SUB-COMMITTEE ON STABILITY AND LOAD LINES AND ON ...

Ref.: 391/09 and 520/09
BSU
Bundesstelle für Seeunfalluntersuchung
Federal Bureau of Maritime Casualty Investigation
3.2.8.5 Effect of free surfaces of liquid on stability
During the course of the investigation, the question of whether it would have been
possible to make the vessel softer by partly filling some of the ballast water tanks
was raised. A reduction in stability usually occurs due to the free surfaces of liquid
and the liquid in the tank can generate additional roll damping when the tank's natural
frequency approaches the roll period of the vessel. This is the principle on which the
so-called anti-rolling stabilisers work successfully when they are adapted to the
vessel's shape in the design process. However, by installing such stabilisers the
operator of the vessel subjects itself to disadvantages from a formal perspective
because the effect of the free surfaces, which are regarded as static, is deducted
from the stability calculation and a dynamic consideration of the positive effect of the
roll damping is not included in the conventional stability assessment. In the present
case, the vessel did not have any anti-rolling stabilisers and the only option was to
partly fill some of the ballast water tanks. However, since there were problems with
the longitudinal strength in any case, removing ballast was not an option. The
flooding of additional ballast tanks to partly fill them would not have achieved
anything in terms of stability because the GM correction via the free surfaces is
compensated by the fact that the centre of gravity shifts downward due to the
additional ballast. The calculations relating to partly filling tanks 4.11 and 4.12 also
delivered no meaningful result since the true tank geometry with all fittings cannot be
logged sufficiently accurately with the calculation software. It was possible to discern
that the water in the tanks ran back and forth, but that the energy was lessened by
local effects without bringing about any significant roll damping. To achieve a
measurable effect, the tanks would have had to cover the entire breadth of the
vessel. Such tanks are not available and in structural terms cannot be retrofitted due
to the continuous pipe tunnel amidships. Therefore, in the course of the calculations
it was assumed theoretically that a bottom tank without fittings ran from one side of
the vessel to the other in order to estimate what effects can actually be achieved by
free surfaces. To that end, a cuboid bottom tank from Frame 84 to Frame 125 with a
length of 32 m and a breadth of 24 m was modelled. The height of the tank
corresponds to the height of the double bottom (1.80 m) and with a permeability of
98%, in the case of sea water, a water mass of 1,398 t is obtained when completely
filled. The moment of inertia of such a tank would be 36,864 m 4 with a theoretical
reduction of the initial GM of about 1.90 m. However, this reduction in GM is only
relevant at small angles as long as water does not hit the top of the tank, which
would definitely be the case with a heel of about 4.2°. In formal terms, the influence
of the free surfaces on the GM reduction would be the same at each stage of partial
filling. However, the effect diminishes as the tank fills because the total mass rises
and at the same time the centre of gravity moves downward. The effect the partly
filled tank has on the static stability is shown in Figure 29. Here, 'Fill Step 11'
corresponds to 100% filled, i.e. 1,398 t. It is also evident from the figure that the
hydrostatic effect of the partly filled tank does not have any material influence on the
stability.
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