MPI Documentation

BWMPI produces a more permanent visualization of SCC and is therefore facilitates easier documentation of SCC colonies than does WFMPI. Documentation using WFMPI requires considerably more effort. Once the SCC colonies have been identified, the colony characteristics should be measured and documented in a consistent format, as outlined in section 7.3.7 and Table 7.10. A photographic record of the detected SCC colonies should be considered.

8.1.1.3 SCC Sizing

Sizing SCC requires the measurement of the SCC length, depth and density. As well, intrinsic to sizing SCC, is a requirement to carefully measure the wall thickness of the pipe at the location of the SCC feature.

Pipe wall thickness, SCC length and SCC density measurements are always quantitatively determined. Conversely, SCC depth can be measured with both qualitative and quantitative methods. Qualitative methods provide an indirect estimate of SCC dimensions. Quantitative methods provide a direct measurement of SCC dimensions. At excavations where large colonies of SCC are found, typically a mix of both qualitative and quantitative methods are required to fully document large areas of SCC in an efficient manner.

8.1.1.3.1 Pipe Wall Thickness Measurement

To obtain an accurate wall thickness of the pipeline being investigated, a calibrated ultrasonic thickness gauge (i.e. D-Meter) typically with a 0.25” or 0.50” diameter compression wave (90°) probe should be used. Calibration utilizes a standard step wedge block. A suitable couplant is used to enable the sound energy to transfer into the pipe wall without scatter or interference, thereby, improving the accuracy of the wall thickness reading.

8.1.1.3.2 SCC Length and Density Measurements

SCC length is measured as the total longitudinal length of the SCC feature before buffing occurs. The measurement is typically made using a calliper tool or ruler with precision down to 1 mm.

There are two aspects to measuring SCC density. The first aspect is to measure the endpoints of an SCC feature in relation to adjacent SCC features in both the

circumferential and axial direction. These measurements are required to assess SCC interaction (section 8.2.1.1.)

The second aspect of measuring SCC density is the assignment of SCC features to one of four possible groups as follows:

1. Toe of the weld SCC. 2. Isolated SCC

3. SCC in sparse colonies 4. SCC in dense colonies.

Toe of the weld SCC refers to SCC located within the heat affected zone of the longseam weld. Isolated SCC refers to individual features that do not fall into either the sparse or dense colony definitions. Sparse and dense colonies are defined as adjacent features with circumferential spacing as described in Table 8.4. [12]

Table 8.4: Definition of SCC Density

SCC Density Approximate Circumferential Spacing Dense < 0.2 wall thickness

Sparse > 0.2 wall thickness

The importance of defining SCC colonies as sparse or dense is related to differences in the incidence of SCC leaks and ruptures as follows [12]:

• 'Sparse' SCC colonies were found at and near the position of failure, and

• 'Dense' SCC colonies were found remote from the failure.

The rational supporting these failure correlations suggest that SCC spaced in close circumferential proximity shield each other from the hoop stress and tend toward dormancy with a final depth of less than 10% of the wall thickness. Conversely, sparsely spaced SCC features have a lesser amount of stress shielding and as a result have an increased potential for continuous growth both by penetration and coalescence. Failure data support that these non stress shielded SCC features are usually responsible for the failure. As such SCC features that are isolated, located at the toe of the weld or are located within sparse colonies should be targeted for detailed sizing when providing an input data source in the calculation of SCC severity.

8.1.1.3.3 Qualitative SCC Depth Sizing

Qualitative SCC depth determination is typically used to document large areas of shallow SCC (<10% of wt) where sizing by buffing removal may not be practical due to time constraints or the proximity of one SCC feature to another.

Qualitative SCC depth determination utilizes the intensity and length of the SCC indication, as determined by MPI, to estimate depth. The short, less intense indications observed in Figure 8.3 typically have a depth less than 10% of wall thickness, while the longer, darker indications have a depth greater than 10% of wall thickness. Observations of intensity should be combined with conservative aspect ratio rules. The minimum aspect ratio that will allow for an estimation of SCC less than 10% should be determined for the pipe segment by sequentially buffing short SCC features until confidence is obtained in the aspect ratio chosen. This value may differ between pipelines with large differences in wall thickness.

Example: Based on a depth and length assessment of short SCC for a pipe segment with a 10 mm wall thickness, an aspect ratio of 8:1 was found to the minimum aspect ratio for SCC less than 10% of the wall thickness. Therefore, SCC with lengths less then 8 mm was estimated to be less than 10% of the wall thickness.

However, there is no limit on the maximum aspect ratio. Maximum aspect ratio’s for SCC found within the base or “toe” of a longseam weld can vary widely and inconsistently, even within the same pipe joint.

Figure 8.3: In-the ditch SCC assessment using BWMPI technique

In qualitatively determining SCC depths, the following points should be considered:

• For shallow SCC in the pipe body (<10% of actual wall thickness), the longest SCC indication typically represents the deepest SCC anomaly.

• For each pipe joint a representative number of the longest SCC indications should be buffed out to verify the specific depth estimation process.

• Where qualitative assessment estimates SCC depth as >10%, the operator should confirm this depth using a quantitative assessment.

• Qualitative depth determination should not be applied to those SCC features located at the toe of the longitudinal weld seam

Qualitative SCC depth determination is technician dependent and pipeline specific, as such, should only be undertaken by experienced technicians that have developed an ability to estimate SCC depths by continually validating their estimations using more accurate methods (i.e. sequential buffing, ultrasonic measurement techniques, etc.)

8.1.1.3.4 Quantitative SCC Depth Measuring

Quantitative SCC depth determination is used to document the depths of isolated SCC, toe of the weld SCC as well as the deepest SCC feature within a colony of SCC features. Although ultrasonic crack sizing is the most common and most mature SCC depth measuring technique, new techniques such as eddy current are being developed and may be commercially available in the future.