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Corrosion Protection Coatings and Sacrificial Anodes used on Offshore Structures

written by: Willie Scott • edited by: Lamar Stonecypher • updated: 9/26/2011

Offshore structures such as oil and gas platforms require protection from corrosion in the marine environment. These huge installations are constructed from carbon steel plates and rolled sections. This renders them particularly susceptible to corrosion in the hostile conditions they operate under.

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    The drilling and production of offshore oil and gas takes place in one of the most hazardous environmental conditions.

    These conditions are detrimental to the structures causing corrosion which will weaken them. To combat this, corrosion protection coatings and sacrificial anodes are used on offshore structures. There are numerous types of anticorrosion methods employed, the majority of which are applied at the construction yards before the transportation of the structure to its offshore location.

    The following sections examine these methods of protecting the structures involved in the offshore oil and gas production against corrosion.

    The first section looks at the different areas which are most vulnerable to corrosion.

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    Areas of Offshore Structures at Risk from Corrosion

    • Steel Jackets

    On offshore platforms a steel jacket is used to support the topsides production equipment and it is the jacket which is most susceptible to corrosion.

    The jacket is constructed from tubular sections made up from rolled steel plates welded together, which be must be protected from corrosion above and below the waterline through the application of paint, epoxy coatings and fitting of sacrificial anodes.

    The area between the sea level and underside of the deck is known as the splash zone. This is exposed to regular soaking by the waves and spray, being partially dried out by the salt-laden air. These are ideal conditions for the promotion of corrosion.

    This area can be protected by convention methods, but lately spraying the structural with a non-ferrous metal has been employed.

    • Production Decks

    The numerous decks are fabricated from steel plates and sections, and are also liable to corrosion as they are exposed to the hazardous environment being constantly assailed by wind borne salt spray from the sea. The deck anticorrosion coatings must also be durable as they get a lot of wear from the personnel operating and maintaining the various processes. Laydown areas are often given extra coatings as they suffer damage from stores and container landing.

    Decks can be protected against corrosion by application of coatings similar to the jacket, but these coatings should incorporate non-slip properties.

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    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.

    Preparation

    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.

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    Corrosion Protection Using Sacrificial Anodes

    This method of anticorrosion to areas below the waterline has been in use for many years being applied to ships before the arrival of offshore structures.

    The system is known as Cathodic Protection (CP) and operates on the principle that electrons are removed from the zinc anode and transferred by the seawater (acting as an electrolyte) are deposited on the steel plate which has now become the cathode thus protecting it against corrosive attack.

    The anodes are welded on at the construction yard. (We used to weld them onto the leg sections before final assembly.) Once they become badly eroded in operation they are difficult to replace under the waterline, however cathodic protection can be maintained by using an anode sledge arrangement. This consists of numerous rows of anodes welded into a steel frame. This frame sits on the seabed and is connected to the structure by thick copper wire strands being welded to the sledge framework and welded or clamped securely onto the jacket leg. Both methods are shown below.