## 4 Conclusions  
This work introduces the concept of different structural systems and presents a design methodology for developing and analysing modular and geometrically complex artificial reef structures. This study is based in a cross-sectional analytical approach based in the principles of structural mechanics supported by numerical analyses to evaluate actions and their effects on the structure. By detailing the geometric design, structural conception, and relevant environmental actions, an approach was proposed for the evaluation of possible structural configurations. The analytical methodology, along with the parametric study, provided crucial insights into the performance and feasibility of different design configurations. In particular, and for the specific case of the cross-sections developed, the main findings were as follows:  
* Reduced fixed diameters establish an important geometric limit for optimizing the cross-section of the RC type. * Tensile strength emerged as the primary limiting factor, particularly in the RC type with variable cross-sections, where the minimum reinforcement was constrained by the larger section. * Flexural strength was not a limiting factor for the CS structural solution, as the variety of structural combinations indicates sufficient robustness under flexural stresses. * In terms of axial and flexural strength, the CS system provided significantly higher design strengths when compared to RC sections. Additionally, the CS structure did not require the use of high-strength concrete, simplifying construction and potentially reducing costs and environmental impact. * Shear strength was not a limiting factor for either structural solution. * Full optimization of the cross-section to withstand the most demanding loading scenario would require a minimum size increase of 23%, making it bulky, heavier, and less attractive. * If cost is not a limiting factor and preserving the original geometry is a priority, the CS solution stands out for its high structural efficiency, reduced carbon footprint, and minimal impact on hydrodynamic and ecological performance. * The methodology needs to include factors such as fatigue or long-term plastic deformation, which are crucial for structures exposed to high load cycles, like artificial reefs. These findings emphasize the importance of a systematic engineering-driven analysis to the design and construction of artificial reefs, as it allows a comprehensive assessment of structural viability before advancing to prototyping and more complex numerical analyses. This stage enhances efficiency and ensures the most promising designs are pursued, optimizing resources and efforts. The design methodology has limitations due to the incomplete consideration of axial and flexural stresses interactions, which may affect performance and lead to an underestimation of the forces acting on the structure. This highlights the need for further analysis in future work to ensure robustness under real exposure conditions. Additionally, in situ monitoring strategies, such as wave height measurements, pressure sensors, strain gauges, accelerometers, or acoustic emission, need to be explored to assess real-time structural performance and detect signs of damage or degradation [72]. Given that marine structures are exposed to dynamic ocean loads over extended periods, future research will address the cumulative fatigue effects. This will require dedicated studies to characterize fatigue resistance as a function of the number of cycles and stress levels in critical sections, based either on the Model Code 90 framework or on experimental data. These investigations will build upon the structural insights developed in this initial study and aim to enhance the durability and reliability of the proposed modular reef system. Conceptualization, F.C., E.B.P., I.B.V., T.M. and D.M. ; methodology, F.C., E.B.P. and I.B.V. ; software, F.C. ; validation, F.C. and E.B.P. ; formal analysis, F.C. and E.B.P. ; investigation, F.C. ; resources, F.C., E.B.P., I.B.V. and T.M. ; data curation, F.C.