js55, I agree, a 1.5" thick coupon is ideal for qualifying 2 separate welding processes (PQR). Depending on the welder qualifications, ultimately a 1/2" thick coupon (whether plate or pipe) will provide unlimited thickness for the welder qualification however, it's important to document that at least 3 layers of weld have been deposited.
We use Super coupons in my shop:
http://www.fischerengr.com/Testmatls.htm or
http://www.trieng.com/They provide maximum thickness and take the welder down to NPS 1"... good stuff and relatively inexpensive.