Sequential buffing

The methodology of sequential buffing is detailed in Chapter 9 (section 9.4.2). 3. Other Methods of Depth Determination

There are continual developments in inspection technologies. For example, laser coupled ultrasonic inspection is currently under development and may provide some advantages over the existing liquid coupled ultrasonic technologies due to the reduced probe contact size.

8.1.1.3.5 Measuring SCC within metal loss

SCC that occurs within metal loss is evaluated based on the summation of the depth of the SCC and metal loss.

Example: A corrosion pit penetrates 25% into the pipe wall thickness. The SCC located at the base of the corrosion penetrates an additional 15% of the pipe wall thickness. The measured SCC depth is then determined to be 40% of the pipe wall thickness (see Figure 8.4)

8.1.2 SCC Pressure Testing Data

SCC pressure testing methodologies are described in section 9.3. SCC pressure testing data available for an SCC condition assessment can be summarized as follows:

• The minimum failure pressure for all SCC features within the pipe segment is determined as being greater than the maximum pressure obtained during the test. In this case, the failure pressure is not a calculated value from SCC dimensions, but is a measured value which is not influenced by the conservatism or non-conservatism associated with engineering calculations.

• Conversely, the SCC features that have not failed can be composed of a multiple combination of SCC lengths and depths as well as unknown variables in pipe wall thickness, toughness properties and interacting features and stresses.

• Single SCC features that fail the pressure test can be completely characterized with respect to the failure pressure, the pipe material properties, physical SCC dimensions and SCC interaction in the depth and length directions. The failure must be removed and examined in a laboratory to achieve this.

• SCC that is most likely to fail is typically composed of several coalesced features with little likelihood of further coalescence.

• The quantity, density and location of SCC features that do not fail within a pipe segment remain unknown.

• In the case where no SCC failure occurs, the determination as to whether any SCC features exist within a pipe segment remains unknown.

This data can be extracted from a successful SCC pressure test only. A successful SCC pressure test achieves a pressure of at least the maximum allowed operating pressure times the company defined safety factor without incurring a failure. However, the successful test may occur after a one or more failed tests.

8.1.3 SCC Inline Inspection Data

SCC Inline inspection methodologies and tools are described in Chapter 6. SCC Inline inspection data available for an SCC condition assessment can be summarized as follows:

• The quantity, density and location of the SCC features are known within the pipe segment.

• The maximum SCC feature length and depths are known within the range “bins” provided by the tool.

• Measured minimum failure pressures are known within the data accuracy, although the failure pressure may not have a well defined minimum value for SCC deeper than 40% of wall thickness. These deeper SCC features are often annotated as “> 40%” on the ILI report. However detection and discrimination capability of most ILI tools is greatest for deeper and longer SCC features.

• Calculated individual failure pressures are known to the accuracy provided by the tool measurements (within the defect groupings and ranges within each grouping).

8.1.4 SCC In-Service Failure Data

SCC failure inspection data available for an SCC condition assessment can be summarized as follows:

• Single SCC features that fail can be completely characterized as to the failure pressure, pipe material properties, physical SCC dimensions and interaction.

• A minimum failure pressure for SCC features can be calculated for the remaining pipe segment by assuming that the most severe feature has failed, however this pressure is typically at or below the operating pressure.

• The quantity, density and location of SCC features that do not fail within a pipe segment remain unknown.

8.1.5 Uncertainty in Data

Lifetime predictions involve subtracting the size of the largest possible remaining flaw from the size that would be critical at operating pressure and then dividing that difference by the crack growth rate. Probably the largest source of uncertainty is from the assumed growth rate. However, errors also can be introduced from the calculations of the crack sizes. To be conservative, it is important not to underestimate the size of the largest surviving flaw and not to overestimate the size of a critical flaw at operating pressure.

When using ILI or in-the-ditch measurements, the size of the largest surviving flaw should, in principle, be determined directly, provided that any uncertainty is in the measurements is taken in to consideration. To avoid overestimating the size of the critical flaw at operating pressure, the minimum reasonable toughness should be used in the calculations.

When predicting lifetime from hydrotest results, it is important to use the same input data and calculation method for determining both the size of the surviving flaw and the critical flaw size at operating pressure. In many cases, any errors that are introduced will cancel out when the sizes are subtracted from each other,

but, when uncertainty regarding the input data exists, it would be prudent to use various reasonable values to ensure a conservative result.

Example: To illustrate this, consider a 25 cm long flaw in a 762 mm diameter, 7.93 mm wall thickness, Grade 359 pipe. Using the log-secant method, the effect of toughness on the depths of flaws that would survive a 110% SMYS hydrotest and that would fail at 72% SMYS were calculated and are shown in Figure 1, along with the differences between those two sizes. The predicted lifetime would be proportional to the difference in depth. 0 10 20 30 40 50 60 70 0 20 40 60 80 100 120

2/3-Size Charpy Toughness (Joules)

SC C dept h (% of wa ll thi c knes s) Depth @ 72% SMYS Depth @ 110% SMYS

Growth from 110% SMYS to 72% S S

Figure 8.5: Effect of Toughness on Depth of a 25 cm-Long Flaw That Would Fail at 110% SMYS and 72% SMYS (Based on Log-Secant Method)

Example: If the actual toughness were 15 joules but was assumed to be 7.5 joules, the predicted lifetime would be overly conservative by a factor of approximately 4. If the actual toughness were 30 joules but assumed to be 15 joules, the predicted lifetime would be correct. If the actual toughness were 50 joules but assumed to be 25 joules, the predicted lifetime would be non-conservative by approximately 20 %. If the actual toughness were 100 joules but assumed to be 50 joules, the predicted lifetime would be non-conservative by only approximately 3 %. Thus, it can be seen that it is not possible to predict, in general, whether a conservative value (e.g., low toughness) of input data would produce a conservative or non-conservative prediction of lifetime.

8.2 SCC Condition Assessment Methodologies

SCC condition assessments are performed within the framework of the SCC management plan to provide the following information: