I'd think current velocities would normally be insignificant when compared to maximum wave orbital velocity.  Use the sum  and see if it makes any difference to the riser design.   

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You should include the current effect.

As you probably know, VIV can happen in the presence of current velocity alone (no wave velocity).

If you refer to DNV-RP-C205, and your main interest is only to analyse the vortex induced oscillations due to the wave then refer to Sec.9.7.
Most discussion on VIV is usually only take current into account, but I would say it will really depend on what the outcome you want to have from your analysis.
You can combine the effect of wave and current if this is want you want to see.

Thanks for all the replies,
My case is subsea riser, and yes the effect of both current and wave should be considered for VIV. However, my question was about DNV-RP-C205 (Apr 2007) approach as I imply that the effect of VIV due to current and wave is evaluated separately/distinctly, as in sec. 9.6 DNV addresses VIV due to only current (with the relevant criteria) and in the next section 9.7 address VIV due to wave (with the relevant criteria).
In other words, I imply DNV evaluates the VIV once there is only current and then once there is only wave, and seemingly without consideration of likely effect of simultaneous current and wave on VIV assessment. (I have noted to sec 9.7.1.6 wave VIV, calling when Kc>40 the VIV is just due to current)
The above is my interpretation from DNV for VIV assessment (e.g. subsea riser), and I would appreciate if somebody could advise /comment.
 

Your note on the Sect.9.7.1.6 is right. Assuming that all data is available then we can distinguish between two categories depending on KC.
If you are using a software to work on this subsea riser analysis, the software usually provides you with the field to input the wave and the current.
You may also look at API RP 16Q, they have example of riser calculation there. Also look at "Fundamentals of Marine Riser Mechanics" by C.P.Sparks. I would say this is 'The Bible' for riser analysis.

My opinion is that we need to consider both wave and current together in the VIV fatigue assessment, and for any particular sea state described by Hs and Tp for waves and current velocity, fatigue damage needs to be computed based on response models (for inline, crossflow and crossflow induced inline VIV) together with force model assessmment for inline VIV.  

Clarification A)
With respect to the replies, it sounds there is a little discrepancy on interpretation of DNV-RP-C205 (Apr 2007).
Considering guideline as DNV still I interpret the VIV due to current and wave are assessed separately not simultaneously, if it is otherwise (i.e. combined current and wave) please address the reference in DNV.
To back up my interpretation, as per sec 9.7 (wave VIV): (Kc is Keulegan-Carpenter)

- If Kc>40, wave VIV does not occur, only current VIV to be evaluated
- If Kc<40, wave VIV plus current VIV to be assessed

In sec 9.6 (current VIV) the criteria is independent on Kc, and in contrast in sec 9.7 (wave VIV) which depends on Kc, it is a function of Vm (maximum orbital velocity due to wave motion, and nothing is mentioned about current effect). In other words Kc is a non dimensional factor which defines only wave effect and independent on current. Even in earlier DNV sections the Drag and Added Mass Coefs (Cd and Ca) are defined based on two cases, i.e. 1st steady current and 2nd steady current + wave effect. (NB, Cd and Ca depends on Kc, Re, and roughness)

(NB, DNV-RP-C205 only address VIV occurrence, and does not provide any guideline for fatigue analysis, and however, I understand a riser may encounter with VIV due to current with a frequency and the same time VIV due to wave with another frequency)

Clarification B)
The next question, I understand since the current and wave is a function of water depth (intuitively VR / Kc/ Ca / etc depends on WD) the average velocity should be considered between two successive riser boundary condition to calculate the required parameters (e.g. VR / Kc / Ca/ me / Ks/ f/ and etc).


Any comment (even minute) on the above helping further clarification is welcomed.