Design of Penstock Steel Pipes for Huangden Hydropower Station

Literature Overview

This paper by Yang Haihong et al. (2019), published in Water Power (水力发电), presents the design of underground penstock steel pipes for the Huangden Hydropower Station on the Lancang River. The project is a major hydroelectric development with enormous penstocks classified as mega-scale pressure pipes. The design was conducted by China Power Construction Kunming Engineering Corporation, with the project owner being Huaneng Lancang River Hydropower Co., Ltd. The paper addresses critical design aspects including structural design, external pressure stability verification, drainage system design, and grouting design.

Project Characteristics and Design Challenges

The Huangden Hydropower Station penstocks are underground, buried-type mega-scale pressure pipes. The design faced multiple challenges arising from the complex geological conditions, the large diameter of the pipes, and the requirement for long-term operational reliability in a seismically active region.

Structural Design Scheme Selection

The selection of the penstock structural design scheme involved a comprehensive multi-criteria comparison considering:

1. Intake arrangement: The intake structure location and orientation influenced the optimal penstock routing
2. Abutment slope excavation: The excavation of the dam abutment slopes created complex stress conditions that affected penstock design
3. Geological conditions: Rock mass quality, groundwater conditions, and fault zones along the penstock alignment
4. Construction convenience: Accessibility for fabrication, transportation, and installation of large-diameter pipe sections
5. Construction reliability: The ability to maintain quality control during construction in underground conditions

The selected design adopted the open pipe in tunnel (洞内明管) configuration, where the steel penstock is installed inside an excavated tunnel with concrete backfill between the pipe and the surrounding rock. This approach provides:

• Direct visual inspection access for maintenance
• Protection of the steel pipe from direct rock loading
• Flexible accommodation of geological variations
• Easier repair and replacement of damaged sections

External Pressure Stability Verification

For mega-scale penstocks, external pressure stability (buckling resistance) is a critical design consideration. The steel pipe is subjected to external pressures from:

• Overburden pressure from the rock mass and concrete backfill
• Seismic loading effects
• Groundwater pressure in adverse conditions
• Construction loads during backfill placement

The stability verification follows the methodology of GB 50634-2011 (Standard for Design of Steel Pressure Pipes) and references international standards such as DNV-ST-F401 and ASME B31.8. The critical external pressure is calculated considering:

• Pipe geometry (diameter-to-thickness ratio D/t)
• Material properties (yield strength, elastic modulus)
• Boundary conditions (support spacing, foundation stiffness)
• Eccentricity of external pressure

Drainage System Design

The drainage system design is essential for the long-term stability of the underground penstock. The system consists of two drainage layers:

Layer 1: Between Rock Mass and Backfill Concrete

This drainage layer intercepts groundwater flowing through the surrounding rock mass before it contacts the concrete backfill. The layer typically consists of:

• Drainage pipes or drainage boards embedded in a gravel layer
• Permeable concrete or open-graded aggregate
• Sufficient permeability to maintain hydraulic gradients below critical values

Layer 2: Between Backfill Concrete and Steel Pipe

This inner drainage layer protects the steel pipe from hydrostatic pressure buildup in the backfill concrete. The design ensures that any water seeping through the outer drainage layer does not accumulate against the pipe surface.

The drainage system design must account for:

• Design groundwater flow rate based on hydrogeological investigations
• Long-term clogging potential due to sedimentation and biological growth
• Maintenance access for periodic inspection and cleaning
• Freeze-thaw resistance in cold regions (not applicable to Huangden but relevant for other projects)

Grouting Design

Three types of grouting are employed:

The backfill grouting ensures full contact between the concrete backfill and the steel pipe surface, providing uniform external pressure distribution and preventing localized stress concentrations. The consolidation grouting improves the rock mass modulus and reduces differential settlement around the tunnel. The curtain grouting reduces the hydraulic gradient driving groundwater flow toward the tunnel, thereby reducing the load on the drainage system.

Standards and Code Compliance

The design complies with multiple Chinese and international standards:

Engineering Practice and Operational Performance

The paper reports that the actual operational performance of the penstock system has been satisfactory. This validates the design approach of combining open pipe in tunnel configuration with comprehensive drainage and grouting systems. Key operational observations include:

• No abnormal deformation or stress concentrations observed during operation
• Drainage system remains functional with no evidence of clogging
• Grouting effectiveness confirmed by post-grouting tests
• Pipe surface condition remains in good condition with no corrosion or damage

Study Insights and Reflections

This case study provides valuable reference for the design of mega-scale underground penstocks in complex geological conditions. The multi-scheme comparison approach, combined with rigorous verification of external pressure stability and comprehensive auxiliary systems (drainage and grouting), demonstrates a systematic engineering methodology. The emphasis on drainage system design is particularly noteworthy, as many penstock failures in practice are attributed to water pressure buildup rather than structural inadequacy. The successful operational performance confirms that the selected design approach provides adequate safety margins for long-term service. Future projects with similar characteristics should adopt this integrated design philosophy, paying particular attention to the interaction between the drainage system and grouting system, and ensuring that both systems are designed to function together over the design life of the structure.