Offshore platform corrosion

Fig. 9.23 Welded areas of an offshore platform structural tubular displaying premature corrosion due to (a) lack of surface grinding (resulting in poor paint adhesion and performance) and (b) the fact that the weld metal is anodic relative to the parent material of the tubular...

Recommended Practice Corrosion of Steel Fixed Offshore Platforms Associated with Petroleum Production, RP-01-76, NACE, Houston (1983)... 

Inco alloy 25-6MO —Used for its corrosion resistance in many environments, this is an austenitic nicket-jron-chromium alloy with a substantial (6%) addition of molybdenum. Especially useful for resisting pitting and crevice corrosion in media containing chlorides or other halides. Applications include equipment for handling sulfuric and phosphoric acids, offshore platforms and other marine equipment, and for bleaching circuits in pulp and paper plants. 

A wide variety of random packing and structured packing are available. For new construction, except for especially corrosive services, pressure drop critical applications, and special cases, economics drives fractionation equipment selection toward trays. Special cases include applications of very low liquid rates, and where equipment size is critical. Examples of critical equipment size include units on offshore platforms, towers in severe earthquake zones, and towers to be housed inside buildings. 

Aluminum and aluminum-zinc alloy anodes have become the preferred sacrificial anodes for the cathodic protection of offshore platforms. This preference is because aluminum anodes demonstrate reliable long-term performance when compared with magnesium, which might be consumed before the platform has served its useful hfe. Aluminum also has better current/weight characteristics than zinc. Weight can be a major consideration for large offshore platforms. The major disadvantage of aluminum for some applications, for example, the protection of painted ship hulls, is that aluminum is too corrosion resistant in many environments. Aluminum alloys will not corrode reliably onshore or in freshwater [37]. In marine... 

Chemical treatment (use of biocides) not only must consider cost effectiveness and broad-spectrum features, but also it must be compatible with environmental rules and regulations. This means that the operators are not allowed to use any concentration of any type of biocide they wish. It must also be noted that as intelligent biocides have not (yet) been invented, the biocide cannot differentiate between corrosion-enhancing bacteria and other types of microorganisms and macroorganisms. This will limit application of biocides in industries such as offshore platforms. 

Significant corrosion and chemical resistance is required (food and chemical processing plants, cooling towers, offshore platforms, etc.)... 

RP0176-83 Corrosion Control of Steel, Fixed Offshore Platforms Associated with Petroleum Production— Item 53031-P... 

Cathodic protection is used widely for the protection of submerged steel in waterfront structures. It also can provide considerable benefit in the intertidal zone and can even reduce the usually high corrosion rate experienced at the boundary between the intertidal zone and the splash and spray zone. Cathodic protection also is used to prevent corrosion of the soil side of steel in marine structures such as sheet steel bulkheads. Cathodic protection also is effective in the control of the corrosion of reinforcing steel in concrete in all exposure zones in waterfiont structures. Particularly for impressed current systems, it is important to select materials for the cathodic protection system components such as rectifiers and junction boxes with consideration of the environment to which they will be exposed. When considering cathodic protection, periodic inspection and maintenance is required for proper system operation. The costs for inspection and maintenance must be considered in the overall cost of cathodic protection. While there are no specific standards for cathodic protection of piers and docks, information in NACE RP0176 (Corrosion Control of Fixed Offshore Platforms Associated with Petroleum Production) and NACE RP-0187 (Design Considerations for Corrosion Control of Reinforcing Steel in Concrete) contain information that is applicable to marine piers and docks. 

As noted above, in the discussion to do with Hazards Analysis, offshore facilities do not generally handle highly corrosive or toxic chemicals, hence reducing the associated integrity problems. However, offshore platforms operate in a marine environment where much of the equipment is exposed to salt water and harsh environmental conditions. 

The anticorrosion protection of offshore platforms subjected to difficult operating conditions is an especially responsible task. Apart from the continuous strongly corrosive interaction of sea water, periodical phenomena are also encountered, such as storms, earthquakes, hurricanes, pressure of ice floats, and other phenomena increasing the corrosion hazard. These interactions cause stress and fatigue corrosion and other forms of corrosion attack. In such conditions, special protective coating systems are applied to protect the objects, coupled with cathodic protection realized by the use of sacrificial anodes or an external power sup-... 

Offshore platforms are, in essence, similar to buried pipelines because in both, external and internal surfaces are exposed to corroding environments in buried pipelines the external surface of the pipe is exposed to the soil (which is a corrosive environment), and its internal surface is under the corrosive impact of the fluid that is going through, either water, oil or the like. In case of offshore platforms, the whole immersed stmcture is exposed to seawater (a corrosive medium), and the internal surfaces of the systems such as seawater injection systems or oil storage facilities can be considered locations at which corrosion is occurring internally. 

No matter how different two systems may seem at first sight, when it comes to MIC, some general patterns of corrosion can be recognised for both. Patterns such as those operative in a buried pipeline may well be similar to those of a submerged steel pipe and its ALWC problem. The stagnant water and the type of problems it produces have the same mechanisms for the involvement of MIC, whether in a firewater pipe or the water-filled legs of an offshore platform... 

Fig. 10.23 The corrosion zones and methods of protection for an offshore platform.

Deep hot gas wells, CO floods, deep water offshore platforms and arctic developments are excellent examples of cases that have provided many material and corrosion problems and are expected to continue to do so. The forms of corrosion of most importance in oil and gas production are ... 

The marine environment is highly aggressive. Materials in marine service are constantly exposed to water, corrosive salts, strong sunlight, extremes in temperature, mechanical abuse, and chemical pollution in ports. This climate is very severe on ships, buoys, and navigational aids, offshore stmctures such as drilling platforms, and faciUties near the shore such as piers, locks, and bridges. 

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