Offshore Pipeline Free Span Analysis

[StevenTran]

Dear Expert,
I'm involved in pipeline free span analysis of a offshore pipeline. I've read DNV-RP-105 and confuse about between In-line (or cross flow) onset span length and in-line (or cross flow) screening span length.
Could you explain to me what is the meaning of in-line onset span length? and what is the different between In-line (or cross flow) onset span length and in-line (or cross flow) screening span length?

Regards,
S.T

 

[bernoullies123]

StevenTran,

There are no screening or onset span lengths. There is really only one, which is the Free Span Length (L) is what is of importance (Yes there are 2 other span lengths defined in 1.14, Leff and Ls).

There are 2 conditions which bring upon or the onset of VIV, which is IL and CF caused by shedding symmetrically or non-symmetrically respectively. For each condition, there are different factors and equations that need to be satisfied in the 1st step in the analysis which is screening. If the conditions are met, then you can bypass to the buckling check as the screening incorporates a fatigue life. If the screening fails, then the fatigue analysis has to then be perform prior to the buckling check. Refer to Figure 2-2, which is pretty clear.