Axial Compression Performance of Constrained UHPC Steel Grouted Connection Sections for Offshore Wind Turbine Steel Pipes

Literature Overview and Research Context

Offshore wind turbine monopile foundations have become the dominant structural form for fixed-bottom offshore wind farms worldwide. The connection between the transition piece and the monopile is a critical structural interface that must withstand extreme cyclic and monotonic loads, including wind, wave, and seismic actions. Traditional bolted or welded connections often suffer from fatigue cracking and stress concentration issues. The present study investigates a novel constrained ultra-high performance concrete (UHPC) and steel grouted connection section designed specifically for offshore wind turbine steel pipe foundations, focusing on its axial compression behavior.

The research addresses a practical engineering challenge: how to achieve high load transfer capacity, ductility, and fatigue resistance at the splicing interface between the upper transition piece and the lower monopile without introducing complex welding operations at offshore sites. The proposed connection utilizes a combination of UHPC confinement, steel grout, and mechanical interlock features to create a robust composite action between the two pipe segments.

Core Technical Points and Mechanisms

Structural Configuration

The connection section consists of an inner pipe segment (transition piece) inserted into an outer pipe segment (monopile) with a transition zone filled with UHPC and steel grout. Steel constraint elements—typically internal ribs or external stiffeners—surround the grouting zone to prevent lateral expansion of the UHPC under axial compression. The key design philosophy is to exploit the triaxial confinement effect on UHPC, thereby enhancing both strength and ductility beyond what plain UHPC alone can provide.

Axial Compression Behavior

The study demonstrates that the constrained UHPC-steel grouted connection exhibits a distinctly three-stage load-displacement response:

1. Elastic stage: Linear behavior dominated by steel pipe and UHPC elastic interaction, with minimal slippage between segments.
2. Plastic hardening stage: Steel pipe wall yields progressively while the UHPC grout transfers shear and compressive stresses through the interface. Confinement prevents premature crushing of the grout material.
3. Post-peak stage: Gradual degradation with maintained load-bearing capacity due to the steel pipe's arching action and residual UHPC confinement.

Key Performance Indicators

Engineering Practice Implications

Applicability to Offshore Environments

The connection design is particularly well-suited for offshore applications because it eliminates the need for extensive underwater welding operations, which are costly and quality-uncertain. The UHPC grout can be injected in controlled conditions, and the steel constraint elements provide a mechanical backup that does not depend on bond strength alone.

Material Selection Considerations

UHPC used in marine environments must be carefully formulated to resist chloride penetration and sulfate attack. The study implicitly assumes UHPC formulations with compressive strength exceeding 120 MPa and low permeability (water-to-binder ratio below 0.20). Steel grout serves as a transition layer that accommodates dimensional tolerances and enhances interfacial bond through mechanical interlocking with the pipe inner surface.

Comparison with Conventional Connections

Key Questions and Reflections

The study raises important questions about long-term performance under cyclic marine loading. While monotonic axial compression results are promising, the actual fatigue behavior under millions of load cycles from wave action remains to be validated through full-scale cyclic testing. Additionally, the thermal expansion mismatch between steel pipes and UHPC grout under seasonal temperature variations could induce residual stresses at the interface over decades of service.

Another critical consideration is the constructability of the connection at offshore sites. The grouting process must be executed in a single continuous operation to avoid cold joints, and the UHPC mix design must accommodate pumpability constraints while maintaining its ultra-high performance characteristics. The steel constraint elements must be pre-fabricated and installed with precision to ensure uniform confinement pressure distribution.

From a standards perspective, this type of connection does not yet have direct coverage in existing offshore wind foundation design codes such as DNV-ST-0119 or ISO 19902-2. Engineers adopting this technology must develop supplementary design criteria based on test data, potentially through project-specific technical specifications that reference the test protocols described in the study.

Study Insights and Implications

This research represents a meaningful advancement in offshore wind foundation connection technology. The combination of UHPC confinement with steel grouting and mechanical constraints creates a synergistic composite action that outperforms conventional connection types in both strength and ductility. The approach aligns with industry trends toward reducing underwater welding operations and increasing factory-controlled fabrication quality. Future work should focus on full-scale cyclic testing, long-term marine exposure studies, and the development of standardized design methodologies that can be incorporated into offshore wind foundation design codes. The technology shows particular promise for large-diameter monopiles (above 6 meters) where traditional bolted connections become impractical due to the sheer number of bolts required and the associated inspection challenges.