Coast to Coast Inspection Services ("Coast") and Kiefner and Associates ("KAI") have jointly developed a software package to facilitate measuring and documenting the interacting length of a colony of stress corrosion cracks. The program default uses the NACE RP02014-2004 interaction rules. However, other flaw combination rules such as API 579 may be used, or the operator can use the software to iteratively identify interaction rules that might better correlate with lengths reported by ILI crack tools.

NACE RP0204-2004 Standard Recommended Practice for Stress Corrosion Cracking (SCC) Direct Assessment Methodology defines interacting cracks as those for which the axial spacing X and the circumferential spacing Y, as shown in Figure 1, are as follows:

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Circumferential spacing of axial crack indications:

Y ? (0.14) (I1 + I2)/2

Axial Spacing of axial crack indications:

X ? (0.25) (I1 + I2)/2

Where: I1 and I2 are the individual crack lengths.


The "interacting length" of an SCC colony is the total axial length of the grouping of interacting cracks and is treated as a single crack for the purpose of calculating the NACE RP0204-2004 "significant" size. The same methodology is often used for the length component in burst pressure calculations for larger colonies.

While this definition of interacting crack length is simple, in practice it is difficult to determine the total interacting length of an SCC colony in the field, particularly for colonies near the NACE "significant" size. It is even more difficult to record the result such that it can be checked by another person or party. The KAI-Coast Crack Interacting Length Software readily facilitates both by importing a photograph of the SCC colony into a software program. The photograph must include a physical scale in either U.S. Standard or metric units. The interacting cracks are identified automatically by using the mouse to click on the end-points of individual cracks and on any point between the endpoints where the crack alignment changes. The scale is set by initially clicking on two points on the physical scale included in the photograph.

The methodology employed is mathematically identical to the NACE RP0102-2004 definition provided above. It is graphically represented by the software drawing a "box" around each individual crack that is 14% wider and 25% longer than the individual crack length. The box is automatically bent to follow alignment changes along the crack. Individual cracks are considered to interact whenever their respective boxes overlap. The overlapping boxes, indicating interacting cracks, are readily identified by the overlapping boxes turning color from yellow to white. A double headed arrow can then be drawn between the outermost crack tips, and both the axial and circumferential lengths of the interacting crack are automatically displayed in the units selected. The circumferential length is included to address the relatively rare case of cracks orientated near 45° that indicate a significant component of circumferential SCC due to the presence of high axial stresses.

The software has a screen capture "save" feature to provide a permanent record of how the interacting length was determined.

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