In a previous paper the authors presented a comparison between a COGAS plant specifically designed and the original prime mover (a two stroke low speed diesel engine) used for the propulsion of a large container ship. The comparison showed some advantages of the COGAS solution from both technical and commercial points of view, advantages that however were counterbalanced at the time by the greater costs of operation (due to the more expensive gas turbine fuel) and also of plant installation.

However the situation is changing very rapidly. The adoption of stringent emission regulations in the shipping sector, particularly with regard to SOx (sulphur oxides) and NOx (nitrogen oxides), will produce an increase in equipment costs of diesel engines and also a significant reduction of the price difference between the diesel heavy fuel oil and the gas turbine distillate fuel. Consequently the COGAS plant option could become in a near future a competitive choice also from an economic point of view.

A feasibility study of the COGAS solution requires the availability of numerical models not only for the plant components design and performance evaluation in steady state operation, but also for the dynamic simulation of the propulsion system in every off-design and transient condition. The last feature is necessary for the prime mover governor development and for the design of the propulsion control system.

In this paper the authors present and describe a COGAS plant dynamic simulator, putting into evidence in particular detail the model of the heat recovery steam generator (HRSG), the more critical component as regards transient situations. For the mentioned large container vessel the prime mover dynamic analysis is combined with the overall propulsion system simulation.

Some significant simulation results relative to acceleration and deceleration manoeuvres of the ship are presented in the paper.