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Corrosion Pathways and Milestones (Coating Breakdown (Main Pipe Coating…
Corrosion Pathways and Milestones
External Corrosion
Concretion and Marine Growth
Buildup of concretion and marine growth will form a protective layer over the pipeline, particularity exposed steel
Pitting
The speed of wall penetration due to pitting of the line will be largely unaffected by the integrity of the coating. The density of pits will be higher in areas where coating breakdown is higher due to the increased number of potential locations. The mass of steel affected by pitting is typically very low for the isolated nature of the corrosion.
General Mass Loss
General corrosion of the pipeline due to the combined affects of oxygen, hydrogen evolution and microbial catalysation of hydrogen evolution will reduce the overall mass of the pipeline.
Corrosion at Field Joints
As the corrosion coating of the field joints is predicted to degrade at significantly higher rates than the general pipeline coating
MILESTONE TWO
Pipe Section Losing 50% of Original Steel Mass
At this point set by this assessment where the pipeline is assumed to have weakened due to the loss of steel mass that fatigue and extreme environmental loading will cause the fracture of the pipe at the corroded section. Due to the elevated coating breakdown of the Field Joints, these are the failure points predicted.
Corrosion of Main Pipe
As the Asphalt Enamel of the main body of the pipeline is expected to degrade at significantly lower rates than the field joints, the mass loss due to corrosion per unit length is significantly lower
MILESTONE THREE
Entire Pipeline Losing 80% of Original Steel Mass
This the point set by this assessment where total structural collapse of the pipeline during a major environmental loading event is likely to occur. After this point the concrete shell remaining is likely to break up, dispersing the corrosion products and coating products.
MILESTONE ONE
Failure of CP System
For the purpose of this assessment it is assumed that no external corrosion occurs prior to the consumption of the CP system in the local area, i.e. the minimum time to CP failure calculated. :
Internal Corrosion
Consumption of Oxygen
Following flooding of the pipeline oxygenated water will cause corrosion. This oxygen will be rapidly consumed for the majority of the line as there are minimal forces driving water exchange. This is also likely to forming a thin concretion layer.
Consequence of External Pitting Corrosion
Following perforation of the pipeline due to pitting corrosion, small volumes of oxygenated seawater will enter the pipeline. This will cause localised areas of increased corrosion but is not expected to cause significant mass loss of the steel. Perforations caused by pitting are likely to be rapidly blocked with concretion and marine growth limiting water exchange to the pipe cavity further reducing the overall corrosion loss.
Oxygenated Seawater Free Enters Pipeline
Following fracture of the pipeline, whether large through thickness cracking or full separation, oxygenated seawater will enter the pipeline relatively unimpeded. Following this the full general corrosion rates are applied to the pipeline, accounting for any remaining protection provided by the internal coating
Seawater Exchange Due to Annual Temperature Fluctuations
Due to the expansion and contraction of the water contained within the pipeline, a volume of water will be exchanged between the internal volume and the general seawater. this will cause additional corrosion at the extreme ends of the pipeline. Due to the low concentration of oxygen in seawater, the mass of steel able to be corroded is low due the small mass of dissolved oxygen entering the pipeline.
Coating Breakdown
Main Pipe Coating Breakdown
Field Joint Coating Breakdown
Internal Coating Breakdown
Coating Breakdown
For this assessment it is assumed that the surface area of the pipeline subject to general corrosion is dependent on the coating integrity. This is supported by research on recent warship wrecks.