Suitability Evaluation of Seamless Steel Pipes After Long-Term Storage

Literature Overview

The paper by Yang Hongli, Deng Li, Ji Bingyin, Xu Aiyuan, and Shi Jiaoqi, published in Steel Pipe (Vol. 50, No. 5, 2021, pp. 68-71), presents a comprehensive suitability evaluation of a batch of seamless steel pipes that had been stored for an extended period. The research was conducted jointly by Xi'an 3D Stress Engineering Technology Co., Ltd. and the Material Supply Management Center of Sinopec Northwest Oilfield Branch. The study addresses a practically significant problem in the oil and gas industry: the assessment of whether long-stored seamless steel pipes can still be safely used for pipeline construction, considering the potential effects of corrosion and property degradation.

Testing Methodology and Results

The evaluation followed a systematic testing protocol covering geometric dimensions, physicochemical properties, full-size hydrostatic testing, and burst testing, all conducted in accordance with GB/T 9711-2011 (Pipelines for the transportation of petroleum and natural gas products). The results indicated that all tested parameters met the requirements of this standard.

The internal wall corrosion analysis revealed that the brown corrosion products were predominantly iron oxide (Fe2O3). Due to the extended storage duration, a layer of loose rust had formed on the internal surface, indicating slight uniform corrosion. The corrosion was classified as mild uniform corrosion rather than localized pitting or stress corrosion cracking, which is a favorable finding from a structural integrity perspective.

Technical Analysis of Corrosion Mechanisms

The formation of Fe2O3 as the primary corrosion product is consistent with the oxidation of iron under aerobic conditions with limited moisture. The presence of loose rust indicates that the corrosion process has progressed to a stage where the oxide layer is not fully adherent to the pipe surface. This loose layer can potentially lead to under-deposit corrosion if the pipe is placed into service without proper cleaning, as the loose rust can trap moisture and create localized corrosive environments.

From a metallurgical perspective, the fact that the physicochemical properties still conform to GB/T 9711-2011 requirements is significant. This suggests that the corrosion depth has not reached a level that would compromise the mechanical integrity of the pipe wall. For seamless steel pipes used in oil and gas applications, the pipe grade (typically API 5L grades such as L245, L290, L360, or L415) determines the minimum yield strength, tensile strength, and other mechanical properties. The conformity of these properties after long-term storage indicates that the corrosion has been superficial.

However, the distinction between uniform corrosion and localized corrosion is critical for fitness-for-service assessment. Uniform corrosion, while reducing wall thickness, does so predictably and can be compensated for through increased corrosion allowance in design. Localized corrosion, such as pitting or stress corrosion cracking, is far more dangerous because it can initiate cracks and lead to sudden failure. The finding of mild uniform corrosion is therefore a positive indicator for continued serviceability.

Fitness-for-Service Assessment Considerations

The evaluation of long-stored steel pipes requires consideration of several factors beyond simple conformance testing. The following criteria are relevant for determining whether long-stored pipes are suitable for continued use:

1. Wall thickness reduction: The depth of corrosion must be quantified and compared against the original wall thickness and the corrosion allowance specified in the design. If the remaining wall thickness exceeds the minimum required thickness plus corrosion allowance, the pipe is structurally adequate.
2. Surface condition: The presence of loose rust indicates that internal cleaning is necessary before installation. Methods such as mechanical scraping, chemical cleaning, or high-pressure water jetting should be employed to remove the loose oxide layer.
3. Coating condition: If the pipes were originally coated for storage protection, the condition of the coating must be assessed. Damage to the external coating during storage can expose the pipe surface to atmospheric corrosion.
4. Storage environment: The conditions under which the pipes were stored (temperature, humidity, presence of corrosive chemicals, stacking configuration) significantly influence the extent and pattern of corrosion.
5. Intended service conditions: The severity of the intended service environment (soil corrosivity, fluid composition, operating temperature and pressure) must be considered when determining the remaining service life of the pipe.

Recommendations for Engineering Practice

Based on the findings of this study, the following recommendations are proposed for the management and use of long-stored seamless steel pipes:

• Pre-installation cleaning: Internal surfaces should be cleaned to remove loose rust and corrosion products before installation. This can be accomplished through mechanical methods (brushing, scraping) or chemical methods (acid pickling followed by neutralization and passivation).
• Enhanced corrosion protection: Additional corrosion protection measures should be implemented during pipeline construction, including improved external coating systems, cathodic protection design, and internal coating or lining where appropriate.
• Periodic monitoring: During the operational phase, regular monitoring of internal wall corrosion should be conducted using appropriate techniques such as ultrasonic thickness measurement, magnetic flux leakage inspection, or ultrasonic phased array testing. The monitoring frequency should be determined based on the severity of the service environment and the initial corrosion condition.
• Storage management: Future storage practices should be improved to minimize corrosion. This includes proper stacking to ensure drainage, use of desiccants, application of temporary protective coatings, and selection of storage locations with low humidity and minimal exposure to corrosive atmospheres.

Key Questions and Reflections

This study raises important questions about the economic and technical trade-offs in pipeline project management. Long-term storage of steel pipes is often unavoidable due to project scheduling delays, supply chain disruptions, or changes in project scope. The question of whether such stored pipes can be economically and safely reused is of practical significance for cost control in large-scale pipeline projects.

The reliance on GB/T 9711-2011 as the sole acceptance criterion, while appropriate for new pipe production, may not fully address the specific concerns of long-stored pipes. The standard does not specifically address the effects of storage duration on pipe integrity, and a more comprehensive fitness-for-service assessment methodology, potentially incorporating risk-based inspection principles, may be warranted for such cases.

From a metallurgical standpoint, the transition from uniform corrosion to localized corrosion is a critical threshold that must be monitored. If the storage environment changes (for example, if the pipes are exposed to chlorides or other aggressive species), the corrosion mechanism could shift from uniform to pitting, which would require a more conservative assessment.

Summary

This study demonstrates that seamless steel pipes subjected to long-term storage can retain their essential mechanical and geometric properties, provided the corrosion remains at a mild uniform level. The conformity with GB/T 9711-2011 requirements provides a solid basis for continued use, but the presence of internal loose rust necessitates pre-installation cleaning and enhanced corrosion protection during construction and operation. The recommended approach of combining cleaning, improved protection, and periodic monitoring provides a practical framework for managing the risks associated with long-stored pipe. For engineering organizations managing large inventories of steel pipes, this study reinforces the importance of proper storage management and systematic fitness-for-service assessment to ensure both economic efficiency and structural safety.