Wind Vibration Response of CFST Lattice Wind Turbine Towers

Literature Overview and Background

This paper by Li Bin, Wen Haotian, and Gao Chunyan, published in Noise and Vibration Control in 2018, investigates the wind-induced vibration response of a concrete-filled steel tube (CFST) lattice-type wind turbine tower located in the Baiyun Obo region of Inner Mongolia. The tower is designed for a 2 MW wind turbine and adopts a novel lattice configuration using CFST members. The study employs the Davenport gust wind spectrum and harmonic synthesis method to simulate the turbulent wind speed time history for the Baiyun Obo site, then uses finite element analysis to evaluate the wind-induced response of the tower. The key finding is that the wind vibration response of the CFST lattice tower is significant and strongly influenced by turbulent wind, and that the current code-specified wind vibration coefficient for CFST lattice towers introduces substantial errors in the response prediction.

Core Technical Findings

Wind Vibration Response Characteristics

The study identifies several important characteristics of the wind vibration response:

Wind Speed Simulation Methodology

The Davenport gust wind spectrum is used to characterize the turbulent wind at the Baiyun Obo site, which is known for its high wind energy density. The harmonic synthesis method generates a realistic turbulent wind time history by summing multiple sinusoidal components with amplitudes and phases determined by the power spectral density function. The simulated wind time history is then applied to the finite element model of the tower to compute the dynamic response.

Code Discrepancies

The study finds that the current code-specified wind vibration coefficient for CFST lattice towers introduces significant errors. This is attributed to the fact that the code coefficients are typically calibrated for solid-walled tubular towers, not lattice configurations, and may not account for the specific dynamic characteristics of the CFST lattice tower.

Connection to Steel Pipe Fabrication and Welding

Steel Tube Members in Lattice Towers

The CFST lattice tower uses steel tubes as the primary structural members, typically in the form of chords and diagonal braces. These steel tubes are fabricated from hot-rolled steel plate using longitudinal welding, and the weld quality directly affects the structural integrity of the tower under cyclic wind loading. The steel tubes must be fabricated to precise dimensional tolerances to ensure proper fit-up at the lattice joints and to provide the intended geometric stiffness.

Welding Considerations for Wind-Loaded Structures

Wind loading on a wind turbine tower is characterized by:

1. Mean wind load: produces static bending and axial stresses in the tower members.
2. Gust loading: produces dynamic stresses that fluctuate at frequencies corresponding to the wind turbulence spectrum.
3. Vortex shedding: produces periodic transverse forces at the Strouhal frequency, which may coincide with the tower's natural frequency.

The combination of these loading components creates a complex stress history at the weld locations, with mean stress, alternating stress, and occasional peak stresses. The welding procedure must produce welds with:

• Adequate fatigue strength to withstand millions of load cycles over the 20-25 year design life.
• Sufficient toughness to resist brittle fracture at low temperatures, as the Baiyun Obo region experiences severe winter conditions.
• Low residual stresses to minimize the interaction between welding-induced stresses and wind-induced stresses.

Material Selection and Welding Procedure

For CFST lattice wind turbine towers, the following specifications are recommended:

• Steel grade: minimum Q355 (ASTM A572 Gr.50) with guaranteed Charpy V-notch impact energy of at least 27 J at -20°C.
• Welding process: multi-pass SAW or FCAW with controlled heat input and interpass temperature.
• Weld geometry: full-penetration butt welds at all critical joints, with ground-flat weld toes to minimize stress concentration.
• Weld inspection: 100% PAUT or UT inspection of all longitudinal and circumferential welds, with acceptance per ISO 17635 Level B or better.
• Post-weld treatment: shot peening of weld toes to introduce compressive residual stresses and improve fatigue life.
• Coating: high-build zinc-rich primer with a total coating thickness of at least 200 μm for corrosion protection in the coastal or high-humidity environments typical of wind farm locations.

Quality Control for Wind Turbine Tower Fabrication

The quality control program for CFST lattice wind turbine towers must address the unique demands of wind loading:

1. All steel tube segments must be hydrostatically tested at 1.5 times the design pressure before leaving the fabrication shop.
2. The lattice joint fabrication must include fit-up gauges to ensure that the chord and brace members align correctly, with angular tolerances of ±0.5°.
3. The welding procedure qualification (WPQ) must include fatigue test data for the specific weld configuration, with a minimum fatigue life of 2 million cycles at the design stress range.
4. The final assembled tower must undergo a dynamic test (e.g., free-decay vibration test or forced vibration test) to verify that the natural frequency and damping ratio match the design predictions.
5. The coating system must be inspected for holiday-free coverage using a holiday detector before installation.

Study Insights and Reflections

The study by Li Bin and colleagues highlights an important gap between current code provisions and the actual dynamic behavior of CFST lattice wind turbine towers. The finding that the code-specified wind vibration coefficient introduces substantial errors has direct implications for the design of the steel tube members and their welds. If the code overestimates the dynamic amplification, the steel tubes may be over-designed, leading to unnecessary material cost and weight. If the code underestimates the dynamic amplification, the steel tubes and their welds may be under-designed, leading to premature fatigue failure and potential catastrophic collapse. For the fabrication engineer, this means that the design forces communicated by the structural engineer must be scrutinized for reasonableness, and the fabrication quality must be maintained at a level that provides adequate margin against the uncertainties in the design methodology. The novel lattice configuration using CFST members offers significant advantages in terms of material efficiency and constructability, but these advantages are realized only if the welds are fabricated to a quality level that ensures the full fatigue life of the tower. The study serves as a reminder that wind turbine towers are not static structures but dynamic systems that respond to the full spectrum of wind turbulence, and that the fabrication and welding quality of the steel components is a critical determinant of the tower's long-term reliability and safety.