NEVER (repeat...NEVER) put a u-lock around the seat tube or any other part of your frame. A dumb crook will attempt to use the frame of your bike as a lever to snap the u-lock.
And a smart crook can use the lock to break your rear rim, then nip through your tire with a small diagonal cutter, if that's all that the lock's holding. Sorry, but homie is gonna keep locking his frame. In two places.
Lift & twist
this.