External Conditions Needed to Develop Low pH SCC.

1. Stress levels in the pipe generally greater than 45% SMYS, and slow cycling of that stress.
2. A coating fault exposing steel to the soil.
3. Coating typically disbonded at fault location.
4. Low pH in surrounding soil at coating fault.
5. No apparent correlation with pipe temperature.
6. Variable correlation with pipe polarised potential. Generally the CP has to be weak .
7. Crack Morphology - Transgranular.

Techniques Available to Find SCC.

The methods employed to locate SCC are Hydrostatic Testing, Acoustic Emission Testing and examination of Operating and Environmental Parameters followed by Excavation and Magnetic Particle Inspection. There are also SCC In-line Inspection Tools but they detect what we consider to be relatively large cracks but can have their interpretation improved by combining inspection data with data from the Operating and Environmental technology recommended here.

Hydrostatic Testing.
Hydrostatic testing is a useful technique for locating metal loss areas or SCC at or near to the point of failure. Unfortunately the pipeline has to be taken out of service, also finding and repairing leaks and re-testing is expensive. In addition there is no guarantee that cracks below the critical crack length will not fail during testing but will continue to grow and rupture shortly after re-commissioning.

Acoustic Emission.
This is a technique more suited to liquid rather than gas pipelines. It has been found that in gas pipelines, because of rapid attenuation there is little difference in signal energy between different defect types, e.g. pitting, laminations, gouges, SCC and background noise. Combining with coating surveys to locate sensors at defects sites improves the accuracy of the technique.

Operating and Environmental Parameters. (PIM Methodology)
PIM's unique approach is to integrate metallurgical, operational and environmental parameters to quantify the possibility of and locate SCC. Studies of the stress level and its relation to pressure and temperature fluctuations plus various parameters of the steel pipe allow a number of factors to be calculated. For example, the %SMYS, the critical crack length, the SCC threshold stress for the onset of SCC, the temperature and stress profile of the pipeline. From samples it is possible to determine factors such as crack velocity and prediction of time to failure. Having determined the risk of SCC failure, the most likely locations are then determined from combined survey data sets.
Studies of the pipe coating system, the level of cathodic protection at individual coating faults, the corrosivity factor which is related to the formation of protective magnetite films on the steel surface, soil resistivity, soil conditions and composition and the pipeline history are some of many additional inputs to a computer program used to select SCC locations. These locations are then excavated and inspected using Magnetic Particle Techniques to identify SCC.