The world is moving towards a cleaner future. With the increasingly stringent emission regulation in maritime field and the limitation of marine diesel engine systems, the shipbuilders may start to seek cleaner solutions rather than modifying the currently prevailing internal combustion engine systems (ICE). According to previous researches, a tandem fuel cell system power plant, which uses LNG as fuel, has a very good overall emission performance in comparison to marine LNG based ICE systems, and can achieve an overall efficiency of more than 60%. To study whether the tandem fuel cell system can be an alternative for marine LNG engine systems, two models are built in this master thesis to study the performance of the system under stationary operation and transients. The first model is built to study the stationary performance of the system, and to verify the result of high efficiency from previous researches. The result shows that indeed an efficiency of almost 60% can be reproduced. The second model is built to study the transient behaviour of the system, and to compare it with that of marine LNG engine systems. The result shows that the system has a better transient response than the recommended transient response for current marine LNG engine systems. Since the SOFC in the tandem fuel cell system has a slow transient behaviour, a hydrogen tank is added between the SOFC system and the PEMFC system as an energy buffer. A design assistance tool is developed to assist in sizing the hydrogen tank and the fuel (LNG) tank in design phase.