P(H) versus Hsis plotted on probability paper for the log-normal dis-
tribution (Fig. 14.26). The interval of measurement is 1 day
1/365 yr. P(Hs) for a return period of 20 years is
P(H)
20
1
365
1.37^10 4.
620 WAVES AND OFFSHORE ENGINEERING
Fig. 14.26 Log-normal distribution (Worked example 14.1, part 2)
Extrapolating the straight line on the probability paper, Hscorresponding
toP(H)1.37 10 4 is the value Hs9m.
Worked Example 14.2
A submarine pipeline of internal diameter 275 mm rests on a rocky sea
bed at a depth of 20 m. The pipe is made of 16 mm thick steel with a
100 mm thick concrete coating. Waves of 6 m height and 9 s period
approach the shore with the crests at an angle of 60° with the pipeline. The
prevailing current of 0.5 m s^1 is along the same direction as that of the
waves. Check whether the pipe is stable against rolling.
Assume the following: CD1.1;CM3.1;CL1.2; density of sea
water1030 kg m^3 ; density of concrete2700 kg m^3 ; density of steel
7800 kg m^3. The coefficient of friction for the rock–pipe interface is 0.3.
