## 2 Materials and Methods
### Cost-Effectiveness  
Figure 16 illustrates the correlation between the weight and cost of the reef module, based on the CS cross-section geometry and the optimized RC cross-section that ensures structural safety under the most demanding loading scenario. The analysis was conducted to assess the cost implications of strengthening the RC modules in comparison to the CS module, addressing concerns related to cost and construction feasibility. In the case of RC solution, only configurations resulting in a sectional increase of less than 35% in the critical area (D\({}_{\text{min}}\)) were considered, and these were identified as the green zone in Figure 16. In addition, the maximum dimension of the limb near the spherical zone was constrained to maintain the original geometric ratio between D\({}_{\text{max}}\) and D\({}_{\text{min}}\), thereby ensuring consistency with the initial design morphology. This aimed to preserve the slenderness of the geometric form, as a bulkier structure could significantly alter the local hydrodynamic behaviour, potentially requiring further studies in that domain.  
Figure 16: Correlation between reef module weight and cost for CS and optimized RC configurations.  
The analysis of the reinforced concrete (RC) and composite steel (CS) cross-sections reveals distinct trade-offs between weight, cost, and structural efficiency (Figure 16). Most RC solutions have a weight of 160 kg, exceeding in 33% the initially desired 120 kg, which requires the use of medium equipment for handling the units. The cost of these RC solutions ranges from 18.7C to 68.8C per module. Conventional reinforcement results in lower costs, while the use of threaded bars significantly increases the unit cost. The RC combination with the best cost-construction feasibility ratio is the one using C90/105, S500, and 1025, which has a weight of 159.8 kg and costs 18.7C per unit. This solution offers an optimal balance between concrete usage and cost-efficiency, making it a more sustainable choice in terms of material optimization.  
In comparison, CS solutions, which utilize a composite concrete and steel structure, present a lower increase in unit weight but come at a significantly higher cost (Figure 16). The lowest cost for CS solutions is approximately double that of the RC unit. Several CS solutions, with weights around 150 kg, have costs ranging from 39.23C per unit to 56C per unit. A notable example is the combination of C30/37, S420, 076.1, and \(\mathrm{t_{w}}\) 4, which weighs 154 kg and costs 39.23C per unit. Whilst these CS solutions may exhibit a reduced weight relative to RC solutions, they necessitate a higher financial investment due to the materials employed. However, they can utilise lower-strength, more readily available concretes. The solution that results in the lowest possible weight for the CS cross-section uses the combination of C30/37, S420, 060.3, with a 5 mm thick wall, which achieves a weight of 139.72 kg (+16%) and costs 37.81C. However, it is important to note that this solution has a slightly lower \(\mathrm{M_{pl,Rd}}\) than required, meaning additional care is needed during application.  
The cost-effectiveness analysis reveals that the optimized RC solution offers the most economical and structurally adequate configuration for the reef module. However, this comes at the expense of increased volumetry, which may alter local hydrodynamic behaviour and potentially impact biological colonization. If maintaining the original geometry is a priority--whether for ecological performance, fluid-structure interaction, or design constraints--the composite (CS) solution becomes the preferable alternative. Despite its higher unit cost, it provides a structurally efficient design while using lower-strength, cement-reduced concretes, contributing to lower carbon emissions and greater sustainability. Thus, the choice between RC and CS depends on project priorities: cost and simplicity favour RC, while environmental performance and geometric fidelity favour CS.