Hydrotesting of LNG Tanks Using Untreated Brackish Water

Hydrostatic testing of oilfield production and storage equipment requires that the damage done to equipment during testing is minimal.  In this case study, the storage tanks at a large liquified natural gas (LNG) terminal were to be hydrostatically tested using the natural brackish water found near the site.  Due to environmental regulations and shedule/cost implications, chemical treatment of the hydrotest water was not possible.  Concerns over both microbiological corrosion and preferential corrosion in the heat affected zone of the tank plate welds were the major technical issues.

The hydrostatic tests were conducted sucessfully using the natural brackish water without chemical treatment.  Post-test visual inspection verified that some flash rusting was evident near the heat affected zones but no significant damage was registered.  A number of mitigating steps were employed, including microbiological testing of the water prior to test, electrical isolation of the stainless steel downcomer pipes inside the tank, and coating of stainless steel fixtures in the tank that could not be isolated.  The primary methods for mitigation were the filtering of the inlet water, rinsing and de-watering of the tank immediately after draining, and minimizing the duration of the hydrotest.