DNV-OS-C201 Structural Design of Offshore Units (WSD method)

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DNV-OS-C201 Structural Design of Offshore Units (WSD method)

Sec.7: Fatigue

A: General

A400: Simplified fatigue analysis

Sec.7
A 400 Simplified fatigue analysis

Sec.7 A
401 Simplified fatigue analysis may be undertaken in order to establish the general acceptability of fatigue resistance, or as a screening process to identify the most critical details to be considered in a stochastic fatigue analysis, see 500.

Sec.7 A
402 Simplified fatigue analyses should be undertaken utilising appropriate conservative design parameters. A two-parameter, Weibull distribution, see DNV-RP-C203, may be utilised to describe the long-term stress range distribution:

raster

n₀	=	total number of stress cycles during the lifetime of the structure
	=	extreme stress range (MPa) that is exceeded once out of n₀ stress cycles. The extreme stress amplitude: is thus given by
h	=	shape parameter of the Weibull stress range distribution
	=	intercept of the design S-N curve with the log N axis
	=	complete gamma function
m	=	inverse slope of the S-N curve
DFF	=	Design Fatigue Factor.

Sec.7 A
403 The simplified fatigue evaluation should be based on dynamic stress from a global analysis, with the stresses scaled to the return period of the minimum fatigue life of the unit. In such cases, scaling may be undertaken utilising the appropriate factor found from the following:

raster

n_i	=	number of stress variations in i years appropriate to the global analysis
	=	extreme stress range that is exceeded once out of n_i stress variations.

DNV-OS-C201 Structural Design of Offshore Units (WSD method)

Sec.7A 400 Simplified fatigue analysis

Sec.7
A 400 Simplified fatigue analysis