Sea-going vessels are often a floating vibration laboratory, and keeping those vibrations at an acceptable level can be a challenge. The solution of vibration problems quite often requires a four-step process:
- Determine the vibration frequency and vibration mode shape of the item that is vibrating over the vessel speed range, and match the vibration to excitation sources;
- Determine from the vibration survey if the vibration is forced or resonant;
- If forced, determine what can be done to repair the source to reduce excitation, or isolate the source;
- If resonant, determine by analysis if the stiffness across the vibrating system can be changed, and/or the suspended mass of the system can be changed to move the resonance out of the operating speed range.
For example, a high performance tug had an unbearable noise level in the Captain’s stateroom located in the deck house just aft of the bridge. A check of the vessel mast revealed that two diagonal mast stays made from steel pipe and welded to both the mast and the deckhouse top-side corners were vibrating heavily in the fore-aft direction. The stay vibration frequency method matched both the noise frequence and the main engine 3rd order.
An analysis of the pipe stays as fix-ended beams confirmed that the engine 3rd order could excite a stay resonance in the fore-aft direction. The problem was solved by changing the stays from fix-ends (welded), to pin-end (yokes and pins), thereby changing the resonant frequency. The noise disappeared, much to the Captain’s approval.