Abstract: Method for freezing a slug of water to form an ice plug in a section of a pipeline for hydrostatic testing purposes. The ice plug is formed by transferring thermal energy from the slug of water through the walls of the pipeline to a cryogenic cooling fluid such as liquid nitrogen. A section of the pipeline is filled with water and is enclosed by an insulated chamber. The chamber is filled with liquid nitrogen to form a pool around the pipeline. The liquid nitrogen is agitated by discharging a stream of nitrogen vapor through the pool. Heat exchange occurs as a result of heat transfer directly from the surface of the pipeline to the liquid nitrogen by the action of convection currents produced by the vigorous agitation of the liquid associated with the motion of the nitrogen vapor. In a preferred embodiment, the liquid nitrogen is diffused over the surface of the pipeline by means of a permeable layer of material which is easily wetted by the liquid nitrogen.