Springing

Springing as a nautical term refers to global vertical resonant hull girder vibration due to oscillating wave loads along the hull of the ship. The hydrodynamic theory of springing is not yet fully understood due to the complex description of the surface waves and structure interaction. It is however well known that larger ships with longer resonant periods are more exposed to this type of vibration. Examples are very large crude carriers and bulk carriers above 250 meter of length. In the extreme cases it may cause severe fatigue cracking of critical structural details, especially from moderate to rough head seas with low peak periods. Ballast condition is normally more easily excited by waves than cargo condition, hence the trade will also matter.

The first experience with this phenomenon is related to fatigue cracking on Great Lakes bulk carriers (700 feet) during the 1950's. Later 1000 feet Great Lakes bulk carriers got the same problems even if the strength requirements became stricter. Ocean going ships have not had this problem until recently when high tensile steel was introduced as a common material in the whole ship to reduce the initial costs. This makes the ships less stiff and the nominal stress level higher.

Todays ship rules does not account for this effect which may dominate the contribution to fatigue for some vessels.