Page 180 - Pipelines and Risers

Force Model and Wave Fatigue 153

- part two calculates the fatigue damage

The FATIGUE program has been compared with fatigue calculations by Fyrileiv (1998), see
Bai et al. (1998).

-m-FrequsmyDomh Wsrrdepm8Dm

f 2.wE.Lu

Figure 10.2 Accumulated Fatigue Damage vs. Span Length based on Time-Domain and Frequency-
Domain Approaches, for Water Depths 80m and 11Om (42” pipe).
It appears that the difference between the results from the time-domain and frequency domain
approaches is not small, and further investigation is required. The time domain approach is
believed to be more accurate than the frequency domain approach because it accounts for the
influence of current velocity and non-linearity’s.

A parametric study on fatigue damage assessment is conducted by Xu et al. (1999).

10.5 Conclusions and Recommendations
1. The chapter presented a methodology for analyzing wave-induced fatigue of free spanning
pipelines.

2. The analytical equations for the dynamic response analysis of free spans in frequency
domain are developed, neglecting current velocity.

3. The equation of motion is solved in time domain for combined regular waves and current
velocities with different probability of occurrence.

4. Fatigue damage is calculated by adding contributions from all sea-states and currents
using joint probability.

5. The numerical examples illustrate that there is a disturbing difference between the time
domain fatigue analysis and frequency domain fatigue analysis. This is due to the non-
linear effects of the Morison equation and current velocity and is a subject for further
investigation.