Tomislav Mrakovčić

Influence of ship behaviour in a seaway on CO2 emissions

A reliable prediction of attainable ship speed at actual seas is essential from economical and environmental aspects. At this paper a methodology for estimating the attainable speed and related fuel consumption and carbon dioxide (CO2) emissions in moderate and severe sea is proposed. The irregular sea is handled as a series of regular waves with different amplitudes and frequencies. The added resistance in regular waves is obtained by either a direct pressure integration method or an asymptotic small wavelength formula. The in-and-out-of-water-effect and ventilation of a propeller in severe seas is accounted for by a quasi-steady averaging of experimental data for different propeller submergences. The propulsion results for regular waves are used in simulating results in irregular waves. It is shown that for higher sea states this effect has much more influence on the speed loss than the added resistance in waves. The speed loss is calculated by taking into account the engine and propeller performance in actual seas as well as the mass inertia of the ship.The numerical model used for main propulsion engine modeling is based on a zero-dimensional model of an internal combustion engine. The main propulsion engine is represented by number of control volumes interconnected with links for mass and energy transfer between them. This model provides excellent prediction of engine dynamic response during transients with rather short computational time. Also, engine fuel consumption can be precisely determined which represents the basic presumption for estimation of carbon-dioxide emission. Furthermore, use of such model can be extended to determination of the lowest fuel oil consumption strategy for given sea condition and ship speed with resulting lowest possible CO2 emissions. The attainable ship speed is obtained as time series. Correlation of speed loss with sea states allows predictions of propulsive performance in actual seas.© 2013 ASME

Comparison of COGES and Diesel-Electric Ship Propulsion Systems

Diesel-electric ship propulsion is a frequent shipowners choice nowadays, especially on passenger ships. Despite many diesel engines advantages, their primary disadvantage is emission of pollutants. As environmental standards become more stringent, the question of optimal alternative to diesel-electric propulsion arises. COGES (COmbined Gas turbine Electric and Steam) propulsion system is one of the proposals for alternative propulsion system, primarily due to significant reduction of pollutant emissions. On the other hand, gas turbines have higher specific fuel consumption in comparison with diesel engines what represents their noticeable disadvantage. However, some analyzes suggested that COGES propulsion system could be still cost-effective in comparison to diesel-electric propulsion, particularly on passenger ships where higher initial investment can be compensated by increasing the number of passenger cabins. This paper shows a comparison of abovementioned propulsion systems, which can be useful for the optimal ship propulsion system selection.