Slow speed engines in LNG Carriers means introduction of an excitation source with forces of considerably magnitude acting over a wide range of frequencies. A vibration study of a large LNG Carrier (235K m3) with spherical tanks reveals that the response of the containment system caused by the main engine is in the same order of magnitude as those excited by the propeller. The analyses also show that in cases magnified vibration level may occur, this is associated with local resonances regardless the source, engine or propeller. Thus the vibration study of the LNG Carrier with spherical tanks reveals that slow speed diesel engine is a feasible solution of propulsion. Detailed vibration studies of local response in the tank area caused by propeller and/or slow speed diesel engines has not been performed for LNG Carrier with membrane containment system. However, as for the spherical tanks, thoroughly investigations with regard to global and local resonances have to be performed. Preliminary investigations of the dynamic behaviour of the membrane containment system itself indicates that internal magnification is low in the actual frequency range for propeller and slow speed diesel engines. However, coupling effects of the membrane system to the ship structure has not been investigated, and further analyses have to be performed. Generally, the analyses reveal that it is necessary to analyse the vibration behaviour of LNG Carriers case by case. The present study of the 235K m3 LNG Carrier was performed assuming two 4-bladed propellers and two 7-cylinder engines. Another number of blades and/or number of cylinders may demand other measures to obtain acceptable vibration levels. In the same manner, different structural design of the membrane tankers (frame spacing, stiffener dimensions etc.) may result in different vibration reducing measures although the excitation frequencies are similar. Introduction of two propellers and/or two slow speed diesel engine means that the phase between the different sources is important with regard to the response at different locations. If two propulsion units are allowed to run freely, modulation and/or beating of the vibration levels will occur. From a habitability point of view this is unwanted. On the other hand by using devices keeping constant phase between the two propulsion units, one may positively utilise the effect of having two sources of equal frequency. By adjusting the phase between the sources lowest possible vibration levels at selected positions may be obtain. For the present study of the LNG Carrier with spherical tanks, phasing of engines/propellers is not found necessary regarding vibration response of the containment system.

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