VIBRATION ANALYSIS OF 140,000M~3 MOSS TYPE LNG CARRIER FOR THE SN?HVIT PROJECT

摘要

It is an important factor for the safe operation of LNG carriers, especially operated in harsh environment, to have a highly comfort accommodation space, so that the ship's crew can take sufficient rest to concentrate in their work. For this purpose, minimizing vibration level is an essential factor. The subject LNG carrier is planned to be operated in harsh environment such as North Sea and North Atlantic Sea, and it is required by the Owner to obtain DNV notation COMF-V(1), which is the highest grade of comfort class with regard to noise and vibration. In order to satisfy the severe vibration requirement, extensive study was carried out during the design stage in view of both hydrodynamics and structure. In case of the steam turbine ship, the main source of vibration is propeller excitation forces. Therefore, it is important to select the most suitable number of blades of the propeller in view of vibration. At the first design stage, 3 types of propellers with 4, 5 and 6 blades, and the suitable superstructure corresponding to each blade were pre-designed and compared, in view of propeller excitation forces, vibration response and also the propulsive efficiency. After the optimum combination of the number of propeller and the superstructure were decided, the more elaborate design of hull form and propeller were carried out, and it was confirmed by model test and calculation that propeller excitation forces were low enough. Then, full ship finite element vibration analysis was carried out by correctly modeling the ship's entire structure, cargo tank structure. As a result, it was confirmed that the vibration levels of accommodation area were within the allowable limit. Finally, the results of the measurements of fluctuating pressure, vibration responses during sea trial are also presented, and show good agreement with predicted values.