Design of Pressure Steel Pipe for Huangdeng Hydropower Station

Literature Overview and Research Background

The study by Yang Haihong, Guan Zhongrui, and Deng Yongjun, published in Water Power in 2019, presents the design of the pressure steel pipe for the Huangdeng Hydropower Station, a large-scale hydropower project on the Lancang River in Yunnan Province, China. Pressure steel pipes (also known as penstocks) are critical components in hydropower stations, responsible for conveying water from the intake to the turbine under high pressure. The design of pressure steel pipes must account for complex interactions between the pipe structure, the surrounding rock mass, and the water pressure, as well as various operational and construction conditions. The researchers from the Kunming Engineering Investigation and Design Institute of China Power Engineering Consulting Group and Huaneng Lancang River Hydropower Co., Ltd. conducted a comprehensive design study that involved multiple scheme comparisons, structural design, drainage design, and grouting design.

Core Technical Findings

Scheme Comparison and Selection

The pressure pipe of the Huangdeng Hydropower Station is buried underground and is classified as a giant pressure pipe due to its large diameter and high operating pressure. The design team considered multiple factors including the arrangement of the intake, the influence of dam shoulder slope excavation, geological conditions, construction convenience, and construction reliability. Through a comprehensive comparison of multiple schemes, the team selected the in-tunnel open pipe design approach. This selection was based on the following considerations:

• The in-tunnel open pipe approach provides better access for construction and maintenance compared to fully buried pipes.
• The approach allows for better control of the pipe-to-rock interaction through the use of backfill concrete.
• The approach facilitates the installation of drainage systems between the backfill concrete and the surrounding rock, as well as between the backfill concrete and the steel pipe.

Structural Design and External Pressure Stability Check

The structural design of the pressure steel pipe was conducted in accordance with relevant standards and codes, considering the following loading conditions:

• Normal operating conditions: Full water pressure, dead weight of the pipe and water, and thermal effects.
• Maintenance conditions: Partial water pressure and the weight of the pipe and backfill concrete.
• Construction conditions: Dry construction without water pressure, considering the weight of the pipe and backfill concrete.
• Seismic conditions: Seismic loads based on the seismicity of the region.

The external pressure stability check was performed to ensure that the steel pipe does not buckle under the combined action of external rock pressure and internal water pressure. This check is critical for underground pressure pipes, as the external pressure from the surrounding rock can cause external buckling of the pipe wall, particularly in the case of thin-walled pipes.

Drainage System Design

A key aspect of the design is the drainage system, which is installed in two locations:

1. Between the backfill concrete and the surrounding rock: This drainage system relieves the water pressure that builds up between the backfill concrete and the rock mass, preventing excessive external pressure on the pipe.
2. Between the backfill concrete and the steel pipe: This drainage system relieves the water pressure that may develop between the backfill concrete and the pipe wall, ensuring that the pipe is not subjected to excessive external pressure during the maintenance condition.

The drainage system design is critical for the long-term stability of the pressure pipe, as it ensures that the water pressure is properly managed and does not compromise the structural integrity of the pipe.

Grouting Design

The study describes three types of grouting:

1. Backfill grouting: Grouting of the space between the backfill concrete and the steel pipe to ensure full contact and load transfer.
2. Uncovered consolidation grouting: Grouting of the surrounding rock mass without cover weight to improve the rock mass quality and reduce permeability.
3. Curtain grouting: Grouting to form a water barrier in the surrounding rock mass to reduce seepage into the tunnel and the pressure pipe.

The grouting design is essential for ensuring the long-term performance of the pressure pipe by improving the rock mass quality, reducing seepage, and providing additional support to the pipe structure.

Integration with Engineering Practice

Design Considerations for Giant Underground Pressure Pipes

The design of the Huangdeng Hydropower Station pressure pipe provides valuable lessons for the design of other large-scale underground pressure pipe projects:

1. Scheme selection: The selection of the pipe installation approach (in-tunnel open pipe vs. fully buried) should be based on a comprehensive comparison of geological conditions, construction feasibility, maintenance requirements, and cost.
2. External pressure stability: The external pressure stability check is a critical design step that should not be overlooked. The check should consider the worst-case combination of external rock pressure and internal water pressure, including transient conditions such as water hammer.
3. Drainage system design: The drainage system must be designed to effectively relieve water pressure in both the maintenance and normal operating conditions. The drainage capacity should be verified based on hydrogeological data and the expected seepage conditions.
4. Grouting design: The grouting design should be tailored to the specific geological conditions and should be verified through grouting tests and post-grouting permeability tests.

Quality Control and Construction Practices

The construction of the pressure pipe requires strict quality control to ensure the design intent is achieved:

• Steel pipe fabrication: The steel pipe must be fabricated in accordance with the applicable standards, with strict control of weld quality, dimensional tolerance, and material properties.
• Pipe installation: The pipe must be installed with proper alignment and support to prevent deformation and stress concentration.
• Backfill concrete placement: The backfill concrete must be placed with proper vibration to ensure full encapsulation of the pipe and good contact with the surrounding rock.
• Drainage system installation: The drainage system must be installed with proper protection against blockage and must be connected to the tunnel drainage system for effective water discharge.
• Grouting execution: The grouting must be executed in accordance with the design specifications, with monitoring of grouting pressure, grout volume, and post-grouting permeability.

Operational Considerations

The actual operation of the Huangdeng Hydropower Station pressure pipe has demonstrated good performance, which validates the design approach. However, the following operational considerations should be noted:

• Regular inspection of the pipe condition, including weld integrity, corrosion, and deformation.
• Monitoring of the drainage system performance to ensure continued effectiveness.
• Monitoring of the surrounding rock mass stability, particularly in the event of seismic events or changes in water level.
• Periodic hydrostatic testing to verify the structural integrity of the pipe.

Key Questions and Reflections

One question that arises is the long-term durability of the backfill concrete and the grouting system. The backfill concrete is exposed to water and potentially aggressive agents in the surrounding rock mass, which may lead to carbonation, sulfate attack, or chloride ingress. The long-term durability of the backfill concrete should be evaluated, and appropriate measures should be taken to protect the concrete from degradation.

Another consideration is the effect of seismic loading on the pressure pipe. The Huangdeng Hydropower Station is located in a seismically active region, and the pressure pipe must be designed to withstand seismic loads without failure. The interaction between the pipe, the backfill concrete, and the surrounding rock mass under seismic loading is complex and should be evaluated through dynamic analysis.

The study also raises questions about the maintainability of the drainage system. Over time, the drainage system may become blocked by sediment or scaling, reducing its effectiveness. The design should incorporate provisions for periodic cleaning and maintenance of the drainage system.

Study Insights and Implications

This study provides a comprehensive case study of the design of a giant underground pressure steel pipe for a large hydropower station. The multi-faceted approach to design, encompassing scheme comparison, structural design, drainage design, and grouting design, provides a valuable reference for similar projects. The successful operation of the pressure pipe validates the design approach and demonstrates the importance of integrated design considerations.

The study also highlights the importance of the drainage system in the design of underground pressure pipes. The drainage system is not merely an auxiliary component but a critical element that ensures the long-term stability and performance of the pipe. The design of the drainage system should be given equal importance as the structural design of the pipe itself.

In summary, this study provides engineers with practical design guidance and lessons learned from the design and construction of the Huangdeng Hydropower Station pressure pipe, contributing to the advancement of underground pressure pipe engineering in hydropower projects.