15. Pipeline elbow

Exponent was asked to consult on issues regarding two twin natural gas pipelines that had just been built. It had been found that these pipelines had been buried with sand backfill material that contained iron pyrite. The pH of the backfill was measured to be as low as 1.7. There was concern that such a low pH would damage the pipe coating, affect the cathodic protection requirements, and require more frequent inspections.

A 20-inch diameter crude oil line hole was discovered in an elbow on the line. This location was excavated and the elbow was cut from the line. This elbow was taken to Exponent in Menlo Park, CA in order to determine the effective orifice area to help determine the amount of crude oil that had leaked from the hole in the elbow.

All modern pipelines are coated with an organic protective coating that is supplemented by cathodic protection (CP) systems sized to prevent corrosion at defect or holidays in the protective coating. CP is an electrochemical means of corrosion control in which the oxidation reaction suppressed by imposing a small current into the pipeline. Excessive CP can lead to coating disbondment. Coating disbondment can increase the demand for protective current and may exceed the capabilities of the system. Increasing the current can generate hydrogen at the disbondment and make CP ineffective no matter how high the current. Microbiological induced corrosion (MIC) has also been observed under disbonded coatings in underground piping systems under CP.

Pipe elbow that leaked (side view); end caps were welded to each end of elbow allowed for pressurized leak testing.

This figure shows that the elbow is still attached to two short sections of straight pipe. End caps were welded to both ends of the pipe to allow pressurized leak rate testing. The elbow surface has pits on both the upper and bottom half, and the hole (deepest pit) is located on the bottom at the 6 o’clock position.

Pitting on straight section of pipe adjacent to elbow.

This image illustrates the initial stages of pitting on the straight pipe section, with many small diameter shallow pits that have not yet coalesced together and deepened. Such pit morphology is consistent with MIC occurring under disbonded tape coating. Clustered hemispherical pits are known as one of the characteristics of SRB-related MIC.

Close-up of hole in bottom of elbow (region surround hole has been painted white).

This shows the location and morphology of the through-wall pit that occurred at the 6 o’clock position of the pipe. Again this pit is very broad and a relatively large area of the pipe wall has been thinned. The center portion of this broad pit had obviously failed when the pipe was pressurized.

Overall chemical composition of corrosion product from an area near hole in bottom of elbow.

SEM and EDS were used to examine some of the corrosion product clearly showed the corrosion product contained iron and sulfur (probably as iron sulfide), as well as some traces of chlorine and oxygen and other elements that were consistent with soil or the steel itself.

In summary, it appeared that this leak developed in the elbow as a result of anaerobic MIC. This corrosion developed underneath the disbonded pipe coating. Such anaerobic MIC has been found to account for 27% of all the corrosion product identified on the approximately 12,000 miles of gas transmission lines. The corrosion product from the elbow contained iron and sulfur. The pit morphology, corrosion product composition high in sulfur, and pit location in a field-coated pipe elbow are consistent with MIC. MIC pitting corrosion rates were reported to be high and ranged from 0.028 to 0.252 inch/year. Furthermore it is not always possible to stop such attack with normal cathodic protection practices. 

Reference: "Cathodic Protection of Gas Pipeline in Acidic Backfill", http://www.exponent.com/cathodic_protection_of_gas_pipeline_in_acidic_backfill1/, February 2015