Underwater Coating Protects FPSO Hull from Corrosion

Offshore and underwater maintenance can be troublesome to say the least! Not only is access to these areas difficult, requiring specialist application personnel, but carrying out repairs can be hampered due to the aggressive environmental conditions. Bypassing this and carrying out maintenance and repairs onshore (in dry-dock) can often be time-consuming and vastly more expensive – meaning that assets have to be taken out of service, causing significant disruption to operations.

Therefore, making offshore maintenance easy and simple is essential, especially for underwater applications. Belzona has a range of systems specifically designed for in-situ repairs. For instance, surface-tolerant repair materials and coatings that have the ability to displace contaminants and bond strongly to wet substrates.

Working alongside the underwater application team at Jones Marine Technologies Ltd., Belzona helped to protect exposed sections on an FPSO hull using an underwater coating – Belzona 5831 (ST-Barrier). This post takes a closer look at this application, including an interview with Michael Jones, Managing Director for Jones Marine Technologies Ltd., about the application and his team’s experiences using the product.

Underwater specialist diving teams were sent to carry out initial inspection of the hull of an FPSO in Nigeria. Following initial reconnaissance, the divers reported that the orginally-applied protective coating was deteriorating in some areas due to fouling, erosion and impact. Over the whole area of the hull, it was estimated that up to 30% of the area of the coating was down to the primer layer, and therefore needed to be repaired before any steel became exposed.

“The idea was to “beef up” (improve) what was there, so we don’t run into future corrosion issues. The work was a long way offshore in very deep water. Luckily, there are number of advantages of working offshore in Nigeria. We have clear visibility and we have warm water, which is great advantage when you’re underwater coating as the material stays runny when you’re applying it.”

Having previously used Belzona solutions, the Contractor (Jones Marine Technologies Ltd.) recommended Belzona 5831 to the client as the appropriate solution to restore the protective layer of the FPSO’s hull.

This 2-part epoxy system is specifically formulated for application to wet and oil contaminated surfaces, as well as for underwater coating applications. The material displaces water from the surface ensuring maximum adhesion to damp or wet surfaces, forming a protective corrosion-resistant barrier for assets operating under immersed conditions.

First, a specialist underwater rotary tool was used to remove fouling and any loosely adhered coating. The surface of the intact underwater coating was then roughened with an underwater abrasive disc.

Once the surface was prepared, Belzona 5831 was applied to the surface by the divers, using a lambskin applicator. The coating thickness was measured using WFT gauges regularly, to ensure the recommended thickness was achieved. The coating was applied in two layers to repair the existing coating, and to ensure that no exposed steel remained.

“We found that the application of one coat [of Belzona 5831] over the thinned existing layer was providing at least 300μm, which was regularly measured by the applicator. The product applied very rapidly, especially with our skilled divers on site. And for those who hadn’t, within the first couple of dives, they were applying the coating as well as the man next to them.”

“It was an extremely strong and durable coating, that bonded to an existing coating very well (having been prepared properly). Overall, the customer was satisfied with the work done and we found the perfect product for the job.”

The customer required an underwater coating due to the shutdown costs associated with accessing a dry dock. Belzona was chosen as competitor underwater coatings were found to be difficult to apply in the specific conditions of this repair; mainly due to the warm temperature of the water. In total, over 1500 litres of Belzona 5831 was used to cover over 2,000 m² (21,000 ft²) of the FPSO hull. This effective underwater coating allowed the application team to apply the system more easily, maximising their time in the water and overall reducing the lead time of the project.