State of the Art
Weight saving through thinner structures and use of high tensile steel (HTS) is of increasing interest in modern cruise ships. Whilst feasibility has been shown in principle, previous research and industrial prototype applications have revealed that production processes have significant impact on the quality of joints and structural distortions, which can decrease the performance of these novel solutions. While low heat input welding for conventional steel has been developed, a lack of experience, quality assurance methods and confidence in the shipyard production processes does not allow for the extensive use of HTS and plate thickness less than 5 mm in critical (load carrying) applications currently.
Objective
Implementation of thin and high strength steel materials combined with advanced manufacturing technology, which enables high-quality production and yields a higher strength for the final product. To obtain these goals, a full-scale demonstrator will be exploited: a block of passenger ship superstructure, in which the selected strength critical structural elements (e.g. joints) are made using lightweight structural topologies, with high strength steel also utilised in load carrying members like longitudinal bulkheads.
Solution
The feasibility of production of large steel sections using low material thicknesses and HTS was demonstrated – including the process chain from butt and fillet welds in pre-manufacturing up to assembly joints in confined spaces or overhead conditions within and outside covered facilities. Process parameters and quality assurance procedures were established under real-life conditions at real scale, providing the basis for approval and wider use of thin plate structures and load carrying HTS structural members under full exploitation of their strength properties.
In the project it was demonstrated that it is possible to use 4 mm-thick deck plates and HTS bulkheads in cruise ships by using state of art production technologies. Small scale specimens were produced to perform hardness measurements and fatigue tests of MAG-welded connections and laser-hybrid welded connections. The results showed very good fatigue life in comparison to current design recommendations for the following configurations: 1) non-load carrying HTS cruciform connections, 2) shear-loaded cruciform connections, and 3) laser-hybrid welded 4 mm thick plates.
Potential
In global competition the knowledge of the production process and being able to produce high quality is a competitive advantage. This does not only apply to the cost of production and maintenance, but also enables production of unique design features. The use of HTS with yield strengths of up to 690 MPa is expected to allow weight savings of 20% for the steel hull in a cost-efficient manner, leading to reduced fuel consumption and increased payload (e.g. more cabins). The demonstrated utilisation potential of HTS in large structures is applicable not only to cruise ships, but also to other highly loaded structures such as cranes, bridges and offshore renewable energy devices.