Measurement and analysis of global hull and superstructure vibrations are carried out in order to investigate and define natural…
Several mechanical quantities, both static and dynamic, can be measured on the ship’s shaft. With the help of tensometer method,…
Measurement and analysis of the vibration of the propulsion engine and diesel aggregate with generator (generator set) is…
Vibrations are measured in local areas where the crew works, performs daily tasks and places of accommodation for them and…
Global hull and superstructure vibration measurement
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.
ISO 20283-2: „Mechanical vibration — Measurement of vibration on ships — Part 2: Measurement of structural vibration”
Power/torque and torsional vibration measurement
Several mechanical quantities, both static and dynamic, can be measured on the ship’s shaft. With the help of tensometer method, with wireless sensor interface nodes, data collection gateway to computer can measure power or torque, but also vibrations which produce additional moment ie. additional torsional stresses. All values are depended on the speed of the shaft or the speed of the propulsion engine or propulsor.
It is possible to read the actual load of the shaft at the measuring point based on the analysis of the measured quantities, ie. the measured power and torque can be compared with those given by the propulsion engine in the whole range of engine speeds. The measurement analysis also shows the values of vibration (additional torsional) stresses, as well as their cause through various harmonics produced by the propulsion engine or propulsor. It can be determined if the total stresses in the shaft are above the allowable in some operating mode and whether they are caused by improper engine operation, propulsor imbalance or shaft misalignment.
„IACS Unified Requirements 2005/Rev.2 2015”
Vibration measurement of propulsion engines and diesel aggregate with generator
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.
1. ISO 10816-6:1995 “Mechanical vibration – Evaluation of machine vibration by measurements on non – rotating parts – Part 6: Reciprocating machines with power ratings above 100 kW”.
2. ISO 8528-9:1995 “Reciprocating internal combustion engine driven alternating current generating sets – Part 9: Measurement and evaluation of mechanical vibrations”.
Vibration measurement in crew and passenger areas
Vibrations are measured in local areas where the crew works, performs daily tasks and places of accommodation for them and passengers. It is determined by whether the total level of vibration transmitted to the body via the deck floor is allowed for a longer stay at that location. Excessive values can affect crew performance, fatigue, and passenger comfort during sailing. The measurement is performed on the nominal regime (nominal speed of the propulsion engines) and the signals are recorded in all three axes of the human body in the frequency range from 1 to 80 Hz.
After the third octave analysis and weighting with coefficients, we give the evaluation for each axis separately. Higher values need to be processed in more detail by spectral analysis in order to determine the dominant frequency or cause (propulsion engine, propulsor, aggregate unit …) of the high total vibration amplitude. In case of impermissible values at a local measured point, it is necessary to limit the retention time, but also at our suggestion in cooperation with the owner to carry out local remediation in order to move the response of the local structure outside the area of the machine or device.
1. Ministartvo mora, prometa i infrastrukture: „Pravila za statutarnu certifikaciju pomorskih brodova, Zaštita pri radu i smještaj posade“, (NN32/2018).
2. ISO 20283-5:2016; „Mechanical vibration — Measurement of vibration on ships — Part 5: Guidelines for measurement, evaluation and reporting of vibration with regard to habitability on passenger and merchant ships“ (zamjena za bivši ISO 6954:2000).