Page 169 - Introduction to Naval Architecture

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Table 7.4 Percentage of time spent at sea in each weather group.

Weather group
1 II III IV V

General routes 51 31 14 3.5 0.5
Tanker routes 71 23 5.5 0.4 0.1

Figure 7.14 Probability curve

bending moment or stress any new ship should be able to withstand. If
the structure is overly strong it will be heavier than it need be and the
ship will carry less payload. If the structure is too weak the ship is likely
to suffer damage. Repairs cost money and lose the ship iime at sea.
Ultimately the ship may be lost.
If a ship life of 25 years is assumed, and the ship is expected to spend
on average 300 days at sea per year, it will spend 180000 hours at sea
during its life. If its stress cycle time is t seconds it will experience:

180000 X 3600/f stress cycles.

Taking a typical stress cycle time of six seconds leads to just over 10 8
cycles. If, in Figure 7.14 an ordinate is erected at this number of cycles,
a stress is obtained which is likely to be exceeded once during the life
8
of the ship. That is there is a probability of 10~ that the stress will be
exceeded. This probability is now commonly accepted as a reasonable
design probability. The designer designs the structure so that the stress
considered acceptable has this probability of occurrence.