Page 382 - Marine Structural Design

358 Part III Fatigue and Fracture

Riser

H

Figure 18.2 Spanned Riser Attached to Platform

The wave force intensity is denoted as F(x). The moment at the span center is given by:
-15
M = f[x-(-2O)]-F(x).dx
-25
The long-term distribution of individual wave heights is given by:

where, H, =2.7, C4.462, D=0.928.

The number of waves exceeding a wave height H per year is given by
N = No[l - PL(H)]
where No is the total number of wave in one year, No=106.72. The wave force is calculated
based on Morison's equation,
1 nDz
F = --pC,D. v2 -+ pC, -a
2 4
where C, =1.0, C, =2.0
Considering the wave: H=l 1 .Om, T=l 1.7sec, the angular frequency is:
2a
w = - 0.537s-'
=
T
Applying linear wave theory, the wave number k is given by:
W2 k . tanh( kd)
-=
g