Increased speed of container vessels in case of one cylinder misfiring

摘要

For large two stroke diesel engines it is common practice to operate the engine for extended periods of time, even in case of one or more cylinder units are taken out of operation. The situation is referred to as misfire. This could be due to any kind of malfunction in the running gear or the fuel equipment for that unit. The upper speed and load limit in case of such an event is determined by the thermal load restriction in combination with any speed restriction as defined by the dynamics of the whole shafting. In particular for large container vessels, it is of outmost importance to be able to keep up with the speed defined by the line in which the vessel is operated. For this type of installation it is commonly observed, that a maximum engine speed is set due to the shafting dynamics. This results in significant reduced speed capability of the vessel. For many vessels this is accepted, but one common way to deal with such restrictions has typically been to modify the shafting by increasing the shaft diameters significantly in order to shift the torsional resonance frequency of concern. Apart from having negative consequences on the installation cost, this solution will also negatively affect the alignment conditions due to the higher bending stiffness of the shafting. In this paper a number of new methods are shown which enables the vessels operators to overcome the limitations set by the shafting dynamics. The methods has been developed, and patented, in co-operation between Mitsui Shipbuilding & Engineering Co Ltd and MAN B&W Diesel A/S. The limit set by the dynamics is typically a barred speed range and/or an upper speed. These limits are defined from a shaft stress analysis, typically a torsional vibration calculation, which will differ significantly between normal operation and misfire operation. The methods presented in this paper deals with ways of modifying the torsional stress in the shafting without increasing the shaft diameters, and in ways which allow higher operational speed in case of misfire. Extensive testing has taken place at the engine builder during shop trial in order to document the system. Furthermore the system has been tested during operation of the engine in a vessel. Thus the system is released as an accepted operational mode for MAN B&W engines. The system is in particular easily implemented for the range of electronically controlled ME-engines, and it can also be used for conventional mechanically controlled engines. It is possible to implement the system on already existing installations.