Page 282 - Practical Design Ships and Floating Structures

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number of situations the ship will experience to those defined by significant wave height, mean wave
period and ship heading towards the waves. The cargo units have been assumed to be lashed containers,
according to the cargo model described above, with F1= F2. All cargo units are assumed to have equal
properties, and the applied pre-tensions are according to the regulations of the Swedish Maritime
Administration (1994). Sea states have been assumed to last for four hours. The probability of
occurrence of the sea states is based on the wave statistics of Hogben et al. (1986), and the wave
simulations are based on the Jonswap spectrum. The motion simulations have been made for four
hours, with an increment of 0.05 rads and about 40 frequencies. In all cases the same wave
realisations and sample of cargo units have been used in order to enable comparison.

In Figure 2 and Figure 3 the risk level is shown for three different shipping zones, which stands for
areas of restricted operation with different cargo securing regulations, according to the Swedish
Maritime Administration (1994). Zone A is sheltered waters, with the lowest requirement on pre-
tension, and zone C is unrestricted operation, which imposes the strongest rules of securing. The route
used for the case studies comprises both zone A and B. Thus the parameter influence is discussed for
cargo lashed according to the regulations of zone B. As can be seen, for a ship operating in zone B
with cargo secured for zone A the risk is significantly larger than when the cargo is secured for
unrestricted operation (zone C). This points at the importance of securing the cargo according to the
area in which the ship will operate. It is interesting to note that for the variation of GMo, the parameter
influence changes when the degree of pre-tension in the lashings is varied.

Figure 2 shows how the GMo of the vessel influences the risk of cargo shifting. GMo, which influences
the stability and natural roll period of a vessel, strongly influences the risk of cargo shifting and an
unfortunate choice of GMo can have a large negative effect on the risk of cargo shifting. As can be
seen the largest GMo results in the largest risk, which can be expected since a large GMo means a very
stable and thereby stiff ship with large roll acceleration. However, a very low GMo (poor stability) will
also result in a relatively high risk, since the roll amplitude is relatively high. The natural roll period is
proportional to the inverse of GMo, which means a vessel with a high GMo will have a low natural roll
period. For the studied case the vessel with a GMo of 2.48 m has a natural roll period of 10 s. Since
wave periods are generally in the range of 5 to 10 seconds, this means the stable ship will more often
be subjected to waves exciting large roll motions.
1
- 0 75 __ Lashed for:
a,
t
3 050
a
u)
0.25
0.00
0 15 0 68 2.48
GMo [ml
Figure 2: Risk of cargo shifting as a function of GMo (route in zone B, at a speed of 15 knots)
In Figure 3 the influence of speed is shown. The influence of speed mainly shows the importance of
operational aspects. If the speed is reduced when severe ship motions are experienced the risk level
can be reduced considerably. For example the risk at 10 knots is only one third of the risk at 15 knots.
Naturally, constant speed in all sea states is unrealistic, especially at 20 knots.