Corrosion control in VEGA gas-condensate pipeline

Vega consists of three separate gas-condensate accumulations, Vega North, Vega South and Vega central.  The field is developed by subsea wells tied in to a host facility, Gjøa, with a 51 km multiphase pipeline.  Internal corrosion control of the carbon steel pipeline shall be done with a combination of pH-stabilisation and film forming corrosion inhibitors.
The aim of the present work has been to qualify a method for internal corrosion control of the Vega pipeline. The main parameter to determine was the concentration of pH stabiliser required for controlling the internal corrosion in the pipeline. The tests have been done using MEG / water mixtures corresponding to the conditions in the pipeline at 115°C. Some tests have also been done at 80 and 40°C, and the test solution has been modified accordingly. The effect of back production of formate completion fluid has also been tested. The requirements have been set to maximum 0.10 mm/year in corrosion rate and maximum 10 µm local corrosion depths after 3 days testing.
The results showed that a pH-stabiliser concentration of 30 mmole/ kg lean MEG (24 to 34 mmole/kg rich MEG) was required to control the corrosion (general as well as localized corrosion).  Furthermore, with the film forming corrosion inhibitors tested herein, a concentration of 290 ppm in the lean MEG (100 to 200 ppm in rich MEG) was needed.  These values were determined in MEG /water mixtures with scale inhibitor.  The scale inhibitor caused higher corrosion rates and also more localized corrosion.  The same values were also valid when formate was present.  No negative effect of the wax inhibitor on the corrosion control was found.