High performance corrosion inhibitors with lower toxicity are now available commercially. For this reason, the performance of these "green" corrosion inhibitors selected carefully in the laboratory and optimized on the field is not questionable, provided field conditions remain stable and adequate for the chemical to be available at the pipe wall. It is therefore the responsibility of the operator to ensure that these conditions are met. This study gives an example where the availability of a "green" corrosion inhibitor at the pipe wall had to be enforced by imposing a tight control on operating conditions. The use of the different tools available to the corrosion engineer is described, and the relevance of shear stresses is discussed.
Numerous papers focusing on the development, the selection, and the field performance of corrosion inhibitors have been published 13. They show that oil operators are now able to choose their corrosion inhibitors from a wide range of very efficient products available commercially, and covering most of their operating conditions. Oil operators have now devised corrosion inhibitor selection procedures including screening test in the laboratory and field trials. However, efficient corrosion inhibition is still not achieved systematically on the field, due to the lack of corrosion inhibitor at the pipe wall.
The poor or even non-availability of corrosion inhibitor at the metallic surface to be protected can be caused by a number of reasons. This paper presents an example of a field where high corrosion rates were induced by the non-availability of corrosion inhibitor at the pipe wall due to excessive shear stresses. In this case, the unavailability of corrosion inhibitor was due to the use of an existing under designed flow line for a new sub sea development. The paper will describe how this problem was addressed, and show that even in such cases, suitable corrosion inhibition can be achieved through the use of modeling and monitoring tools and the strict definition of allowable operating conditions. This case study also shows that whatever progress is achieved by corrosion inhibitor manufacturers and research and development laboratories, it is ultimately the operator, and the operator only, who has got the responsibility and the relevant data to ensure that suitable corrosion inhibition is adequate in the field.
GRANT EARLY PRODUCTION SYSTEM
Grant is a gas field lying about 9 kilometers from the Dunbar platform situated in the UK sector of the North Sea, 150 kilometers east of the Shetland Islands. The Dunbar platform is a wellhead platform, and is linked to the Alwyn North platforms by a 16" multiphase export pipeline (Figure I). All process operations, including gas dehydration, are situated on the Alwyn North platforms. Dry gas is exported from the Alwyn North platforms via the Frigg platform to the gas terminal situated at St Fergus (Scotland), where it is further processed to deliver sales gas. Liquids are exported via the Cormorant Alpha platform to the Sullom Voe oil terminal on the Shetland Islands (Scotland).
About 500 meters from the Grant field lay another satellite field called Ellon. This field was the subject of a sub sea development, and started producing in January 1995. The Ellon field consisted of two sub sea wellheads, tied back to the Dunbar platform via two 6" carbon steel flow lines, put in service in January 1995 and June 1997 respectively. At the end of 1997, it was decided that the Grant field would be developed initially as an Early Production System (EPS), by drilling one well and tying back its production to the second Ellon flow line. At the same time, the two wells of the Ellon field wo
