Examination of the intact stability and the seakeeping behaviour

6 Detailed examination
The evaluation discussed in chapter 5 discloses, that further investigations are necessary for the
better understanding of the high transversal acceleration problem. For this reason the following
detailed examinations are carried out.
At rst, the inuence of the ship's stability on the transversal accelerations is analysed. Furthermore
it has to be examine, if the large vessels, Vessel No. 11 and Vessel No. 15, are also
endangered to encounter high acceleration and rolling angle values in a loading condition similar
to the one of Vessel No. 14 (Chicago Express) when it experienced severe ship motions in
accident.
6.1 Variation of the GM values
In this chapter the inuence of the initial stability (namely GM) on the occurring transversal
acceleration for three out of the 15 examined vessels is determined. In order to consider the
probable inuence of the ship's size, a small, a mid-sized and a large vessel are chosen.
Based on the ballast arrival loading condition, the value of GM solid is varied by changing the
vertical center of gravity KG of the vessel. The calculations are performed without considering
the inuence of free surfaces, for the reasons named in chapter 3.6. The displacement as well as
the ship's trim of the analysed ballast arrival condition are kept constant. For each GM value
all three accident situations according to chapter 3.12 are simulated.
In addition it is estimated, if such a GM solid or respectively KG value is achievable from a
technical point of view. For instance a total ship's center of gravity KG at height of the inner
bottom is not realistic. The feasible minimum KG depends on the light ship's vertical center of
gravity, the position and lling of the ballast water and fuel oil tanks in the lower part of the
ship as well as cargo in the holds.
A shifted GM directly aects the seakeeping behavior. Hence to represent well the dierences,
a new set of transfer functions (RAOs) according to chapter 2.1.1 has to be calculated for each
GM value.
6.1.1 Small vessel
Vessel No. 13 was chosen as smallest vessel for the detailed stability analysis. Its relevant data
are listed in table 6.1. More main dimension data of this vessel can be found in the appendix A.13
on page 71. The GM is varied around the ballast arrival GM solid = 6.98 m within a range of
about GM ∼ = 3 ... 12 m.
In gure 6.1 the resulting transversal accelerations on the bridge are shown related to the
dierent GM solid values. All three accident situations result in a comparable curve slope. In
accident situation 1 and 2 the highest accelerations occur for the GM value of the ballast arrival
loading condition. In accident situation 3 the highest acceleration occurs for a higher GM of
about 8 m. With a higher or a lower stability the vessel would experience signicantly smaller
accelerations. Though just a GM solid value of 8 ... 9 m is realistically reachable for this vessel by
e.g. lling ballast water in tanks in low positions. Higher values in this simulation can only be
reached by decreasing the vertical center of gravity of the lightship weight, which is considered
being impossible for an existing vessel.
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