BİLGE AKUSTİK
Noise and Vibration Control Engineering
“I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”
Local Vibration
In addltlon to hull girder vibration problems, there may be problems with major substructures such as the deckhouse, mast, propulsion system, or large portions of the deck structure. Such large sections of the ship, when they vibrate, will affect the vibration of the hull girder Itself. In an analysis, the hull model should Include a representation of that structure. One rule of thumb is that if the mass of a substructure is greater than 1/2 percent of the displacement of the ship and has a natural frequency close to a hull frequency, it should be represented as separate from but attached to the hull girder.
The term "local vlbratlon" usually refers to the vibration of smaller structures or pieces of equipment, small enough so that its vibration will not significantly effect the hull girder vibration.
Examples include:
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Plates and panels - Bulkheads, webframes, deck or bottcm sections,
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Beams - Shafting, masts, cranes, antennas, pipes
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Plate and beam assmblies - section of deck, bottom, bulkhead, shell, or superstructure
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Equipment mounted on elastic foundation - boilers, condensers, turbines, auxiliary machinery, electronic equipment (may be mounted on resilient mounts ‘or hard mounted to structure)
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Appendages - rudders, skegs, roll fins, propeller blades
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1. Excitation of Local Building Elements
The sources of excitation for local structures include all of the sources mentioned for hull girder excitation: propeller, engines, and waves. These excite Iocal structures through the hull girder motion. Even if the hull motion is acceptable, a local structure mounted to it may resonate at one of the
frequencies of excitation. In addition, auxlllary equipment may excite itself or nearby structures or equipment. In the case of appendages, the flow of water past the appendage and Its interaction with the structure can excite vibration.
2. Evaluation
Like ship hull vibration, local vibration will be considered unacceptable if it causes crew discomfort, structural fatigue, or equipment failure. Often, however, an issue must be addressed and evaluated on an individual basis.
The natural frequency of the local structure is probably the most important feature in evaluating a problem. In general, if the natural frequency of a structure is about 15 percent above or below an excitatory frequency, there will be no problems. The natural frequency can be known from measurements made while the ship is underway or when a machine is running. Another method of finding the natural frequency is to artificially stimulate it and measure the resulting vibration. Hitting the object with a hammer (usually a rubber or leather hammer is preferred to minimize high frequency effects) will cause it to "ring" at its natural frequencies. Large structures may require heavy wood to obtain adequate stimulation. A more controlled method is to use a vibration generator to provide a sinusoidal force at various frequencies. The weight of the vibration generator should not be so much as to affect the vibration of the object.
If system hardening or other correction is envisioned, it's usually a good idea to find the natural frequency available, compare it with the calculated natural frequency to see how good the model is, and then calculate the frequency of the object with the added element.
Often changes in structure have less of an impact than expected. It would also be prudent to test the modified build to verify the affected fix.
Related formulas can be used for calculations. Many local issues are due to panels. Most panels on ships (decks, bulkheads, etc.) will have natural frequencies somewhere between simple supported and clamped states.
3. Corrective Techniques
A large part of "fixing" vibration problems consists of redoing something that wasn't done properly in the ship's design. Therefore, it is often appropriate to review certain design guidelines to see if they are valid. The following may help:
• Vertical bulkheads (longitudinal and transverse) should be constructed as continuous as possible from deck to deck. Where this is difficult, perhaps struts can be used for continuity.
• Make sure heavy equipment is mounted on beams, bulkheads, frames or nets, preferably in both directions (longitudinal and transverse).
• Avoid consoles to support equipment (unless designed with vibration in mind).
• Reinforce carrier foundations.
• In cases where large forces are involved between machines and foundations (engines, turbines, gears, thrust bearing), manufacturers should be consulted regarding the stiffness of the foundations.
Often the solution to local vibration problems is to increase the stiffness. The figure shows a few examples and some possible solutions.
In Figure (a), a piece of equipment is mounted on a foundation whose feet do not rest on the deck's support beams. The deck flexes and the equipment wobbles or vibrates vertically. The solution may involve moving the equipment so its legs rest on the joists, adding additional legs resting on the joists, or adding a cross brace (above or below the deck).
In Figure (b), the feet of the foundation are very flexible. This can be corrected by installing cross braces between or on the outside of the legs or by installing planks along the legs and from the deck to the platform.
In figure (c), a panel is vibrating too much and needs to be cured. Natural frequency is controlled by both dimensions, but primarily by the shortest aperture. In hardening the panel, efforts should be made to reduce the shortest span, as shown in the last two solutions.
