Page 391 - Marine Structural Design

Chapter 19 Simplified Fatigue Assesment 367

where the incomplete Gamma functions are defined as:

(1 9.1 3)

(1 9.14)
.=(:) e
and

(19.15)

where S, is the stress range at the crossing of two S-N curves (e.g. corresponding to fatigue
life of 10' 1.
The formulation for simplified fatigue assessment based on bi-linear S-N curves was derived
by Wirsching and Chen (1987), and appeared in DNV Classification Note 30.7 for ship
structures and DNV (2000) for steel offshore structures.
Tables of Gamma function and incomplete Gamma function are given in BV (1998) fi
convenience of fatigue damage estimation.

19.5 Allowable Stress Range
A fatigue check format based on the simplified fatigue assessment is:

'0 '0 nllormble (1 9.16)
where the design stress range So is the local stress range related to a given probability of
occurrence during the design life. The allowable extreme stress range So is determined
by solving Eq.(19.12) using the appropriate S-N curve, allowable cumulative damage ratio and
knowledge about stress distribution.
For prompt fatigue assessment, usually allowable extreme stress ranges have been pre-
calculated and listed in fatigue guidance documents as functions of the types of S-N curves,
Weibull shape parameter and the environment.
Reference is made to Zhao et a1 (2001) for formulations for the strength and fatigue
assessment of converted FPSOs.

19.6 Design Criteria for Connections Around Cutout Openings

19.6.1 General
Cracks around cutout openings (also known as slots) are often seen in many types of ship
structures, see Figure 19.2. Past studies (Bea, et al., 1995) have concluded that single-hull
tankers experienced most of these cracking in the side shell and bottom shell areas due to