Mechanical vibration is a ubiquitous source of pollution on board ships. Vibration is defined as the variation over time in movement or position of a mechanical system, with an amplitude that is alternately larger and smaller than a reference value. The parameters involved in vibration are frequency of vibration (in Hertz/Hz), amplitude or displacement (m), speed (m/s), acceleration and direction of displacement (m/s2).Some studies consider impedance, which is the dynamic force to which the structure is exposed over speed (Z = F/v N s/m).“Jerk” (m/s3) can also be defined as the derivative of acceleration with respect to time.

It is considered that:

- very low frequency vibrations correspond to frequencies of between 0 and 2 Hz

- low frequency vibrations correspond to frequencies of between 2 and 20 Hz

- high frequency vibrations correspond to frequencies of between 20 and 1000 Hz and more.    

They can be either continuous or periodic, and can occur randomly or transiently.

 

On board ships, personnel are subjected to whole-body vibration, in other words exposure of the whole body to vibration on all three axes (horizontal, vertical and lateral). Acceleration is generally between 0.006 and 0.6 m/s2 along each axis in response to movements of the ship, and are very variable depending on sea conditions, wind direction and the position of the subject on board (in the centre or nearer the sides).

Given the complexity of the phenomenon, there are no satisfactory threshold exposure values. An evaluation of exposure of individuals to whole-body vibration is given in international standard ISO 2631.

 

Equivalent acceleration

Annoyance

Less than 0.3 m/s²

No discomfort

Between 0.3 and 0.6 m/s²

Slightly uncomfortable

Between 0.6 and 1.0 m/s²

Relatively uncomfortable

Between 1 and 1.6 m/s²

Uncomfortable

Between 1.6 and 2.5 m/s²

Very uncomfortable

More than 2.5 m/s²

Extremely uncomfortable

 

Relationship between various levels of discomfort and equivalent acceleration (ISO 2631)

 To evaluate the severity of vibration exposure, the acceleration equivalent (aeq) is calculated, which is the effective acceleration value measured at the point of entry into the body along the three orthogonal directions. The acceleration signal is weighted for frequency and direction in order to take into account human sensitivity to these parameters. An equivalent value over 8 hours (aeq(8h)) is obtained by multiplying the acceleration equivalent as measured by the square root of the ratio of daily actual exposure/8 hours.

 In 2002 Europe adopted a Directive concerning minimum health and safety requirements regarding the exposure of workers to the risks arising from vibration. This Directive defines:

  • a level of exposure action value set at aeq(8h) = 0.5 m/s2 over 8 hours. If this value is exceeded, employers are asked to assess and monitor the risks, to reduce vibration levels, to inform and train workers and to arrange health surveillance.
  • a daily exposure limit value set at aeq(8h) = 1.15 m/s2, above which it is considered that regular exposure to vibration presents such a risk to health that vibration levels should immediately be reduced.