Long stroke and low speed diesel engines with six or more cylinders have a likelihood of vibration problems related with resonant response of engine due to the guide force moment which can causes H-mode and X-mode vibration. Top bracings are installed between the main engine and the hull structure to reduce the dynamic vibrating characteristics of the onboard engine body and coupled effect on ship structures. Without considering a natural frequency of coupled main engine and ship hull structures, installation of top bracings can deteriorate the vibration problems.
In this study, a resonant response of engine and global ship structures with and without top bracings was evaluated using a global vibration analysis by finite element models (FEM). The resonant response from global vibration analysis (FEM) is compared with a measured resonant response during sea trials. The results showed that the top bracing controls a natural frequency of engine and coupled hull structure to avoid a critical vibration. In addition to the evaluation of resonant response, guideline for installation of top bracings on specific type engine (MAN B&W 6S70MC engines) of suezmax tankers is suggested.