Shipbuilding

Measurement and analysis of global hull and superstructure vibrations are carried out in order to investigate and define natural frequencies of the ship’s hull girder and superstructure. The measurement is carried out in the full range of revolutions of the propulsion engine in small steps, in order to determine whether any mode of operation of the propulsion (speed) is unfavorable for the structure of the ship. In this way, we can find out whether the hull or superstructure is in resonance with a frequency produced by the propulsion machine (usually a diesel generator) or a propeller (usually a propeller). If the natural frequency of the hull or superstructure is found in the range of speeds, not only the structure of the hull and superstructure is loaded, but also all installed equipment and devices.

We record the signals with the help of an acceleration transducer at one solid point of the hull (stern end above the propulsion) and the superstructure (highest, most often in the wheelhouse), in all three axes of the ship, because each axis has its own form of vibration or vibration response. After spectral analysis, we create a precise picture of the vibration orders (harmonics) and their amplitudes, which excite the global ship structure and through the report we provide useful information to both sides the Survey and the ship’s crew. If in any of the operating modes of the propulsion engine it is determined that there is a significant resonance, this mode should be avoided or repairs/reconstruction in overhaul should be made that would change the frequency of the hull or superstructure and move it from the main engine range of speeds.

Measurement and analysis of the vibration of the propulsion engine and diesel aggregate with generator (generator set) is performed in order to determine their total vibration levels (“vibration severity”) and assess whether the machine is suitable for long-term operation. We performe measurement at the nominal speed of the propulsion engine, and for the gen-set at their nominal speed and with the electric generator connected to the ship’s network. According to standards or manufacturer recommendation we perform engine and gen-set signal recording at solid points in all three axes.

The evaluation is performed after the analysis in effective vibration velocity amplitudes and if the values are to high it is necessary to make a spectral analysis so that the condition of the engine and gen-set can be described in more detail (cylinder operation, mounting, rotating and oscillating masses, forces from the moment of inertia …). It is important not only to confirm that a machine is new and work properly, but to make a decision for the service and repair of worn parts before failure or breakdown of the engine itself and the components on the same shaft line as well.