Offshore Pipeline Corrosion Prevention

Corrosion of offshore oil and gas pipelines is a critical problem that can lead to catastrophic failure if not properly managed—from the design stage of the pipeline system through ongoing monitoring and maintenance. Pipeline corrosion is a natural deterioration and destruction of pipe material and essential properties due to electrochemical and other ingredient reactions of pipeline materials with their environment - on the inside as well as outside surfaces. Like any other naturally occurring hazard, a pipeline corrosion can result in a life threatening failure and expensive damage to the pipeline and related system.

Corrosion understanding and mitigation methods have greatly improved over time, but significant challenges remain in the difficult subsea environment, where assets can be located up to a mile deep or more.

1. Methods to Prevent or Mitigate External Corrosion

Pipeline external corrosion

-  Coatings
Since the 1950s, corrosion coatings have continuously improved. During the period of 1950 to 1960,
common coatings included coal tar-based coatings, asphalt coatings, early grease coatings, early cold and hot-applied tapes, and the first coal tar enamel coatings. All of these coatings were designed to
isolate the pipe from its environment and to prevent water from reaching the pipe surface.

In the 1960s, newer and more long-lasting coatings were produced and applied, and the use of certain
kinds of grease wraps was discontinued during this time. Also, the adhesive capability of coal tar products was improved. In the 1970s and 1980s, the early epoxy products and new cold-applied tapes were introduced. New wax or grease products were introduced to accommodate irregularly shaped features such as valves and fittings.

In the 1990s, better and more effective dry powder and wet-applied epoxies appeared and were
widely utilized. These coatings possessed good dielectric properties and degraded relatively slowly
over time from environmental exposure. However, they lacked mechanical strength. Rough handling
would cause coating voids; moisture would subsequently penetrate the voids, forming blisters. Fusion-bonded epoxy (FBE) coating, sometimes called thin-film epoxy, is an epoxy-based powder
coating that is currently widely used to protect pipelines, including valves. FBE coatings are thermoset polymer coatings. The name is derived from resin cross-linking and method of application,
which is different from that of a conventional liquid paint. FBE coatings are made from dry powder. The resin and hardener parts of the dry powder remain unreacted at normal storage conditions. At
typical coating application temperatures, usually in the range of 180° C to 250 °C (350° F to 480 °F),
the contents of the powder melt and transform to a liquid.

Fusion Bonded Epoxy (FBE) powder application schematic
Source: Peabody’s Control of Pipeline Corrosion, 2nd Edition, p. 18

Offshore pipelines also incorporate concrete coatings to offset buoyancy as well as to prevent fatigue damage and flow-induced vibrations due to wave or current forces by providing stability across irregular seafloor topography or where there is scouring or local differences in load bearing capacities of the soil. The use of thermal sprayed metallic coatings (Aluminum and Zinc) has been gaining popularity in submerged offshore applications, including some pipelines. This coating offers the benefits of low cost with excellent corrosion protection over a wide range of temperature and conditions. While thermal sprayed coatings do provide a measure of cathodic protection, they still require supplemental anodes but the quantity can be significantly reduced. These coatings are still under development for broad-based pipeline applications but expect to see continued progress.

Concrete coating

 - Cathodic protection
There are two types of cathodic protection systems: sacrificial anode and impressed current anode.
Sacrificial anode systems utilize an externally connected sacrificial metal with a relative activity value greater than steel (iron) and thereby protect steel from corrosion. Alloys of zinc and magnesium
are the sacrificial metals most commonly employed. The sacrificial anode is connected to the pipeline
via a wire and placed some distance from the pipeline. The current flows from the anode into the
surrounding soil (electrolyte) and is picked up by the pipeline at coating holidays. The circuit is completed by a wire that connects the anode to the pipe. The number and placement of anodes is
based on the site-specific requirements of the particular pipeline that is to be protected. A well-coated
pipeline with a few small holidays does not require many anodes.

Basic concept of cathodic protection
Source: Steel Structures Painting Council

Impressed-current anode systemsinvolve the application of direct-current voltage between an anode and the pipeline. Impressed-current anodes can be made from graphite, high-silicon cast iron, lead-silver alloys, precious metals, mixed-metal oxides, or steel. As with sacrificial anodes, the shapes, locations, and number depend on the geology of the area and the nature of the pipeline system.

Impressed-current cathodic protection of a buried pipeline

2. Methods to Prevent or Mitigate Internal Corrosion

Dehydration is the most commonly applied measure to protect against internal corrosion in gas
pipelines (and also in liquid pipelines that contain oil with free water or other electrolytes). Dehydration removes condensation and free water that, if permitted to remain, would allow internal
corrosion to occur at points where water droplets precipitate from the gas stream to either form liquid
puddles at the bottom of the pipe, or adhere to the top of the pipe. Where the gas stream is usually
dry, topside corrosion rarely takes place. Complete dehydration is very effective, but, because the
systems are neither 100 percent effective nor 100 percent dependable, there always is the potential to
introduce water and other electrolytes into a gas pipeline.

- Inhibitors
Inhibitors are chemicals that can be added to a pipeline to reduce the rate of corrosion. They can
adsorb onto the metal surface or react with it to form a protective film, or they may react with the corrodent to make it less corrosive. Many different chemicals are available commercially. The choice
will depend on the type of product in the pipeline and the type of corrodent. Other considerations
include cost, availability, toxicity, and environmental friendliness.

Corrosion inhibitor injection

- Coatings
Internal coatings have been used on some gas transmission pipelines to improve product flow by
reducing drag and eliminating dust. Such coatings can be somewhat effective in controlling internal
corrosion, but they are very difficult to apply uniformly, which impacts their effectiveness. In lieu of
coatings, some operators have attempted to install plastic or high-density polyethylene liners or
inserts in their pipelines. Plastic liners are an effective barrier against corrosion but are not fail-safe.
Problems occur if pinholes are present and allow corrosive materials to migrate behind a pipe’s liner.
Relying solely on liners or coatings may not be prudent since it is very unlikely that the problems
associated with each can be remedied. Many operators who do use liners or coatings also apply
additional preventive measures.

- Buffering
In principle, buffering agents that change the chemical composition of fluids that remain in the
pipeline can be utilized to prevent internal corrosio. The introduction of a
buffering agent, such as a mild or dilute alkaline mixture, can significantly reduce the corrosivity of
any standing liquid, predominantly by raising its pH value above seven (neutral), so that it turns from
acidic to alkaline. Alkaline liquids cause virtually no harm to steel. In general, buffering is not very
effective because it is difficult to cover the entire pipe surface.

- Cleaning Pigs
There are many types of cleaning pigs. The choice of which type of pig to use depends on the product carried by the pipeline and the contaminant to be removed. Although their application may preclude the use of internal corrosion direct assessment (ICDA) models, cleaning pigs can effectively direct both liquids and corrosive solids to pig traps for removal from the pipeline.

Cleaning pig

Routine pigging will channel any liquid pools away from low points and, if performed properly, out
of the entire pipeline. Cleaning pigs also will displace the solids and remove them from the pipeline
via the pig trap at the end of the pipeline, provided that the pipeline is properly configured (i.e. no
dead legs or other features that would trap liquids or solids and prevent cleaning pig access). Pigging
effectiveness is a function of the pigging velocity, pigging distance, and characteristics of the
materials targeted by the pig run.



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