# Risk-Based Hybrid Light-Weight Ship Structural Design Accounting for Carbon Footprint
## 2 Sandwich Structures: Light-Weight Solution  
Introducing alternative lightweight materials in shipbuilding is one of the paths to promote a reduction of the environmental impact due to weight reduction and attention to the consequences during the ship's life cycle, including dismantling. The structural design addressed weight minimisation would result in several advantages [22; 23] in energy efficiency, which include required power and consequent fuel consumption reduction, capital expenditure (CAPEX) and operational expenditure (OPEX) decrease. In addition to weight reduction, practical benefits in terms of environmental impact can be achieved by addressing the environmental consequences of the whole ship's life cycle, from raw materials production to dismantling (see Figure 1).  
Sandwich structures represent one of the most common solutions the marine industry adopts to address weight-saving. In addition to high stiffness-to-weight and strength-to-weight ratios, sandwich structures may benefit marine applications since they can provide excellent crashworthiness, good damping properties, economical assembly, and reduction of secondary stiffening. When applied in the maritime industry, sandwich structures should fulfil different requirements such as manufacturing feasibility for large structures, economic boundaries or resistance to corrosion [24]. The latter is a key feature that makes sandwich structures competitive with traditional materials such as steel.  
Viable sandwich structures for shipbuilding include polymer-based solutions and all metal sandwich structures. The selection and comparison among alternatives for a specific application can be based on several approaches and criteria. As reported in [6; 10], a preliminary mechanical comparison among several solutions could be based on their bending stiffness, a valuable parameter in ship structural design. According to the results  
Figure 1: Key points to address to achieve environmental impact reduction in the shipping industry.  
reported in [6]--which compared GFRP (glass-fibre reinforced plastics)-PVC foam sandwich structures, GFRP-balsa wood sandwich structures, web-core steel sandwich structures and AHS--given a specific bending stiffness, AHS is potentially the solution able to provide the highest weigh reduction (\(-\)76.7%) in comparison to traditional steel plate structures. The fulfilment of the required bending stiffness may lead to a higher thickness of AHS panels compared to steel plates or other heavier sandwich structures.  
The direct environmental benefits deriving from the ship's structural weight reduction could be estimated according to simplified approaches as those reported in [25; 26], who respectively suggested that fuel consumption depends on \(\Delta^{2/3}\) (where \(\Delta\) is ship displacement) and that CO\({}_{2}\) emissions are linearly proportional to fuel consumption. It follows that the highest the weight reduction, the lowest the CO\({}_{2}\) emissions. Hence focusing on AHS, which has excellent potential for weight reduction, means investing in an attractive option to mitigate the environmental impact of the operational phase in the ship life-cycle.  
In addition to emissions due to operation, the environmental consequences of other activities, such as raw materials extraction, manufacturing, disposal, and reuse, influence ships' environmental impact. Therefore, a crucial aspect to consider in selecting alternative materials and structures is the environmental impact of their production, manufacturing, transport, and disposal. In this scenario, aluminium is recognised as a highly recyclable material [27], and its production from recycled sources (secondary aluminium) is reported to save between 93% and 95% [28; 29] of energy than the primary production. It can be concluded that aluminium-based sandwich structures are excellent candidates to address sustainability issues in shipbuilding.