Introduction. This paper covers a brief description of the Troll field off and on-shore process facilities and the design facilities required to meet the effluent discharge limitations.
Gas from the reservoir, at a design rate of 100 × 106 Sm3/d, enters the four gas-liquid separation trains at theplatform. In the separators a three phase separation takes place between the gas, the condensed water and the liquid hydrocarbons. The aqueous phase is cleaned in the Oily Water Treatment facilities before it is discharged to sea. The hydrocarbon condensate is re-injected into the main gas stream for further stabilisation on-shore. The total fluid is transported through two 36" carbon steel pipelines to the onshore processing facilities. To avoid hydrate formation and excessive corrosion in the transport lines, caused by the equilibrium water and CO2 from the gas, Mono Ethylene Glycol (MEG), which is regenerated at the on-shore plant, is injected into the two gas transport pipelines to shore.
Gas and liquids from the two wet gas pipelines arrive at the on-shore plant in two slug catchers at a pressure of approximately 95 bar and a temperature of 3°c. Gas from theslug catchers is discharged to three identical gas conditioning trains, where it is treated to sales gas specification by a cooling process and compressed by means of five export compressor trains for further transport into the Zeepipe and Statpipe export systems. Glycol, with a concentration of 75 wt % is injected in the gas processing trains to prevent hydrate formation.
The liquids (hydrocarbon condensate and glycol) from the slug catchers mixed with the glycol and hydrocarbon condensate from the gas trains are separated in a glycolcondensate separator. Hydrocarbon condensate from this separator is further processed to sales specification in acondensate stabilisation unit. From the stabiliser the condensate is pumped into the condensate export pipeline to the Norske Hydro oil terminal at Sture. Flash gas which isgenerated at the different stages of the condensate treatment facilities is recovered and recycled into the gas processing trains. The glycol is regenerated to 90 wt % in three identicalglycol regeneration units before it is recycled to the platform for re-injection into the wet gas pipelines. The water from the re-boilers in the glycol regeneration units is sent to the oilywater treatment facilities for further treatment before being discharged to sea (fig. 1).
Emission objectives. The design for the Troll on-shore plant and off-shore platform had to fulfill the discharge limitations as imposed by the Project for hydrocarbon emission to air andhydrocarbon and glycol emissions to sea. These limitations meant that the hydrocarbon emissions to air, under normal operation, had to be restricted to an absolute minimum for both the platform and the on-shore facilities. Only those off gases which technically could not be re-routed to the process and /or were not suitable for reprocessing were allowed to be flared or vented.
