Page 33 - Marine Structural Design

10 Part I Structural Design PrincipreS

Weibull parameter for wave heights may be found from API RP2A for Gulf of Mexico
offshore.
There are three approaches for predicting accumulated fatigue damages accounting for wave
scatter diagrams, namely:
0 Frequency-domain (e.g. spectral fatigue analysis based on Rayleigh model or bi-
model)
Time-domain (which could account for non-linearities and contacVIiiction due to soil-
0
structure interactions)
A mixture of frequency-domain and time-domain approaches (e.g. use stress range
spectrum from frequency-domain fatigue analysis and rain-flow counting approach to
sum up the fatigue damages due to individual sea-states).
As an alternative to the S-N curve approach, fracture mechanics has now been used for
evaluation of the remaining strength of cracked structural connections and in planning
inspections of welded connections. There is an approximate linear relationship between the
crack growth rate and AK on a log-log scale. This is generally characterized by the Paris
equation:
da
- C(AK)"
=
dN
where
AK = K,, - K,,

K,, and Kh are the maximum and minimum values of the stress intensity factor, at the
upper and lower limit stresses during a cyclic loading. The values of material properties C and
m may be found fiom design codes for typical materials used in marine structures and other
types of steel structures. The stress intensity factors may be available fiom handbooks for
simplified structural and defect geometry's and loads.

1.3 Structural Reliability Applications
1.3.1 Structural Reliability Concepts

Component reliability concerns the failure probability modeled by a single limit-state function.
It is a fundamental part of the structural reliability analysis since all marine structures are
composed of their components.
The concept of structural reliabiIity is illustrated in Figure 1.4, where load and strength are
both modeled as random variables. Failure occurs when load exceeds strength. Denoting the
probability density function for load and strength as F, (x) and FR (x) respectively, the failure
probability may then be expressed as:
F,
P, = P(S 2 R) = 1 (X)F, (1.7)