Dual Unit of Measure Transaction for Steel Pipes

I have a client requirement which is as follows:

-Pipes are purchased through vendor on a PO with UOM of Pieces.

-Each Piece has a standard length of 18 Feet.

-The PO is priced by a per Foot unit cost.  Therefore, the List Price on the Item is based on the price for 18 Feet.

-The item needs to be tracked on hand by UOM Pieces

Here is the challenge:

-The supplier may send the item with a variation in size.  For example, if the PO was for 10 pieces, the supplier may send 5 pieces that are 18 Feet and 5 pieces that are 16 Feet.