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External Corrosion Control of Offshore Pipelines

All offshore pipelines are protected from seawater corrosion in the same way. The primary corrosion control system is pipeline coating. This is supplemented with cathodic protection (CP) to provide protection at coating defects or "holidays." In the Gulf of Mexico, the pipeline coatings used until the early to mid-1970s were either asphaltic/ aggregate, "Somastic"-type, coatings or hot-applied coal tar enamels. Since then, the trend has been to use fusion-bonded epoxy powder coatings. In the earlier days, the trend in cathodic protection (CP) was to rely on impressed-current systems. In the 1960s and early 1970s, zinc bracelet anodes attached to the pipe were widely used. Since then, more efficient aluminum alloys have surpassed zinc as the preferred material for offshore galvanic anodes. There are, however, still some operators using impressed current systems and some using zinc anodes.

Bracelet Anodes

Virtually all new pipelines installed in the Gulf of Mexico are equipped with aluminum bracelet anodes. There are two basic types, square shouldered and tapered.
The square-shouldered anodes are typically used on pipe that has a concrete weight coating. When installed, the anodes are flush with, or slightly recessed inside, the outside diameter of the concrete.
The tapered anodes are designed to be installed on pipelines with only a thin film corrosion coating. The whole idea is to protect the bracelet anodes during the pipe-laying process. The anodes are particularly at risk from mechanical damage when the pipeline travels over the stinger on the back of the lay barge.
Even with these tapered designs, non-weight-coated pipelines still sustain anode damage, which can in turn cause coating damage. Several methods are being used to combat this problem. The use of cast-on polyurethane tapers is gaining popularity, and mounting both halves of the bracelet on top of the pipe is a common technique when pipe is laid from a reel barge and the anodes have to be attached offshore (Figures 1 and 2).

Figure 1.

Six-inch pipe reeled on the barge Chickasaw

Figure 2.

Tapered bracelet anodes installed on top of pipe

Designing CP Systems for Offshore Pipelines

When designing a cathodic protection system for a pipeline, the corrosion engineer has to consider the following variables, all of which will have an impact on the final anode alloy and size selection:

• Design life required - (minimum is 20 years)
• Pipe diameter length and to-from information
• Geographic location
• Type of coating
• Pipe-lay / installation method
• Water depth
• Burial method
• Product temperature
• Electrical isolation from platforms or other pipelines

The smart cathodic protection designer will look early on at the intended pipe installation method, as this will have a direct impact on the amount of coating damage one may expect (there is also a risk of having anodes detached during the lay process). In all pipeline design guidelines, the conservative approach is advised. For example, the majority of early Gulf of Mexico (buried) pipelines were designed on the basis of 2 mA / ft. of bare steel and 5% coating failure. In essence, this means taking 5% of the total pipeline surface area, and applying 2 rnA / ft. of cathodic protection current to it. This may sound reasonable, until one looks at what 5% bare means:
On a 40 ft. joint of 12 in. pipe, 5% bare coating would have 2 square feet of bare steel, or to express it another way. 4 linear feet of pipe would have the coating gone from 180° of the circumference. This is an extremely conservative figure. As a result, the early pipeline system designs would appear to be very conservative.


Source:  http://stoprust.com/technical-papers/26-offshore-pipeline-integrity/