Abstract: The assembly comprises a pair of semi-circular steel shells. The inner surface of one shell forms a circular recess. A flat cylindrical annular seal element is seated in and fills the recess. The element is resilient and deformable. It has concentric protruding ribs on its top and bottom surfaces. The seal element is positioned to circumscribe a leaking small hole or crack in the wall of an internally pressurized pipeline. The shells are then circumferentially clamped against the pipeline to cause the element to seal around the leak and to effect tight shell-on-pipe contact. The steel side wall of the recess confines the compressed seal element. The side edges of the shells are then welded together longitudinally and circumferentially to create a permanent pressure vessel sealing and containing the leak.