Page 168 - Pipelines and Risers

Force Model and Wave Fatigue 141

Stress Transfer Function: The load transfer function per unit wave amplitude as a function
of wave frequency is multiplied by the stress transfer function per unit load to determine
the stress transfer function per unit wave amplitude as a function of wave frequency.

Stress Concentration Factor: The geometric SCF should be considered in the fatigue
assessment. The SCF is determined by Finite Element Analysis.

Hotspot Stress Transfer Function: The stress transfer function is multiplied by the stress
concentration factor to determine the hotspot stress transfer function.

Long-term Stress Range: Based on the wave spectrum, wave scatter diagram and hotspot
stress response per unit wave amplitude, the long-term stress range can be determined.
This is done by multiplying the ordinate of the wave amplitude spectrum for each sea state
by the ordinate squared of the hotspot stress transfer function to determine the stress
spectrum. The stress range distribution is assumed to follow a Rayleigh distribution. The
long-term stress range is then defined through a short-term Rayleigh distribution within
each sea state for different wave directions. This summation can be further used to fit the
Weibull distribution.

S-N Classification: For each critical location considered, S-N curves will be assigned
based on the structural geometry, applied loading and welding quality.

Based on the long-term hotspot stress distribution and the S-N classification, the fatigue
analysis and design of free-spanning pipeline may be conducted.

10.2.3 Fatigue Damage Acceptance Criteria
The design philosophy is that vibrations due to vortex shedding and oscillations due to wave
action are allowed provided that the fatigue acceptance criteria are satisfied for the total
number of stress cycles. The fatigue damage assessment is to be based on the Miner's rule:
(10.1)

where Dfat is the accumulated life time fatigue damage, q is the allowable damage ratio and Ni
is the number of cycles to failure at stress Si defined by the S-N curve on the form:
N, = c*s;- (10.2)

m is a fatigue exponent and C is the characteristic fatigue strength constant. The number of
cycles n, corresponding to the stress range block Si is given by
n, = p(*)fJ,,,, (10.3)