Page 283 - Practical Design Ships and Floating Structures

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1 .oo
- 0.75 Lashed for:
> 0 zone A
a,
a,
3 0.50 zone B
a =zone C
ln
0.25
0.00
5 10 15 20
Speed [knots]
Figure 3. Risk of cargo shifting as a function of speed (route in zone B, at GMo 0.68 m)
Calculations have also been made varying the cargo weight, the pre-tension in the lashings, and the
coefficient of friction. The intervals of variation have been chosen based upon a study performed by
Andersson et al. (1986). Figure 4 shows the relative risk as a function of the expected values of the
cargo parameters. The relative risk is expressed as a percentage of the risk level when the actual cargo
parameters are equal to the expected values. On the x-axis the cargo parameter values are given as a
percentage of the expected value. It can be seen that even moderate changes in the cargo parameters
can have a large influence on the risk of initial cargo shift. This points at the importance of proper
lashing of the cargo, and of controlling the cargo parameters, such as friction.
5 150
- -weight
>
a,
r pre-tension
.- ln 100
L ----friction
> .-
a,
-
t;j 50
0 2L \. -* --.
--.
0 50 100 150 200
% of expected value
Figure 4: Relative risk of cargo shifting as a function of cargo parameters
(route in zone B, GMo = 0.68, 15 knots)
4 DISCUSSION
In this paper a methodology for estimating the risk of initial cargo shift for a vessel carrying unit cargo
has been presented. The benefit of the method is that the risk of initial cargo shift is evaluated in a
large number of different situations that are weighted with respect to their probability of occurrence.
The results from the case studies show that the method can be used for comparing risk levels. They
also show how loading condition and speed have a large influence on the risk of cargo shifting. It is
also shown that controlling the cargo parameters, and establishing routines for appropriate lashing of
the cargo is an effective means of reducing the risk. Examining the risk of cargo shifting, and trying to
optimise the vessel and its cargo with respect to low risk may have a large positive effect on the
overall safety of the vessel.
As can be seen from the results the level of the risk of initial cargo shift is rather high. However, it
should be noted that the risk of initial cargo shift is really the risk of getting potentially dangerous
situations. The shift mode is set to an initial motion of the cargo, and the risk presented in the results is
the probability that at least one such situation will occur during the studied time interval, which
explains the rather high risk levels in the results. Furthermore, if the risk were calculated for the
lifetime of a vessel instead of one year the risk would be one in all cases. This would imply that
changes in ship handling, and cargo securing would have no effect on the risk of cargo shifting in the