Noise and vibration on maritime vessels
Encyclopedia
Noise and vibration on maritime vessels are not the same but they have the same origin and come in many forms. The methods to handle the related problems are similar, to a certain level, where most shipboard noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...

 problems are reduced by controlling vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...

.

Sources

The main producers of mechanically created noise and vibration are the engines, but there are also other sources, like the air conditioning, shaft-line, cargo handling and control equipment and mooring machinery.

Diesel engines

When looking at diesel
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

 driven vessels, the engines induce large accelerations that travel from the foundation of the engine throughout the ship. In most compartments, this type of vibration normally manifests itself as audible noise. The problem with diesels is that, for a given size, there is a fixed amount of power generated per cylinder. To increase power it is necessary to add cylinders but, when cylinders are added, the crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...

 has to be lengthened and after a very limited number of additions, the lengthened crankshaft begins to flex and vibrate
Torsional vibration
Torsional vibration is angular vibration of an object—commonly a shaft along its axis of rotation. Torsional vibration is often a concern in power transmission systems using rotating shafts or couplings where it can cause failures if not controlled....

 all on its own. This results in an increase of vibrations spread all over the ships structure. Crankshaft vibration can be reduced by a harmonic balancer
Harmonic balancer
A harmonic balancer is a device connected to the crankshaft of an engine to reduce torsional vibration and serves as a pulley for drive belts....

.

Turbines

Steam turbine
Steam turbine
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....

s and gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

s, on the other hand, when new and/or in good repair, do not, by themselves generate excessive vibration as long as the turbine blades are in a perfect condition and rotate in a smooth gas flow. But after some time microscopic defects appear and cause small pits to appear in the surface of the intake and the blades which set up eddies in the gas flow, resulting in loss of performance and vibrations. Vibration levels may change with different loading conditions or when doing a manoeuvre.

Other sources

Besides mechanical produced vibrations, other sources are caused by the motion of the sea, slamming of the vessel on the waves and water depth to mention just a few. The main problem here is that they are less controllable.

Exposure Limits

Exposure to noise and vibrations is regulated and limits for maritime vessels are given in the ISO standard 6954: Guidelines for permissible mechanical vibrations on board seagoing vessels to protect personnel and crew.

Because there are different noise regulations from country to country, the International Maritime Organization
International Maritime Organization
The International Maritime Organization , formerly known as the Inter-Governmental Maritime Consultative Organization , was established in Geneva in 1948, and came into force ten years later, meeting for the first time in 1959...

 (IMO) sets some standards for vessels. The table bellow gives some comparisons of preferred maximum noise levels on board of vessels and onshore levels.
Area Noise limit land (dB) IMO noise limit (dB)
workshop 70 85
kitchen 60 75
control rooms 55 75
offices 55 65
dining rooms 55 65
sleeping area 45 60

Prevention

At the design table, the naval architect makes the necessary choices concerning the ship's structure to achieve an optimized design towards noise and vibration control. Decisions are made about the engine and shaft, what kind of instruments and material can be used to reduce noise and vibrations throughout the vessel and what is the best way to implement these. Advanced computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

 technology tries to simulate these vibrations under different ship conditions to provide an overview of weak spots. The generated vibrations are also compared with the natural frequencies of the different parts/sections and adaptions can be done to the structure.

Control at source

To control the vibrations at the origin, isolating
Vibration isolation
Vibration isolation is the process of isolating an object, such as a piece of equipment, from the source of vibrations.-Passive isolation:Passive vibration isolation systems consist essentially of a mass, spring and damper ....

 fittings, elastic mounting of engines, elastic holding of pipes or dampers can be installed. These will absorb a part of the vibrations (and the noise) produced by the machines.

Maintenance

Regular maintenance will have a major influence on the performance of instruments and machines. Lubrication
Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...

of the joints, tightening of the bolts, good alignment of stern contour of the vessel, adjusting of variables following the weekly and monthly schedule are the most effective routes to noise and vibration control.
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