have been with the oil and gas industry since it started, The use of the technology however, was limited to short gathering systems and it is fair to state that the underlaying technology was poorly understood, In recent years the potential of exploring the use of multiphase transport and in particular the transport of unprocessed well streams has been clearly seen. In connection with wellhead platforms and sub sea developments especially the potential savings are most significant. For multiphase transport lines having a starting point on shore or at a manned offshore platform it is possible to control the liquid accumulation through pigging. In this way production at low flowrates where liquids tend to accumulate can be tolerated as regular pigging (e.g daily) can be carried out to remove excessive liquid. Multiphase transport lines originating subsea can normally not be pigged on a regular basis as the pigging operation requires the use of a drill rig, For such lines the importance of detail understanding of the multiphase flow is paramount, and the determination of a proper operational envelope is one of the most important tasks of the early design. The TOGI development. The TOGI production system will transport gas from the Troll field through a 20" pipeline 48 Km to the Oseberg field where the gas is processed, compressed and injected into the gas cap of the Oseberg reservoir (Fig l). The gas injection improves the total oil recovery by in excess of 7% over the originally planned water injection system. The TOGI subsea station is located at 303 m of water depth at the south western part of the Troll East field. There are five wells remotely controlled from Oseberg by an electro-hydraulic control system. Methanol is pressurized at Oseberg and injected through a service line into the wells at Troll. The pipeline has a 0.47 m inner diameter. The riser has an inner diameter of 0.46 m. From the Oseberg B-platform where there are facilities for receiving pigs, there is a 18" 0.41 m inner diameter line approximately 225 m long connecting the pipeline to the slug catcher located in the process facilities on the A-platform (Fig. 2). Peak daily production is between 10.5 and 12 106Sm3/d. Minimum production rate is 5 106Sm3/d. The TOGI fluid is at its hydrocarbon and water dew points at reservoir conditions. Maximum liquid drop out is around 3 % (mass) or 0.4 % (vol). No liquids are produced from the reservoir. The reservoir pressure will fall around15 bar over the 11 year of planned production. There are several potential problem areas associated with transport of unprocessed well fluid.