Abstract: An underwater pipeline apparatus for delivering a pig unit through an initially air or gas-containing sea-bed pipeline from an entry point toward an exit point. The apparatus includes an inlet conduit for coupling to a pipeline inlet port at a location behind the pig unit in its intended direction of travel, from the entry point to the exit point, having an opening to the sea to admit pressurised water into the pipeline under the head of water above the pipeline and means to regulate flow of water through the inlet conduit into a pipeline to regulate the speed of delivery of the pig unit along the pipeline.