RIASSUNTO
ABSTRACT
Hydrostatic pressure testing, or hydrotesting, 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 liquefied natural gas (LNG) terminal were to be hydrotested using the natural brackish water found near the site. Due to environmental regulations and schedule/cost implications, chemical treatment of the hydrotest water was not possible. Concerns over both microbiologically influenced corrosion and preferential corrosion in the heat affected zone of the tank plate welds were the major technical issues. The hydrotests were conducted successfully using 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. The primary mitigation step taken was temporary cathodic protection of the stainless steel internal piping, ladders, and other fixtures using sacrificial anodes and protective coatings. Additionally, the duration of test was minimized through efficient execution as well as potable water rinsing post-test and de-watering and drying steps immediately after draining. Microbiological testing of the brackish water was conducted prior to and at intervals throughout the test duration to determine if risk of microbiological activity was present.
INTRODUCTION
A major concern when commissioning oil and gas production and transportation equipment is corrosion damage to the equipment prior to start-up. One of the activities of greatest concern is the hydrotesting of pipelines, tanks, and vessels. The hydrotest process normally is conducted with some kind of water. The water can be fresh, potable, brackish or even come from the sea. A general industry rule of thumb for hydrotesting is that test durations lasting beyond 30 days should have pre-emptive chemical treatment to control corrosion. The corrosion could be general or localized, and may result from microbiological activity. A typical corrosion prevention “cocktail” for hydrotesting includes biocide, oxygen scavenger, and corrosion inhibitor. After the hydrotest is completed, disposal of the test water (plus neutralization of any treatment chemicals) is generally required. The standard method of water disposal is to empty the water back to its source. When the water has been chemically treated, this necessitates that the residual level of treatment chemical in the water meet regulatory allowable limits, in some cases, addition alone can limit disposal options regardless of residual testing. In this case study, no chemicals were allowed to be added to the water if returned to its original source. Corrosion mechanisms in a closed system (i.e. hydrotest environment) are usually limited to dissolved oxygen corrosion and microbiologically influenced corrosion (MIC). To control dissolved oxygen corrosion, oxygen scavenger such as sodium sulfite is often added. Biocides used for hydrotesting include glutaraldehyde and Tetrakis Hydroxymethyl Phosphonium Sulfate (THPS)1, both of which will break down into nonhazardous by-products if given reaction time. Dosages of biocides are significantly higher than corrosion inhibitor dosages. By their very definition, biocides may impact existing ecosystems if introduced into a body of water before they have been chemically neutralized or have bio-degraded into non-hazardous materials.