Simulation based design of a fleet of ships under power and capacity variations

Achieving the right balance between fixed and operating costs is a key but non-trivial task for optimizing the characteristics of a fleet of ships for a new trade at a very early stage of the design. A wide range of design choices as well as a significant number of uncertainties related to the operation of the fleet affects those two expense items. In the proposed analysis, operative uncertainties rising from rough sea conditions are taken into account in a Simulation Based Design (SBD) framework for fleet generation and evaluation. The costs-benefits analysis is aimed at understanding the effects of variations in the installed power and in the ship capacity on the overall fleet performances. These two major characteristics of the ship design (and, accordingly, of the fleet of ships realizing the trade) are introduced in the numerical model considering the sea margin and the nominal capacity as free design parameters instead of fixing them as commonly done in practice. From a design point of view, this corresponds to the introduction of two new variables, enlarging the dimensions of the design space to be explored, better than relaxing the boundaries of a simpler design space. The validity of this approach is demonstrated by a practical application to the design of a fleet of Compressed Natural Gas (CNG) carriers. In this context, the effects of variations in the fuel price and in the voyage length on the optimal fleet characteristics are analyzed and the results are discussed in comparison to those coming from the reference scenario.