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Thixotropic Gel Halts Subsea Corrosion of Carbon Steel

The deployment process to install FlexGel in the annulus between an I-tube and riser. Image courtesy of Flexlife.

A recently released gel system from Flexlife (Aberdeen, United Kingdom) is designed to halt the corrosion of carbon steel (CS) in subsea flexible and steel risers in locations, such as outer sheaths or other areas with damaged coatings. The system displaces oxygenated seawater from the region of the damage while replacing it with a noncorrosive gel.

Known as FlexGel, the products can also be used inside tubes to eliminate the air-water interface and coat the damaged riser, thus protecting both the riser’s outside diameter and the inside diameter of the tube. The gels can also be injected into flexible riser repair clamps used in subsea locations to form a flexible seal and prevent further corrosion. They can be installed both as a preventive measure for new assets, or as a method to extend the life of aging assets.

According to the company, the gel system’s thixotropic nature allows it to be pumped easily. Yet within 48 hours of application, the system’s viscosity increases significantly to prevent splashing or a loss of gel due to waves and vessel motions. For some applications, biocides can be added to the gel mixture to prevent microbiologically influenced corrosion (MIC).

The system was one of 10 winners at the 2019 MP Corrosion Innovation of the Year Awards, honored in March 2019 in Nashville, Tennessee, USA. Further technical details on the gels are available at the awards web site.

Previous Problems

Historically, having pipeline risers pass through the splash zone inside I-tubes or pull-tubes provides some protection from the surrounding environment, explains Kirk Francis, regional director of the specialist provider.

However, this also creates a situation where the inspection and repair of coatings and sheaths is not possible with the asset remaining in service. And when a riser is pulled into the platform through a pull-tube, the coating can be scraped and damaged, with no method for repair.

According to Francis, the gels solve the problem of not being able to inspect or repair coating or sheath damage in these situations by removing splash zone considerations.

The thixotropic gel is installed inside the tube or other structure to eliminate the corrosive air-water interface. This prevents the corrosion of CS pipeline risers or other structures inside the tube, where coating repair and clamps cannot be performed due to lack of access, and where cathodic protection is not very effective, the company says.

How the System Works

Since the gels are non-aqueous and density tuned to be slightly lighter than seawater, it sits on top of the seawater and displaces it out of the tube, thereby replacing the air-water interface.

The product then increases in viscosity and stays static within the tube, despite external waves and platform motions. The gel solution has been designed to be compatible with steels and polymer materials by coating them and preventing further corrosion.

A biocide in the form of tetrakis-hydroxymethyl phosphonium sulfate (THPS) can be added to the mixture for extra protection against MIC, Francis notes.

The gel can be installed quickly and requires no hot work, divers, or other safety-critical tasks, thus allowing production to continue during installation. According to the company, it is relatively low cost when compared to existing inspection and repair tools. Further, it does not face the issues associated with lack of access inside a tube. This mitigates the potential problem of not being able to inspect in the region and also provides a repair solution for risers or structures that already experience corrosion problems in this area.

Promising Results

Francis explains that the system has been tested both in laboratory settings and in the field.

In the lab, tests were performed to prove the CS samples did not experience any degradation in the gel, and to illustrate the difference between high-strength CS samples immersed in seawater vs. the same steels immersed in the gel. According to the company, the samples showed significant corrosion in seawater and no corrosion in the gel.

Testing of the gel system has been conducted in both laboratory and field environments. Image courtesy of Flexlife.

Additionally, material compatibility testing was performed for the gel system in contact with common outer sheath materials and coatings found on risers. Further testing considered the vibration frequencies at which the gel lowers in viscosity, to prove that floating platform motions would not affect the viscosity of the gel system while in service.

In the field, Francis says the company has a client who experienced coating disbondment on two steel risers. One of the risers had already experienced a failure due to corrosion, and concerns remained about the safety of an adjacent gas-lift riser.

In response, the company installed 5,600 L (1,479.4 gal) of the gel inside the tube to displace water from the critical area. Two years later, the area showed no further corrosion after the installation of the gel. More recently, after an additional three years, the client says the gel continues to perform as desired and the riser remains in service, according to the gel provider.

The gel system is approved by the Centre for Environment, Fisheries and Aquaculture Science (Cefas), an executive agency of the U.K. government, for use in ocean environments. According to the company, it has already been installed in the North Sea and Gulf of Mexico.

Patents for the system, which is now commercially available, have been granted in Australia, Denmark, the United Kingdom, Malaysia, the Netherlands, Norway, and Africa.

Source: Flexlife, www.flexlife.co.uk. Contact Kirk Francis, Regional Director—e-mail: Kirk.Francis@flexlife.co.uk.

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