BİLGE AKUSTİK
Noise and Vibration Control Engineering
“I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”
Hull Girder Vibration
Hull Girder Springing
A 1,082-foot long oil tanker on trials was ballasted down to approximately two-
thirds of its full load displacement in order to conduct a 4-hour economy run. Once on
course and making about 16 knots, the hull began to flex or spring in its two-noded
vertical mode. Seas were approaching from about 10 to 15 degrees off the bow and
were averaging approximately 40 feet between crests.
Symptoms
The hull girder was experiencing visible two-noded vertical vibration with an
estimated amplitude of 12 to 18 Inches. This motion was especially visible when
standing on the main deck aft and slightly forward along the deck edge. The frequency
was low enough to be counted and was about 32 cycles per minute. The springing was
very noticeable In the pilothouse seven levels above the main deck where the vertical
motion of the stern was translated Into a fore and aft motion. The springing was also
very noticeable In the lower engine roan as a vertical heaving motion.
Problem
Springing occurs when the range of frequencies of the wave energy includes the
two-noded vertical natural frequency of the hull girder. The waves, even though they
may not be very large, act as a somewhat random excitation at the bow of the ship
causing the hull girder to vibrate. This phenomenon most often occurs in large ships
with low resonant frequencies, such as Great Lakes bulk carriers. However,
occasionally It occurs In large oceangoing vessels as In this example. Figure 1 shows
the hull girder mode shape and vibratory response at the stern of the tanker in this
example. For fast ships with relatively light scantlings, hull girder stresses due to
springing can be of the same magnitude as those caused by wave bending and thus
may be of concern from the point of view of structural fatigue and buckling. Springing is
a function of ship speed, the heading, sea conditions, bow and stem lines, ship length,
distribution of cargo or ballast weight, and the amount of energy contained In the sea.
All ships experience springing to some extent. However, it only becomes a concern
when the frequency of the first hull mode Is within the range of frequencies of the wave
spectrum. This occurs primarily with ships over 900 feet In length. Some recent
experience indicates the existence of a similar phenomenon occurring In ships which
are very flexible In torsion, such as container ships. In this case, torsional resonance of
the hull girder may be excited in oblique waves.
Table and Figure 1. Problem Locations and Springings Phenomena
Solution
Springing response can be reduced by changing the frequency of encounter with
the waves by changing speed or heading. Usually, significant changes In these
parameters are required In order to reduce springing. Another potential solution Is to
change the ship’s distribution of cargo on ballast In order to change the hull girder
natural frequency, although very large changes are required to gain only small
reductions in springing. This is usually not feasible once at sea, and may not be feasible
at all for most ships.