Can  prequalified procedure be employed in this case if for a longitudinal seam? 

21 inch pipe (B-L2a-S)

Or does the clause below disallow this:
3.13.4 Tubular Butt Joints: For Tubular Groove welds to be given prequalified status, the following conditions shall apply:

(1)  Prequalified WPSs. Where welding from both sides of welding from one side with backing is possible, any WPS and groove detail that is appropriately prequalified in conformance with Clause 3 may be used, except that SAW is only prequalified for diameters greater than or equal to 24 in.

I'm leaning toward "no"  but if there is a loop-hole here that says yes, please shine a light on it for me.

Would the "longitudinal" seam make this "not" a tubular weld?  But rather a longitudinal weld on a pipe?