Hard1 is right
attach them to something
In permanent mooring terms best bet is to have two anchors attached to each other by a long length of heavy chain. Place this horizontally on the sea bed in line with the current or wind, which ever is stronger, one anchor you can pull in the other you have to dig in. from the middle of the chain take your riser up to the mooring buoy
good but more speny than a truck tyre
Des