Industry Standards and Classification Society Guidelines

Classification societies and regulatory bodies emphasize the importance of preventing gaps or crevices between a welded doubler plate and the original tank steel, as these can trap oil and gas, posing serious safety and corrosion risks. Regulatory guidance notes that such gaps may create inaccessible “gas pockets,” increasing the likelihood of corrosion and potential hazards. Consequently, the use of doubler plates in cargo oil tanks is generally discouraged or subject to strict approval and additional precautions.

Classification society regulations typically classify doubler plates as temporary repairs unless specifically engineered and approved. When permitted, best practices dictate that the doubler must be fully sealed through welding:

•             Continuous Perimeter Welds: A continuous fillet weld along the plate’s edges ensures an oil-tight boundary. Corners are rounded (≥50 mm radius) to reduce stress concentration and allow continuous welding.

•             Slot/Plug Welds: An array of slot welds securing the doubler plate to the original plating is recommended. These effectively stitch the doubler to the base plate, minimizing unsupported areas and eliminating internal gaps. Guidelines specify slot sizes and spacing (typically 150–200 mm apart) with a weld throat thickness of approximately 60% of the doubler plate thickness.

By implementing these measures, repairs become essentially solid-welded, minimizing any voids where crude oil could seep. However, minor crevices or porosity can still exist at weld toes or between weld passes. Epoxy encapsulation serves as an additional safeguard to seal any remaining micro-gaps, ensuring no fluid entrapment. Classification societies recognize the use of sealants or fillers in such repairs, provided structural integrity is assured through welding.

Some advanced repair techniques, such as the Sandwich Plate System (SPS), intentionally weld a doubler plate in place and inject the cavity with resin, bonding the plates and eliminating voids. This composite approach has been successfully used in FPSOs and tankers to restore strength and corrosion resistance, with studies showing improved stress performance compared to void-filled spaces. Industry standards consistently emphasize that cargo oil tank repairs must not leave crevices for oil entrapment, whether through welding or approved filling and encapsulation methods.

Suitability of Epoxy Coatings for Corrosion Prevention and Sealing

Epoxy-based coatings are widely used in marine tanks due to their excellent corrosion resistance and ability to create an impermeable barrier against liquids. International protective coating standards for cargo oil tanks specify the use of high-performance epoxy coatings designed to maintain integrity for at least 15 years in service. These coatings are engineered to withstand continuous exposure to crude oil and seawater without degradation.

Solvent-free epoxy linings exhibit superior chemical resistance, making them highly effective in crude cargo environments. When properly applied and cured, epoxy forms a hard, inert barrier that prevents oil and water from reaching the steel, thereby halting corrosion and crevice attack.

Epoxy coatings also possess the ability to penetrate and seal rough surfaces or small gaps. Composite epoxies often incorporate fillers such as glass flakes, ceramic, or metallic powders, enhancing mechanical strength and impermeability. This makes them particularly suitable for encapsulating welds, as the epoxy can be worked into the fillet weld toe and any slight gaps, curing into a solid mass that bridges the doubler plate and base plate. The result is a smooth, sealed fillet with no crevices where oil could accumulate.

Many marine epoxy systems are specifically formulated for crevice-sealing and corrosion prevention. High-build epoxy coatings are commonly used for “stripe coating” weld seams in tanks, providing extra protection to critical areas. Epoxy encapsulation serves the same function by covering the weld and adjacent area with a durable, chemically resistant layer. This approach not only prevents crude oil entrapment but also provides an additional corrosion barrier over the weld itself, which is often a high-stress and high-corrosion area.

It is crucial to select an epoxy formulated for marine immersion service. Standard epoxy paints or adhesives may not withstand exposure to hot crude oil, but marine-grade epoxies, including epoxy phenolic and novolac formulations, are engineered for such conditions. Additionally, composite epoxy systems with reinforced materials, such as glass-flake epoxies, significantly slow the penetration of water or oil, providing superior long-term performance.

