RISER - Narrow Band Fatigue Damage Assessment
RISER - Narrow Band Fatigue Damage Assessment
(OP)
Ref: DNV-RP-F204 (July 2005) App. A (narrow band fatigue damage assessment)
Regarding fatigue damage calculation, in Eq A.3 the effect of wave probability is not considered, while I expect the wave probability should be accounted. (NB, in Eq. 2.8 the effect of wave probability has been considered)
Could somebody please advise?
Regarding fatigue damage calculation, in Eq A.3 the effect of wave probability is not considered, while I expect the wave probability should be accounted. (NB, in Eq. 2.8 the effect of wave probability has been considered)
Could somebody please advise?





RE: RISER - Narrow Band Fatigue Damage Assessment
RE: RISER - Narrow Band Fatigue Damage Assessment
Ref. DNV-RP-F204
You mean that EQ. A.3 is unit time fatigue damage for a sea state and based on wave probability and each sea state duration the cumulative damage fatigue should be calculated.
To extend my matter, to use EQ A.3 it is required the coefficients (i.e. m and sigma) are extracted from EQ. A.9 / A.8 / A.7 , However I do not know what the meaning of Stress Response Auto-Spectral Density (i.e. S(w)) is?, and how it is calculated?
Could you please shed light.
RE: RISER - Narrow Band Fatigue Damage Assessment
Please refer to Sec 5.2.6 of DNV-RP-F105, 2006 for stress response spectral density. (Note that this is applicable for pipelines and not risers and hence need to be interpreted with care).
RE: RISER - Narrow Band Fatigue Damage Assessment
In general fatigue is due to two effects, VIV and Direct Wave. VIV effect is named as Response Model and Direct Wave as Force Model (ref. DNV-RP-F105).
Stress Spectral Density in F105, Sec. 5.2.6 is for Force Model (i.e. Direct Wave fatigue), and based on your advice I interpret that Appendix A of DNV-RP-F204 calculated only Direct Wave fatigue damage NOT VIV fatigue damage. Is it correct?
(Moreover, the fatigue damage due to VIV should be calculated separately (as per F204 sec 4.3). Then the final fatigue damage is summation of Direct Wave fatigue damage and VIV fatigue damage)