Offshore Pipeline Corrosion Prevention
Monday, February 15, 2016
Infrastructures such as steel pipelines are susceptible to corrosion. Offshore steel pipelines are normally designed for a life ranging from 10 years to 40 years. To enable the pipeline to last for the design life, the pipeline needs to be protected from corrosion both internally and externally. Corrosion is a deterioration of a material due to reaction with its environment (oxidation or chemical reaction). It is a natural tendency of a refined material (steel) to return to its original state (iron ore).
External Pipe Coatings
This first external pipe coating
layer is used to protect the pipe against corrosion. A singlelayer coating is
used when the installed pipeline is always in a static, laterally stable condition
lying on soils such as clay or sand. Additional layers of coating are used for additional
protection, for weight to help the pipeline remain laterally stable on the
seabed, or for providing insulation. A multi-layer coating is generally used in
cases where the external environment tends to easily wear out the external coating
(e.g., pipeline lying on top of rocky soil, calcareous material, etc.).
Insulation is provided to maintain a higher temperature of the flowing internal
fluid compared to the ambient.
The properties that are considered
desirable for deepwater pipeline coatings are:
- Resistance to seawater absorption
- Resistance to chemicals in seawater
- Resistance to cathodic disbondment
- Adhesion to the pipe surface
- Flexibility
- Impact and abrasion resistance
- Resistance to weathering
- Compatibility with cathodic protection
Single-Layer Coating
The most common choice for
single-layer coating for deepwater pipelines is Fusion Bonded Epoxy (FBE). For
deepwater pipelines where there is no other requirement on the external
coating, FBE is most frequently used. The FBE field joint coating is carried
out using the same coating material as millapplied coating. Further advantages
include:
- Easy to repair
- Easy for coating application
- High adhesion to steel
- Good for pipeline operating temperatures
Multi-Layer Coating
Dual-layer FBE coatings are used
when additional protection is required for the outer layer such as high
temperature, abrasion resistance, etc.
Three-layer PP coating consists
of an epoxy or FBE, a thermoplastic adhesive coating and a polypropylene top coat.
These coatings are used for additional protection against corrosion and are
commonly used for dynamic systems like steel catenary risers and where the
temperature of the internal fluid is high.
Concrete weight coating is used when
stability of the pipeline on the seabed is an issue.
Cathodic Protection
Corrosion can be reduced or
prevented by providing a direct current through the electrolyte to the structure.
This method is called cathodic protection (CP).
There are four components in CP
system as follows:
- Anode (lower electrical potential) – the point that corrosion occurs (oxidation or production of electrons)
- Cathod (higher electrical potential) – the point that consumption of electrons occurs
- Electrolyte – electrically conductive fluid (water or air)
- Return Circuit (metallic path) – electrons move from anode to cathode
In order to conduct a CP design
for a deepwater pipeline, the parameters that need to be known are:
- Service/design life (years)
- Coating breakdown (%)
- Current density for protection (mA/sq.m) buried or unburied
- Seawater resistivity (ohm-cm)
- Soil resistivity (ohm-cm)
- Pipeline protective potential (normally, 900 mV w.r.t Ag/AgCl)
- Anode output (amp-hr/kg)
- Anode potential (mV w.r.t. Ag/AgCl)
- Anode utilization factor (%)
- Seawater temperature
- Pipeline temperature
- Depth of pipeline sinkage/burial
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Source:
Lee, Jaeyoung. Introduction to Offshore Pipelines and Risers. 2007.
Guo, Boyun, Shanhong Song, Jacob Chacko, Ali Ghalambor. USA. 2005.
Bai, Yong and Bai, Qiang. Subsea Pipelines And Risers. USA: Elsevier Inc. 2005.
Lee, Jaeyoung. Introduction to Offshore Pipelines and Risers. 2007.
Guo, Boyun, Shanhong Song, Jacob Chacko, Ali Ghalambor. USA. 2005.
Bai, Yong and Bai, Qiang. Subsea Pipelines And Risers. USA: Elsevier Inc. 2005.
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