Examination of the intact stability and the seakeeping behaviour

5.3 Recommendations
accelerations seems not to be a pure stability problem. Rather high accelerations occur for a
wide scope of examined GM solid values (see gure 5.1).
Regarding the occurring maximum transversal accelerations, a comparison with an usual value
for the dimensioning of the container lashing equipment is interesting, since a reference value
for the maximum transversal accelerations acting on humans on the bridge does not exist. For
example according to the DNV rules [12], the transversal dynamic acceleration taking eect on
container lashings on deck, shall be taken not smaller than ∼ 0, 5 g. The calculated accelerations
on the bridge partly exceed the triple of that value. In the same time the normalized mean value
of the occurring acceleration amplitudes mostly exceeds a value of ∼ 0, 5 g (refer to the respective
histograms in chapter 4). Such high transversal accelerations are considered being denitely not
acceptable.
5.3 Recommendations
The simulation results show, that the ballast arrival loading condition of container vessels is
not a safe seagoing condition. The risk of encountering excessive rolling angles and very high
transversal accelerations on the bridge in heavy sea is increased signicantly. Based on the
simulation results, the following approaches to reduce the risk of accidents can be exemplied.
5.3.1 Stability
Concerning the stability of the vessels, no universally valid GM value, which reduces the risk of
accidents, can be derived from the analysis. The seakeeping behavior of a ship apparently has
a lot of important additional inuence factors. For instance two of the factors which have to
be considered, are the ship's trim and the hull form. Altogether the factors can form a critical
ship situation consisting of the ship's stability, the ship's trim, the ship's hull form and so on.
To identify such critical situations for each loading condition, seakeeping calculations have to
be done with adequate methods for each vessel individually. A general prediction of a critical
situations is not possible until now. For three of the vessels in ballast arrival loading condition,
the most critical situation is determined in chapter 6.1.
5.3.2 Roll damping
The enlarging of the roll damping, no matter how the damping is done, reduces the roll motions
and transversal accelerations on the bridge. There are dierent ways to increase the roll damping
of a ship. According to Abdel-Maksoud [11] the following possibilities t for this purpose:
ˆ Enlarge the bilge keel area → roll damping by ow separation at the bilge keels
ˆ Increase the ship's speed at low speeds → roll damping by shear stress on the hull, angular
incoming ow on the rudder and immersed transom
ˆ Integrate a roll damping tank → roll damping by e.g. a sloshing uid
For a vessel already in service, the bilge keels could be modied easily. Though such a modication
would not have a deciding inuence on the transversal accelerations. In appendix B.1 a
graph can be found, where the bilge keel area is changed for Vessel No. 13, being the vessel
with the highest occurring transversal acceleration value during the examination. In the graph
the vessel in ballast arrival condition encounters the seaway of accident situation 2. it follows,
that the enlarging of the bilge keel area by 50 %, just provides a reduction of the transversal
accelerations of about 10 %. A general advantage of bilge keel is, that they also function with
zero speed.
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