Abstract: The plug includes a rigid tapering frustum with an upstream end having a larger diameter than a downstream end. A bladder is located outboard of the frustum and preferably fastened to the frustum. The bladder preferably has a toroidal form. A fluid fill system extends into an interior of the bladder to provide fluid for inflating or deflating the bladder. When the bladder is inflated with fluid passing into the interior, an outer wall of the bladder is expanded against a wall of a pipe where the plug is located, to hold the plug in place. As pipeline fluid forces act downstream on the frustum of the plug, the frustum moves slightly downstream, exerting a radial force through the bladder and against the wall of the pipe, so that pressure within the pipe and upstream of the plug enhances radial force with which the bladder engages the pipe wall.