RISER - Narrow Band Fatigue Damage Assessment

[atedionyahoo]

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?

 

[nietzche]

Eq A.3 is for a single sea state (one needs to use the wave probability while estimating the cumulative damage for a long term distribution of stress cycles); whereas Eq 2.8, if used for short term fatigue damage in stationary environmental conditions (characterized by unchanging Hs,Tp and theta)involves probability distribution of current only.

 

[atedionyahoo]

Thanks,

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.

 

[nietzche]

Regarding eq A.3, now i feel your interpretation is right.

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).

 

[atedionyahoo]

Thanks for reply,

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)