Application Methods and Longevity of Epoxy Encapsulation

Surface Preparation

Proper surface preparation is critical to ensuring epoxy adhesion and long-term performance. The weld and surrounding steel must be thoroughly cleaned and roughened to promote bonding. Ideally, the area should be grit-blasted to near-white metal (Sa 2½) and degreased. Any residual oil, grease, or cargo deposits must be removed, as epoxies do not adhere well to contaminated surfaces.

Given that the application occurs inside a cargo oil tank, special attention must be paid to cleaning. Common methods include solvent wiping, followed by blasting or mechanical cleaning. If blasting is impractical, a surface-tolerant epoxy should be used, though proper cleaning remains essential. Inadequate surface preparation is a leading cause of epoxy failure, so meticulous attention to this step is crucial.

Application Process

1.           Weld Inspection: Before coating, fillet welds should be tested for leak-tightness using methods such as vacuum box testing or dye penetrant inspection. Epoxy can seal minor porosity, but structural defects should be repaired through welding before encapsulation.

2.           Epoxy Filling: A thick epoxy paste is applied to form a smooth fillet over the weld, eliminating sharp corners and sealing any undercuts or gaps at the weld toe. In some cases, a low-viscosity epoxy may be injected into accessible gaps to completely fill voids, ensuring no liquid pocket remains.

3.           Coating Application: After initial filling, the entire repair area should be overcoated with a high-performance epoxy system. The process typically involves applying a stripe coat by brush to all weld edges, followed by a full spray or brush application. Epoxies should overlap onto the surrounding steel and doubler plate to ensure a seamless, durable bond.

4.           Curing and Inspection: The epoxy must be fully cured per the manufacturer’s specifications before exposure to cargo. Environmental conditions such as temperature and humidity should be controlled to ensure proper curing. Once cured, the encapsulated weld should be visually inspected for continuity and tested for pinholes using holiday detection methods.

Expected Service Life and Maintenance

A properly applied epoxy encapsulation can provide long-lasting protection. Cargo oil tanks coated according to international standards typically maintain good coating integrity for 15 years. Similarly, an epoxy encapsulation repair using equivalent materials should offer protection for 10–15 years, assuming no mechanical damage or excessive thermal exposure.

Routine inspections during scheduled drydockings or tank entries are recommended. The encapsulated weld should be checked for any signs of epoxy cracking, disbondment, or underlying corrosion. If the epoxy remains intact, the steel beneath is likely well-protected. If damage is detected, the epoxy can be removed and renewed as part of maintenance procedures.

Potential Limitations and Considerations

While epoxy encapsulation offers significant benefits, it is essential to acknowledge its limitations:

•             Surface Preparation Sensitivity: Poor cleaning or inadequate profiling of steel surfaces can lead to adhesion failures. Cargo tanks present challenging environments, and ensuring proper substrate preparation is critical.

•             Epoxy Degradation Over Time: Exposure to aggressive crude oil components, high temperatures, or flexing can affect the epoxy’s long-term performance. Regular inspections and potential reapplications should be planned.

•             Not a Substitute for Structural Repair: Epoxy does not restore the base steel’s thickness in areas of significant corrosion. If the doubler plate covers extensively deteriorated steel, a long-term plan for steel renewal should be considered.

•             Inspection Challenges: Once covered by epoxy, welds and joints cannot be visually monitored for underlying deterioration. Indirect inspection methods or periodic removal of epoxy in critical areas may be necessary.

•             Application Constraints: Epoxy requires specific curing conditions and controlled environments. Temperature and humidity must be managed, and safety precautions must be taken during application.

Conclusion

Epoxy encapsulation of fillet welds in cargo oil tanks is a proven method for preventing crude oil ingress and mitigating corrosion risks. By sealing crevices and providing an additional protective layer, epoxy encapsulation aligns with industry best practices for ensuring tank integrity. When combined with proper welding and class-approved procedures, this approach significantly enhances the longevity and reliability of cargo tank repairs.