Design space exploration of gas turbine based ship propulsion systems

Abstract

The conceptual design of a ship propulsion system, developed during the early stages of the overall ship design process, has a very large impact on the overall design and performance of a ship. Gas turbines are often utilized for warship propulsion systems designed to fulfil requirements of high ship speed and power density. To achieve a high overall system-level efficiency, gas turbines are often used in combined architectures with diesel engines along with geared, electric or hybrid transmission systems driving multiple propulsors. The process for the development of the conceptual design for such ‘combined’ systems, designed to achieve multiple and often conflicting design objectives, is significantly more complex compared to that for systems where a single-engine drives a propulsor. A number of approaches are currently used in practice towards the conceptual design of a ship, ranging from manual ‘design lanes’ based iterative approaches, to computerized overall ‘ship as a system’ design synthesis approaches. Towards the conceptual design of the propulsion system, both these approaches pose their own limitations, wherein the approach of the manual iterative design process relies heavily on the preferences of experts and approximations based on past experience; while the ship design synthesis approach usually yields good results generally if the candidate propulsion architectures are based on existing designs. This research work proposes a model-based process for the design space exploration using a model-based ‘Techno-economic & Environmental Risk Assessment’ (TERA) approach. To undertake feasibility and performance analysis of the candidate propulsion system architectures, the process involves building performance models of the candidate propulsion system architectures at this very early stage the overall design process of the ship and utilizing the generated results as the basis of design related decisions. For undertaking the performance modelling of the multiple candidate propulsion system architectures, an agile modelling and simulation framework was considered essential that could handle the complexity of ‘combined’ propulsion plants. To achieve this, a component-based modelling software, ‘Poseidon +’, has been developed as a part of the present work which enables 0-D modelling of gas turbine engines as well as the entire propulsion system in the same framework. A key aspect of this work was the development of an algorithm that analyses the direction of the torque transmission across the complex transmission system of ‘combined’ plants, based solely on the based on the states of the engine and clutches. For undertaking TERA, a suite of ‘Multiple-Criteria Decision-Making’ (MCDM) methods were selected and applied to select a compromise solution from competing propulsion system architectures, using a combination of performance data generated from simulation of developed models, and comparative expert opinions-based metrics for information not available early in the ship design process. To execute the MCDM procedure, a methodology of deriving weights of the competing design criteria using a combination of hierarchal and network structure, considering the degree of relationships between the design criteria, has been demonstrated. The overall proposed approach for design space exploration of gas turbine based ‘combined’ ship propulsion systems has been demonstrated towards the conceptual design analysis of two notional ship designs. The results of the analysis show the effectiveness of the proposed procedure for design space exploration of ship propulsion systems.