In this paper, the objective is to develop new graphical tools that allow straightforward identification of the optimal design of a Rankine cycle that uses LNG being regasified as a heat sink. To achieve that objective, a numerical model is developed in order to predict the value of the specific work output produced by the system. Then, numerical optimizations are performed and the results are reported in a graphical form. These new figures provide the values of the maximal specific work (objective function) for a wide range of operating conditions. These figures also provide the corresponding values of the four optimized design variables. The main result is that ethylene is the best working fluid for most of the operating conditions considered. Furthermore, a sub-optimal analysis is performed for each fluid investigated and the resulting figures allow to identify the fluids that can provide a specific work output close to that provided by the best working fluids. Finally, optimal designs identified in this paper are compared with optimal designs of the direct expansion (DE) and of the double direct expansion (D2E) systems. (C) 2019 Elsevier Ltd. All rights reserved.