An assembly and a method for reinforcing or repairing one or more segments of an existing pipeline, without taking the pipeline out of service and without the need for disassembling the existing pipeline segments that are to be repaired. The assembly incorporates a metallic reinforcement pipe, a front seal ring, a rear seal ring, a front seal ring retention assembly, a rear seal ring retention assembly, and a self-hardening medium receiver. The reinforcement pipe and the seal rings create an annular space between the respective ends of the reinforcement pipe and the external surface of the existing pipeline. The seal rings, seal ring retention assemblies, and the flexible reinforcement pipe, accommodate out-of-round and joint angular displacement conditions of the existing pipeline. A self-hardening medium is injected into the annular space and pressure is maintained on the medium until the medium has set.