Stress Corrosion Cracking (SCC) Direct Assessment Methodology
|Publication Date:||14 March 2015|
This standard covers the NACE SCCDA process for buried steel pipeline systems. It is intended to serve as a guide for applying the NACE SCCDA process on typical petroleum (natural gas, crude oil, and refined products) pipeline systems. Background information may be obtained from NACE Publication 35103.3
SCCDA as described in this standard is specifically intended to address buried onshore petroleum (natural gas, crude oil, and refined products) pipelines constructed from line pipe steel.
This procedure is designed to be applied to both forms of external SCC (near-neutral-pH SCC and high-pH SCC) on these pipelines.
SCCDA requires the integration of data from historical records, indirect surveys, field examinations, and pipe surface evaluations (i.e., direct examination) combined with the physical characteristics and operating history of the pipeline.
This standard was written as a flexible guideline for an operator to tailor the SCCDA process to specific pipeline situations. Nothing in this standard is intended to preclude modifications that tailor the SCCDA process to specific pipeline situations and operators.
SCCDA is a continuous improvement process. Through successive applications, SCCDA should identify and address locations where SCC has occurred, is occurring, or might occur.
SCCDA provides the advantage and benefit of indicating areas where SCC might occur in the future rather than only areas where SCC is known to exist.
Comparing the results of successive SCCDA applications is one method of evaluating SCCDA effectiveness and demonstrating that confidence in the integrity of the pipeline is continuously improving.
SCCDA was developed as a process for improving pipeline safety. Its primary purpose is to reduce the threat of external SCC on pipeline integrity by means of condition monitoring, mitigation, documentation, and reporting.
This standard assumes SCC is a threat to be evaluated. It can be used to establish a baseline from which future SCC can be assessed for pipelines on which SCC is not currently a significant threat.
SCCDA is complementary to other inspection methods such as in-line inspection (ILI) or hydrostatic testing, and is not necessarily an alternative or replacement for these methods in all instances. SCCDA is also complementary to other direct assessment procedures such as those given in NACE SP0206.4
ILI or hydrostatic testing might not be warranted based on the initial SCCDA assessment of the pipeline system.
SCCDA can be used to prioritize a pipeline system for ILI or hydrostatic testing if SCC is found that is sufficient to warrant general mitigation as defined by Paragraph 6.2.3.
SCCDA may detect other pipeline integrity threats, such as mechanical damage, external corrosion, etc. When such threats are detected, additional assessments or inspections shall be performed. The pipeline operator shall use appropriate methods such as ASME B31.8S,1 ASME B31.4,5 ASME B31.8,6 API(3) 1160,7 NACE standards, international standards, and other documents to address risks other than external SCC.
SCCDA can be applied to most onshore petroleum pipelines, regardless of the coating system. Precautions should be taken when applying these techniques just as with other assessment methods.
Given the diversity of pipelines and their operation, this standard recognizes that SCCDA may be inappropriate for some situations because of the complexity of conditions to which buried pipeline systems are exposed.
The provisions of this standard shall be applied under the direction of competent persons who, by reason of knowledge of the physical sciences and the principles of engineering, geosciences, and mathematics, acquired by education and related practical experience, are qualified to engage in the practice of corrosion control, integrity management, and risk assessment on buried steel pipeline systems.
Relationship between SCCDA Process and SCC Integrity Management
Initial selection of segments of gas pipelines for assessment of risk of SCC should be based on Part A3 of ASME B31.8S,1 Section A3.3. Part A3 considers the following factors: operating stress, age of pipeline, and coating type.(4) It is recognized that these screening factors can identify a substantial percentage of the susceptible locations, but not necessarily all of them.
Part A3.3.1 of ASME B31.8S (SCC Threat Criteria) specifies that a gas pipeline segment is considered susceptible to SCC if all of the following susceptibility criteria are met:
The operating stress exceeds 60% of specified minimum yield strength (SMYS).
The age of the pipeline is greater than 10 years.
The coating type is other than plant-applied or field-applied fusion-bonded epoxy (FBE) or liquid epoxy (when abrasive surface preparation was used during field coating application).
High pH SCC: Part A3.3.2 of ASME B31.8S provides guidance for high-pH SCC. A gas pipeline segment should be evaluated for high-pH SCC if all of the susceptibility criteria listed in Paragraph 220.127.116.11 are met and the following additional susceptibility criteria also are met:
The operating temperature exceeds 38 °C (100 °F).
The segment is less than 32 km (20 mi) downstream from a compressor station.
ASME B31.8S addresses gas pipelines, but the same approach can be used for liquid petroleum pipelines. When applying guidance in B31.8S to liquids pipelines, the characteristics of the pipeline segment must be considered in order to establish appropriate assessment intervals and mitigation activities.
Part A3 of ASME B31.8S considers service incidents and hydrostatic test breaks or leaks caused by SCC. If the susceptibility criteria in Paragraph 18.104.22.168 are met or the segment has a previous SCC history, then the pipeline segment is considered to be at risk for the occurrence of SCC.
Primary guidance for managing the integrity of a natural gas pipeline that has a risk of containing stress corrosion cracks is provided in Part A3 of ASME B31.8S,1 which identifies several options for assessment and mitigation. Additional guidance for management of the integrity of natural gas and liquid petroleum pipelines subject to near-neutral-pH SCC is provided in the CEPA Stress Corrosion Cracking Recommended Practices.2 When applying guidance found in these documents to liquids pipelines, the potential for fatigue and/or corrosion fatigue must be considered in order to establish appropriate assessment intervals and mitigation activities.
This standard provides guidance for Part 3.4 of ASME B31.8S on prioritization of potentially susceptible segments, dig site selection within the potentially susceptible segments, dig site verification, inspection of the pipe at a dig site, data collection at the dig site, and subsequent data analysis.
(3) American Petroleum Institute (API), 1220 L St. NW, Washington, DC 20005.
(4) These selection criteria for SCC-susceptible segments are based on the criteria in the most recent edition of ASME B31.8 in effect at the time of publication of this edition of SP0204. If the criteria are revised in future revisions of ASME B31.8, the criteria used in this standard shall remain unchanged.