Multi-Objective Optimization of Section Dimensions for Large-Diameter Hollow Sandwich Steel Tube Concrete Composite Monopile Foundations

Literature Overview

This paper addresses the multi-objective optimization of section dimensions for large-diameter hollow sandwich steel tube concrete (HSTC) composite monopile foundations used in offshore wind power installations. The sandwich configuration consists of an outer steel tube, an inner steel tube, and a concrete layer sandwiched between them, creating a composite cross-section that provides enhanced structural performance while managing material costs and construction complexity. The optimization targets include minimizing structural weight, reducing material cost, ensuring fatigue resistance, and satisfying buckling constraints.

Structural Configuration and Design Parameters

The HSTC composite monopile cross-section is characterized by the outer tube diameter (D_o), inner tube diameter (D_i), outer wall thickness (t_o), inner wall thickness (t_i), and the concrete layer thickness (t_c). The optimization problem involves finding the combination of these parameters that simultaneously satisfies multiple objective functions. The paper formulates the problem using a weighted sum approach combined with Pareto frontier analysis, where the objectives include minimum total weight, minimum steel consumption, and maximum fatigue life.

Optimization Methodology

The paper employs a multi-objective genetic algorithm to explore the design space, with constraints derived from Eurocode 3 and DNV-ST-0119 for offshore wind turbine support structures. The constraints include global buckling under combined axial load and bending moment, local buckling of the steel tubes, fatigue verification for the weld joints, and the ultimate limit state for the composite section. The analysis reveals that the concrete sandwich layer contributes significantly to the torsional stiffness and shear capacity of the monopile, allowing for thinner steel walls compared to a conventional solid steel tube of equivalent bending capacity.

The Pareto optimal solutions show a clear trade-off between weight minimization and fatigue life maximization. The optimal solutions cluster in a region where the outer wall thickness is in the range of 35 to 50 mm and the concrete layer thickness is between 400 and 600 mm. The inner tube provides additional buckling resistance and facilitates the construction sequence by serving as a temporary form for concrete placement.

Engineering Practice and Construction Considerations

The construction of HSTC composite monopiles presents unique challenges compared to conventional solid steel monopiles. The sandwich concrete must be placed through the gap between the inner and outer tubes, which requires careful management of concrete flow and compaction. The paper discusses the use of self-compacting concrete with a slump flow of 650 to 750 mm to ensure complete filling of the sandwich layer without mechanical vibration. The interface bond between the steel tubes and the concrete is critical for the composite action, and the paper recommends surface treatment of the steel tubes with mechanical roughening to enhance the bond strength.

Fatigue performance is a dominant design consideration for offshore wind monopiles subjected to cyclic wave and wind loading. The sandwich configuration provides inherent fatigue advantages because the concrete layer shields the inner tube from direct wave impact and reduces the stress amplitude at the weld joints. The paper reports that the optimized HSTC design achieves a fatigue life exceeding 50 years for the critical weld joints, compared to approximately 30 years for an equivalent solid steel monopile design.

Study Insights and Reflections

This research demonstrates that the hollow sandwich configuration offers a technically sound and economically viable alternative to solid steel monopiles for large-diameter offshore wind foundations. The multi-objective optimization framework provides engineers with a systematic approach to balancing competing design requirements. However, I note that the long-term durability of the sandwich concrete under marine environmental conditions, including chloride ingress and carbonation, requires further investigation. The concrete sandwich layer is essentially a confined concrete element that is not readily accessible for inspection or repair, which poses a challenge for maintenance and life extension strategies. Future work should develop non-destructive evaluation techniques capable of assessing the condition of the embedded concrete layer throughout the service life of the structure.