Page 455 - Pipelines and Risers

422 Chapter 22

REDUCED VELOCITY
VR = U/(fnD)

Figure 22.3 Typical amplitude response due to reduced velocity.

22.6 Fatigue Life

22.6.1 Estimate of Fatigue Life
Fatigue damage from first order response due to individual seastates and from VIV generated
by individual current profiles may be summed using the Miner’s rule. Consideration should
be given to the distribution of fatigue damage around the riser circumference in order to avoid
unnecessary conservatism and the damage calculated at 8 circumferential points.
Bending is an important consideration for fatigue. Indeed, the wave induced bending
moments in the splash zone are important for fatigue consideration.
Fatigue assessment based on S-N curves shall be applied.
Fatigue damage shall be estimated using Miner’s rule summation as given below:

where, D is accumulated fatigue damage ratio, ni is the stress range cycle numbers in stress
block i, Ni is the allowable stress cycle number at constant stress range, q is the allowable
fatigue damage ratio given as:
0.1 no inspection
‘= { 0.3 with inspection
Normally, the riser should be designed without consideration of inspection. If inspection is
included in fatigue evaluation, the inspection shall detect the critical cracks.
Fracture mechanics may be applied for the estimate of fatigue life using Paris’ law. The initial
crack size must be selected based on statistics.

22.6.2 Effect of Inspection on Fatigue Analysis
The effect of inspection on fatigue evaluation is included by selecting the allowable fatigue
damage ratio q=0.3. This factor might be released a slightly by using fracture mechanics.