One of my old neighbors put a several-ton boulder on his lawn. After a collision shot the boulder into his kitchen like a billiard ball, he added a 3-foot tall, 2-foot thick reinforced concrete curb up in front of the boulder.
Like a billiard ball is the appropriate analogy, too.
That's the reason to put 2/3 of the bollard in the ground. Around these parts the drill pipe would be 4 1/2" or 5" in diameter, with a 1/2 or better thick wall. It would take bending two if you spaced them right to even get enough of a ramp to go over. Not likely, 'cause I'd fill them with concrete, too.
