Methods Used to Combat Corrosion
As we have seen, the jacket is built from mild steel rolled plates in the form of tubular sections welded together. These are fabricated from mild steel which is very susceptible to corrosion by salt water. The structure therefore requires extensive protection against corrosion, and can be carried out using the following methods.
This along with the priming of the steel is one of the most important stages in the process, being carried out after all welds have been checked by Non Destructive Test. (See my article on NDT).
This preparation involves shot-blasting the steel back to bare metal to ensure there is nothing remaining on the steel surface that would impede the adherence of the protective coating employed.
Application of Anticorrosive Coatings
Anticorrosive coatings used to be of specialized zinc-based paints along with acrylic and rubber binding compounds. However modern coatings are apt to be epoxies, having a much superior adhesion property required for offshore environment. Urethane and vinyl coatings are preferred by some offshore oil and gas clients, being cheaper and easier applied but these are used to a lesser extent.
At my old offshore construction yard they sometimes applied a combination of coatings to the structures. This consisted of a primer of zinc, followed by a coat of epoxy then a final coat of urethane.
The zinc primer ensured a good key for the epoxy anticorrosive, the final urethane coating providing a visible and enduring color finish.
This method of coating can be used for the jacket both above and below the waterline and also for the protection of the deck surfaces.
The splash-zone is constantly being sprayed with seawater or covered by waves so it is a special case which can be protected by the aforementioned techniques but other methods include the spraying or splattering of molten aluminum onto the surfaces in this area. This is known as Thermal Spray Aluminum (TSA) and has proved a very effective means of protection, its only drawbacks being the requirement of specialist equipment for initial application and repair problems.