Abstract: An ice plug for defining a barrier in a pipeline section filled with water, comprises a hollow, cylindrical body enclosed at a front end and provided with guide means for sliding movement of the plug along and within the pipeline. The body has a diameter smaller than that of the pipeline to define with the latter an annular interspace enclosed at opposed axial ends of the plug. The thickness of the annular space is selected to allow free movement of the body through curves in a pipeline and to form a continuous, annular ice layer firmly engaging the inside wall of the pipeline and the outer surface of said body to thus form, together with the body, a temporary water impermeable barrier within the pipeline. A water supply system fills the interspace with process water.