Background
A company in the oil and gas industry specializing in wellsite wastewater removal and processing was experiencing through-wall failures and leaks of oily water in one of their storage tanks. The tank was approximately 40 ft. in diameter and featured a bolted, rolled, tapered plate design. The company reached out to Advanced FRP’s team of experts to help them determine what was causing the through-wall failures and resultant leaks. The company wondered if Advanced FRP could offer a repair solution that would stop the leaks, prevent further corrosion, and preserve their storage tank.
Immediate Challenge
Bolted tank designs often include an internal coating when the tanks are built. Unfortunately, these internal coating systems are often inadequate for long-term water-immersion service. Over time, asset owners will need to turn to other solutions to mitigate or repair the damage resulting from a failed or inadequate coating.
Two of the more common reasons leaks can develop on bolted tanks are internal corrosion and issues with the gaskets between plates. Applications with high microbial activity, like oily water storage, frequently see accelerated corrosion located at any pinhole, void, or holiday on the internal coating causing through-wall failures to occur. Seals on bolted connections can fail for various reasons: the bolts themselves loosened over time; the tank was left empty for a period that allowed the gaskets to dry out, or the bolts have corroded, and the material loss loosens the tension on the gasket. Finding the root cause of a leaking tank is difficult to determine without a detailed inspection that can only be performed after a storage tank is drained.
Designing a Solution
After the asset owner drained the storage tank, the contractor performed a thorough cleaning and issued a detailed inspection. They found the tank was experiencing multiple leaks caused by both the expected pathways and an unforeseen issue, which manifested in the following ways:
- Significant degradation of the bolts, despite the presence of plastic bolt caps. The bolt caps were designed to fit closely over the bolts but could not seal out water. Most had water inside when they were removed.
- There were isolated areas of significant corrosion throughout the tank.
- Previous “molecular weld” repairs were no longer adhered to the tank floor, allowing active leaking. Molecular welding is a repair method where a steel plate is “glued” over a hole with a specialty epoxy.
Upon close analysis of the corrosion patterns, locations, and service type, the Advanced FRP team determined the cause of the tank and bolt degradation. Most likely, microbes in the tank accelerated the corrosion of the tank and bolts.
Microbially Induced Corrosion (MIC), also known as microbial corrosion or microbially influenced corrosion, is corrosion caused or accelerated by microorganisms and can lead to the deterioration of both coatings and underlying substrates. MIC can quickly lead to through-wall failures if left untreated.
Advanced FRP designed a repair solution to:
- Include a decontamination step to kill any microbes on the surface to address the treatment of MIC.
- Repair exposed through-wall failures with carbon fiber composite patches.
- Fully encapsulate the bolts on the tank floor with immersion-grade epoxy.
- Prevent further corrosion with a MIC-resistant novolac epoxy coating system over the entire tank surface.
Installation Process
As surface preparation is the first step to a successful coating project, special attention was given to properly preparing the tank surface and bolts. Any remaining oily water was cleaned and removed before grit blasting. The surface was then checked for the presence of chlorides and other ions.
Despite holding freshwater, the chloride level on the surface was higher than the acceptable amount. So a rinse to remove chlorides was required before final grit blasting. In the process of grit blasting, it was found that many of the molecular-weld repairs were not well adhered to and came off with the minimal impact. After a consultation, the tank owner requested all molecular-weld repairs be removed and replaced with carbon fiber composite patches to provide more adhesion and greater long-term reliability.
After completing grit blasting, further visual inspection of the floor bolts showed the full extent of degradation. Of the two-hundred bolts exposed and inspected on the tank floor, approximately 20% were significantly degraded. A fiberglass encasement was made to seal the bolted connections and protect the bolts from further corrosion. This encasement allowed for the injection of an immersion-grade epoxy system into an entire row of bolts simultaneously, providing both a seal and long-term corrosion protection of the bolt heads.
Finally, Advanced FRP’s immersion-grade, chemically-resistant epoxy novolac coating system, HP-410 GF, was installed inside the tank. This system was applied via airless spray at 15 – 20 mils per coat. The welds and corners were stipe coated, followed by two layers HP-410 GF in alternating colors for a minimum-film build of 30 mils DFT. The coating was allowed to cure at room temperature, and the entire tank was subjected to high voltage holiday detection. Any holidays were repaired, and the tank was returned to service.
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