Page 85 - Marine Structural Design

Chapter 3 Loah and Dynamic Response for offshore Structures 61

Table 3.8 Long-term Extreme Values of Dynamic Stress for Deck Plate
(Zhao, Bai & Shin, 2001)
I I I* beriod Number of Cycles (1/ hour) I
b
Wave Spectrum
50 I 100
t
W156 JONSWAP 2476.9 509.2
W156 Bretsch. 2166.4 500.9
W391 JONSWAP 1751.6 694.0
1 W391 1 Bretsch. 1 1676.6
(stress in unit: Kgf/cm2)

0 W156, JONSWAP W156,Bretschneider
A W391, JONSWAP o W391, Bretschneider
o 20 years 50years
x t 00 years

Figure 3.15 Long-term Extremes of Dynamic Stress Responses for Deck
Plate (return period = 20,50, and 100 years) (Zhao, Bai &
Shin, 2001)

3.5.4 Prediction of Most Probable Maximum Extreme for Non-Gaussian Process
For a short-term Gaussian process, there are simple equations for estimating extremes. The
Most Probable Maximum value (mpm), of a zero-mean narrow-band Gaussian random process
may be obtained by Eq. (3.6), for a large number of observations, N. In this Section, we shall
discuss the prediction of most probable maximum extreme for non-Gaussian process based on
Lu et a1 (200 1,2002).
Wave and current induced loading is non-linear due to the nonlinear drag force and free
surface. Non-linearity in response is also induced by second order effects due to large
structural motions and hydrodynamic damping caused by the relative velocity between the
structure and water particles. Moreover, the leg-to-hull connection and soil-structure
interaction induce structural non-linearity. As a result, although the random wave elevation can
be considered as a Gaussian process, the response is nonlinear (e.g., with respect to wave
height) and non-Gaussian.