# Risk-Based Hybrid Light-Weight Ship Structural Design Accounting for Carbon Footprint
## 1 Introduction  
Light-weight materials and structures in shipbuilding have gained significant attention recently in reducing fuel consumption and emissions while maintaining structural integrity and load-carrying capacity. The International Maritime Organization (IMO) [1] reported that in 2018 the global shipping energy reached nearly 11 exajoules (EJ), which is about 1 billion tonnes of carbon dioxide (CO\({}_{2}\)), resulting in about 3% of the annual global greenhouse gas (GHG) emissions. According to the International Energy Agency [2], the estimated emissions could further increase because of the rapid increment of maritime freight traffic. One possible solution in mitigating the environmental pollution due to the activities in the shipping industry is to reduce ship weight--and consequently the required power and fuel consumption--and to increase the share of more sustainable and recyclable materials in construction maintenance during the ship's life cycle. Implementing lightweight materials in ship structures presents a promising solution to the abovementioned challenges. The sandwich structures consisting of two thin, stiff outer skins separated by a lightweight core material demonstrate a good advantage. Thesematerials result in a light structure, making it an attractive option for reducing the weight of different ship structural components. Most sandwich types of structures are destined for small boats and generally comprise glass fibre-reinforced plastic (GFRP) skins with a polymeric foam core. However, as reported in [3], the disposal of marine composite sandwich panels at the end of their life are not economical and energetically expensive, leading to very poor recyclability. This further supports the need to address sustainability issues in the design process, promoting more effective integration of lifecycle-based analysis [4] and a green design approach that combines structural requirements, reuse and recycling of materials and weight reduction. Recent analyses, such as in [5], recognised the significance of limiting structural weight by combining fuel consumption reduction with light displacement limitation to achieve effective environmental benefits. [PERSON] et al. [6] provided an overview of the relevant lightweight materials that can be used for marine structural applications and compared their potential environmental impact. It was observed that aluminium sandwich structures represent an attractive alternative to composite sandwich materials in maritime structures. Aluminium is recognised as a sustainable and light material due to its high degree of recyclability [7] and low weight density. In this scenario, aluminium honeycomb sandwich (AHS) panels can provide sustainable and lightweight marine structures and excellent impact absorption capabilities. Some applications of AHS for marine structures are reported in [8; 9; 10; 11]. A preliminary evaluation of the potential advantages of integrating AHS in shipbuilding was performed in [12], where AHS was suggested as an alternative to the steel inner side shell of the ship hull. A key point to address to support the actual use of AHS in shipbuilding concerns their suitable integration in risk-based design approaches, whose importance is recognised by IMO [13; 14] in their guidelines on Formal Safety Assessment (FSA). Risk-based methodologies are driven by the need to address safety issues as part of the design objectives from a cost-effective perspective [15]. The research project SAFEDOR testifies the efforts toward a more comprehensive integration of risk assessment approach in the design of all marine vehicles. Oil tankers are one of the main categories where the relevance of risk-based design is well-recognised, as highlighted in tan [16]. They focused on developing a preliminary design procedure accounting for oil outflow risk after collisions, addressing the consequences of relevant structural details. However, economic impacts and cost-benefit considerations were not included in the process.