In this paper a procedure is presented for the conceptual design of a crashworthy side structure of a liquefied natural gas (LNG) tanker. The crashworthiness of an LNG tanker is assessed taking into account the deformation restrictions of the containment system. A particle swarm algorithm is used for the structure optimisation. The classification society compliance of the conceptual design is checked for a characteristic service loading condition. Collision simulations are carried out with a non-linear finite element solver in order to assess crashworthiness. An element length-dependent constant strain failure criterion is used to model possible rupture. The resulting optimised conceptual side structure is compared with the initial rules-based concept and with a minimum weight concept, demonstrating the influence of the structural concepts and the containment system restrictions on the crashworthiness.