Vibrations in main structures and equipment on board
In the previous blog, we addressed the different tests we carry out in the sea trials of a ship. And we make a brief explanation of the tests:
A) Power measurements to the propulsion shaft with torque meters and ship speed.
B) Consumption and temperature tests according to the driving regime in the primary and auxiliary propulsion machinery.
In this blog, we will address the different types of solutions that must be carried out at the ship design stage to minimize onboard vibrations to the point that they can be within the parameters specified in the IMO and ISO 4867-ISO 4868 standards.
Subsequently, these parameters are verified or analyzed in sea trials to measure vibrations at different points of the ship with special equipment and sensors for each case. At the same time, this type of verification is carried out by companies certified by the Class Companies, and they have qualified and trained personnel to collect the different types of readings in:
C) Vibrations of the Main Machinery and Auxiliary Equipment.
D) Measurement of reactions in axle and horn line bearings.
E) Measurement of torsional, lateral, and axial vibrations.
Conceptual design is where the vibration prevention process should begin. If vibration problems, repeatedly identified by experience as the most important, are addressed in the early stages of design, serious problems involving significant remediation efforts and costs can be avoided.
The focus is on vibration planning in the early stages of conceptual design, where specific shipbuilding programs help naval architects simulate the stresses in the different structures on the ship. Consequently, apart from calculating the different types of stress in the structures, these programs give us readings of the different types of vibrations present according to the different frequencies and conditions the ship will encounter. Naval architects and engineers make the pertinent modifications to the projects before their development.
The four most important elements to consider in ships according to vibrations are:
1. Reduction of the excitation amplitude force F.
In propeller-induced ship vibration, the excitation can be reduced by changing the unstable hydrodynamics of the propeller. This may involve changes to lines or clearances to reduce wake non-uniformity or geometric changes to the propeller.
2. Increase in stiffness K.
Stiffness is defined as the spring force per unit of deflection. In general, stiffness should increase rather than decrease when variations in stiffness produce variations in natural frequency. It is not recommended to reduce system stiffness to reduce vibration.
3. Avoid radio frequency values near the unit. ω/ωn = 1
It is the resonant condition. At resonance, excitation is only opposed by damping. Note that ω/ωn can be varied by varying the excitation frequency ω or the natural frequency ωn. The spectrum of ω can be changed by changing the RPM of a relevant rotating machinery source or, in the case of propeller-induced vibration, by changing the propeller’s RPM or its number of blades. ωn changes due to changes in the system’s mass and/or stiffness; increasing stiffness is the usual and preferred approach.
4. Increase in damping ζ.
The damping of structural systems, mainly ships, is small. Therefore, the vibrational amplitude is approximately independent of damping, except for very close resonance. Additionally, significantly increasing damping in systems such as ships is challenging. It is generally the least effective of the four parameters available to the designer for implementing changes in the ship’s vibration characteristics.
At SOUTHERN OCEAN MARINE ENGINEERING SOLUTIONS, we ensure that the vessel, from its construction stage, is built under the standards of the class and flag, meeting the requirements of the Shipowners and Shipping Companies.
In the next blog, we will continue addressing the different tests on ships in sea trials.
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