Michael Harrison, Belzona, UK, discusses the importance of surface-tolerant repair and protection in offshore operations.
Historically, in all corrosion repair and protection applications, surface preparation has been key when it comes to assuring the best possible outcome. Offshore environments are no exception. They are notoriously susceptible to problems stemming from their corrosive salt-water environment, difficult-to-access locations and contaminated surfaces, which are unlikely to see perfect conditions. This contrasts to external or internal areas which can be tented, cleaned and prepared, providing the asset owner with the choice of a plethora of environmental barrier options. Yet, in areas where surface contamination cannot be removed, corrosion is still a major problem. For example, areas near the waterline pose many issues and unique complications for coating manufacturers to face.
It is all in the preparation
Traditionally, achieving adequate bond strengths required a highly adhesive, chemical and mechanical bond between both the coating and the substrate. A standard epoxy material, for example, relies heavily on a strong mechanical bond typically achieved by creating a rough angular profile, allowing the epoxy material to physically grip the substrate. This grip is proportional to the contact area of the bond, achieved by this rough profile. Common standards for coatings and repair materials require a rough, angular profile of around 50 – 100 microns. In addition, SSPC-SP3 or SP6 are among the minimum standards required, although SSPC-SP10 is more commonly required. Overall, a dry and contaminant-free surface is required to ensure good adhesion is achieved. However, many available repair solutions are not compatible with minimal surface preparation, and therefore, can often lead to failure.
Limitations of common materials
Some of the common problems with available materials result in longer downtime intervals, higher costs, and higher incidence of failures. Commercial adhesives do not tend to perform well when bonding to oil contaminated substrates in wet or aquatic environments, especially at low temperatures.
Traditional acrylic adhesives have longer set times which slow assembly processes and lower impact resistance, resulting in brittle bonds. These materials also have a limited shelf life and high odour levels that can cause Health and Safety concerns. And most polymer repair solutions are not compatible with minimal surface preparation and can often lead to failure.
These systems fall short for use in adverse conditions for a variety of reasons. In two-component adhesive systems, the chemically reactive components needed for curing generally react low in the presence of water and oil, and thus create poor-quality bonds. Light curable adhesives generally require external heat sources, making them less desirable from a logistical standpoint. Few thermally cured adhesives result in rapid, high-strength adhesion, and even fewer function in high-moisture environments. Moisture reduces adhesion and low water temperatures reduce exothermic cure for underwater adhesives. As a result, underwater adhesives either cure poorly or require long cure times to obtain sufficient adhesive properties.
Unfortunately, being able to achieve the optimum bonding mechanism is virtually impossible in an offshore environment area close to the waterline. Surface contaminants can reduce the contact area preventing the coating from penetrating, wetting out and achieving the intimate contact with the substrate required for optimum adhesion. Modern technology has led to the creation of new protection systems which will bond to surfaces which have differing levels of contamination. These materials are designed to be effective at displacing surface contaminants, such as water and oil, from the substrate and are suitably named ‘surface-tolerant’. As well as this, the technology can achieve excellent bond strength using hand or power tools to prepare the surface.
Displacing contaminants through strong electronic affinity
So, for a surface-tolerant product to be able to bond to a contaminated surface it must first be able to displace contaminants. This can be achieved by innovative formulations and technology which allow the material to effectively absorb or displace surface contaminants. Once both the electronic and adhesive bond have taken place, the solution will create a strong mechanical bond once cured. This can be seen effectively via the offshore riser leg example (Figure 1).
Figure 1. Belzona 5831 (ST-Barrier) application, in the splash zone.
Tensile shear adhesion
With a high level of tolerance to surface contamination and excellent adhesion to manually prepared substrates, Belzona 1212 allows the epoxy to penetrate the profile and create a strong mechanical bond.
Figure 2 compares the tensile shear adhesion values of Belzona 1212, a surface-tolerant repair composite, as per ASTM D1002 standards. The data indicates the material can adhere strongly to contaminated surfaces. Overall, an abrasive blasted surface will offer better adhesion due to its irregular profile. However, this is closely followed by a manually prepared surface. This is mainly due to the technology’s innovative chemical formulation which effectively wets out the substrate. The data also indicates this technology is capable of adhering strongly to contaminated surfaces. Indeed, the adhesion of Belzona 1212 has been shown to be more effective when adhering to an oily surface than to a clean and dry surface. This is still not yet fully understood and will be investigated in future testing.
Figure 2. Comparison of the tensile shear adhesion values of Belzona 1212, a surface-tolerant repair composite, as per ASTM D1002 standards.
In order to minimise downtime, it is important that a repair material cures and develops mechanical strength quickly. This system will cure rapidly after mixing the two components even in low temperatures, without compromising on durability or strength. Figure 3 shows the development of adhesive strength with time at a range of low temperatures.
Figure 3. Belzona 1212 cures rapidly even in low temperatures.
A timed load test was conducted to demonstrate the speed of cure and strength of the system:
Following a 90 minute cure at 20°C (68°F), a one-inch square bracket bonded with Belzona 1212 was able to support a tensile load of greater than 200 kg (440 lbs.).
After a 4 hour cure at 20°C (68°F), the same bracket was able to support a load of greater than 1000 kg (2204 lbs.).
The material continued to develop mechanical strength and once fully cured, the bracket supported a load in excess of 1800 kg (3968 lbs.).
Gulf of Mexico application
An excellent example of this technology can be seen on an oil platform in the Gulf of Mexico, which was protected by using a Belzona solution. Due to long term salt water contact and impact damage, the protective paint system encasing the platform legs had failed, allowing corrosion creep, erosion and loss of structural integrity. The substrate was grit blasted prior to the application of Belzona 1161 (Super UW Metal), a surface-tolerant paste and Belzona 5831 (ST-Barrier), a surface-tolerant fluid, to bond a steel casing around the leg in the contaminated splash zone. The application involved bonding and sealing the outer edges of the split casing with Belzona 1161, before Belzona 5831 was injected through four ports, filling the entire cavity between the platform leg and steel casing. Thus, the possibility of corrosion was eliminated. The customer has now completed 32 successful leg repairs, with many more scheduled.
Even submerged, the Belzona surface-tolerant repair solutions offer outstanding corrosion protection and completely encapsulate the repair area. With a high level of tolerance for surface contamination and good adhesion to minimally prepared substrates, Belzona surface-tolerant products provide an effective repair solution that can reinstate equipment for years of service.
Advancements in the splash zone
Product suppliers, applicators, maintenance engineers and asset owners alike have long faced the challenge of reducing the level of surface preparation without degrading performance of the repair material. To overcome this apparent disparity between levels of cleanliness and product performance, many new products are being developed.
New technology in polymeric materials has led to the development of a fast cure epoxy-based product which opens new opportunities when ideal surface preparation of the metallic substrate cannot be achieved due to lack of time or resources.
Where conventional repair composites would be ineffective and perform poorly, the new technology, such as Belzona 1212, bonds tenaciously onto steel substrates even when surface preparation is minimal or if the substrate is heavily contaminated with oil or water. This innovative technology is now enabling effective repair and maintenance to be carried out in environments such as splash zones and underwater.
Read the article online at: https://www.oilfieldtechnology.com/offshore-and-subsea/19042018/splash-zone-solutions/