Section 5 Damping

5.1.1 Damping of ship vibration is complex and represents the greatest weakness in a dynamic analysis, particularly as most vibration problems occur in way of resonances, where the response is inversely proportional to damping.

5.1.2 This parameter has largely defied attempts to formulate a reliable theoretical predictive method, and measurements have indicated significant variations for similar and sister ships, without reasons being precisely quantified.

5.1.3 Damping occurs by way of dissipation of energy in various ways, which is finally converted into thermal energy.

5.2.1 Hydrodynamic damping is important for ship motions. However, this external damping is generally considered to be insignificant in relation to ship vibration.

5.2.2 Internal damping is generally considered to be predominant for ship vibration, for which the primary constituent is the behaviour of material under stress: essentially energy absorption by material hysteresis. This is increased by stress concentrations and residual stresses from welding. Hence, some variation in workmanship and welding quality may account for apparent differences in damping between similar ships. This assertion may be strengthened by measurements that have been taken on naval ships that tend to indicate lower levels of damping than those for merchant ships. This may be in connection with the higher standards of detail design, workmanship and welding quality required for naval ships.

5.2.3 Damping indicates a rising trend with increasing frequency. This may be explained by the effect of shear stresses becoming more significant for higher vibration modes.

5.2.4 Cargo damping is another constituent of internal damping, which, of course, varies according to the type of cargo. Reliable quantification of this effect is not presently available.

5.3.1 Critical damping is defined as the value of damping such that the response of a given mode to an impulse returns to equilibrium just without oscillating.

5.3.2 Damping is normally expressed as a percentage of critical damping. Ship structures are lightly damped; thereby a small percentage of critical damping applies.

5.4.1 Structural damping generally increases with increasing frequency.

5.4.2 Damping levels can vary significantly, even between sister ships, without precise reasons being quantifiable. Hence, LR's policy is to use damping levels that are ‘realistic to pessimistic’ in dynamic analyses, generally such that actual vibration responses are similar to, or less than, predictions.

5.4.3 As mentioned in Ch 1, 1.1 General description of document, the most notable frequency range in relation to global ship vibration is usually 5 – 20 Hz. LR uses damping, as shown in Table 1.5.1 Damping levels and their associated frequency ranges.