Application of Bimetallic Composite Pipes in Hutubi Underground Gas Storage Project

Literature Overview

The paper titled "Application Research on Bimetallic Composite Pipes in Hutubi Gas Storage" addresses the practical challenges encountered during the construction and operation of the Hutubi underground gas storage facility located in the Altay region of Xinjiang, China. This facility operates under extreme seasonal temperature variations and cyclic pressure loading, making it an ideal case study for evaluating the long-term performance of bimetallic composite piping systems. The research team conducted a comprehensive investigation covering material selection, manufacturing process optimization, in-service inspection, and failure analysis, providing valuable data for engineers working on similar high-pressure storage applications.

Core Technical Content

The study focuses on the application of stainless steel-lined carbon steel composite pipes used in the wellhead and gathering systems of the Hutubi gas storage. The base material is typically 20# carbon steel or Q345R low-alloy steel, while the cladding layer is 304 or 316L austenitic stainless steel with a minimum thickness of 3 mm. The composite pipe diameter ranges from DN50 to DN300, with operating pressures up to 12 MPa and cyclic pressure variations between 4 MPa and 12 MPa during injection and withdrawal cycles.

The manufacturing process employed in this project is primarily electroslag welding (ESW) overlay combined with hot-rolled strip cladding for smaller diameter sections. The ESW overlay process parameters were carefully controlled to ensure a metallurgical bond strength exceeding 95% of the cladding layer's tensile strength, as required by NB/T 47015. The dilution ratio between the cladding layer and the base metal was maintained below 10% through multi-pass welding with appropriate consumable selection.

Process and Quality Control Analysis

The study highlights several critical process control points that are essential for ensuring the integrity of the composite pipe system. First, preheat temperature control is vital, particularly given the extreme ambient temperatures in the Altay region. A preheat temperature of 100–150°C was maintained for the base metal prior to cladding to prevent cold cracking in the heat-affected zone. Second, interpass temperature was controlled below 250°C to avoid excessive grain growth and ensure proper microstructural development in the weld overlay.

The dilution control strategy involved using a combination of consumable types: the first pass utilized a nickel-based welding wire (such as ER309L) to create a transition layer with lower dilution sensitivity, followed by 304L or 316L consumables for subsequent passes. This approach effectively reduced the dilution ratio from approximately 25% in a single-pass approach to below 10% in a multi-pass approach, which is critical for maintaining the corrosion resistance of the cladding layer.

Non-destructive testing (NDT) was performed according to JB/T 4730 standards. Ultrasonic testing (UT) was used to detect bonding defects between the cladding layer and base metal, while penetrant testing (PT) was applied to detect surface cracks in the cladding layer. The acceptance criteria required zero bonding defects larger than 5 mm in area and no surface cracks exceeding 0.5 mm in width.

Engineering Practice and Failure Analysis

During the field application phase, the research team monitored the composite pipes over a period of three years, encompassing approximately 200 injection-withdrawal cycles. The monitoring program included periodic ultrasonic thickness measurements, magnetic particle testing of the weld seams, and intergranular corrosion testing of coupon samples extracted from representative locations.

The results showed that the composite pipes performed satisfactorily under the cyclic loading conditions, with no significant bond degradation observed. However, the study identified a potential concern regarding hydrogen-induced cracking (HIC) in the base metal near the weld heat-affected zone, particularly in areas where the carbon equivalent (CE) exceeded 0.45. This finding underscores the importance of base metal selection and hydrogen control measures during welding operations.

The study also reports on a specific failure case where a composite pipe section experienced cladding delamination at a weld joint. Metallographic analysis revealed that the root cause was insufficient preheat temperature combined with excessive cooling rate, which resulted in the formation of martensitic structures in the dilution zone. This case study provides a valuable lesson for process control: even when dilution ratios appear acceptable, the microstructural evolution in the dilution zone can significantly affect long-term bonding performance.

Study Insights and Reflections

The most significant insight from this study is the recognition that bimetallic composite pipe performance in cyclic pressure applications is governed not only by the initial manufacturing quality but also by the cumulative effects of thermal-mechanical fatigue. The cyclic pressure variations in gas storage applications create repeated stress concentrations at the interface between the cladding layer and base metal, which can promote crack initiation and propagation over time.

From a standards perspective, the study raises questions about the adequacy of current design codes for cyclic loading applications. GB/T 150 and NB/T 47002 provide guidance for static pressure design but offer limited specific provisions for cyclic fatigue assessment of cladded components. Engineers working on gas storage projects should consider supplementing standard design calculations with fatigue life assessments based on ASME VIII Div.2 Part 5 or equivalent methodology.

The economic analysis presented in the study indicates that while bimetallic composite pipes have a higher initial cost compared to solid stainless steel pipes (approximately 30–40% higher), they offer significant advantages in terms of weight reduction, corrosion resistance, and overall lifecycle cost. For the Hutubi project, the use of composite pipes reduced the total material cost by approximately 25% compared to an all-stainless steel alternative, while maintaining the required corrosion resistance for the hydrogen sulfide-containing gas mixture.

Summary

The Hutubi gas storage project provides a well-documented case study demonstrating the successful application of bimetallic composite pipes in demanding cyclic pressure environments. The key takeaways for engineers include the critical importance of dilution control through multi-pass welding strategies, the necessity of adequate preheat and interpass temperature management in extreme ambient conditions, and the value of comprehensive in-service monitoring programs. The study also highlights the need for enhanced design methodologies that account for cyclic fatigue effects in cladded components, which remains an area requiring further standard development and engineering guidance.