Structural response of the ship hull elements subject to excitation generated by the main engine

Abstract

The central subject of the investigation in this paper is real existent bulk carrier m/s Miedwie (DWT 30000). During exploitation in certain operation condition excessive vibration levels were observed in the engine room and reported by the crew. Moreover, structural failures occurred in the form of extended cracks along foundation of hull supporting elements connected to the engine. Due to all these facts vibration of the main engine and the hull supporting structure in the engine room has been studied in detail. The bulk carrier m/s Miedwie is equipped with the main engine of 6RTA48T-B type produced by Wartsila Company. It is a low-speed direct-reversible two-stroke engine with 6 cylinders. In the configuration there are also two lateral side stays of friction type installed on exhaust side of the engine. Their role is to reduce the engine vibration and the vibration transmission to the ship’s bottom and side structure. Numerical analysis has been performed to find out the reasons of the excessive vibration problem. For this purpose a very accurate finite element model of the aft part of the ship including engine room was created. The procedure of model generating was divided into two parts. First step is geometrical and initial FEM modeling of the ship structure in Poseidon GL software. The second stage is mesh modification, adding of the propulsion system and other debugging of the numerical model in ANSYS software. The main engine has been modeled with maximum concern about its stiffness and mass distribution. Interaction of the intermediate shaft is expected to be important, thus the whole simplified shaft line is represented in the model. The superstructure has been incorporated in the model in approximate way to represent only mass inertia interaction. Forced harmonic vibration analyses have been performed in ANSYS APDL software. Studying of the forces induced in this type of engine was done. Measurements carried out on board the ship showed vibration of the engine with the 6-th order frequency dominating component. Therefore it has been concluded that the mode of the occurred lateral engine vibration (rocking) was of so-called “H-type”. This type of excitation is caused by lateral guide forces and the value of that forces are known from Marine Installation Manual for engine. Firstly numerical analysis without influence of lateral stays was performed. The natural frequency of “H-type” vibration and corresponding amplitude were found. The influence of the double bottom structure stiffness on the engine natural frequency was determined. Comparison between service engine speed and engine speed when resonance effect occurred was done. Second analysis was dedicated to modeling of forced vibration with presence of the active lateral friction type stays. Due to high friction side stays were not installed properly, making joint between the engine block and the hull structure almost stiff. Resonance frequency and amplitude were also found and the comparison with service speed was performed. The formation of local stress concentration areas, which is able to cause the fatigue crack in short time, was observed. After all numerical simulations conclusions about influence of the hull supporting elements stiffness were made. Importance of the correct friction type stays installation was shown. Several recommendations about avoidance of the dangerous resonance effects during exploitation period were given.