A method and device for preventing water from penetrating into the rising part/intermediate section (12) of a pipe (6) when laying the pipe (6) on or near the seabed (8) by means of a pipelay vessel (1), and where a plug (22) that seals against, travels and locks in the inside passage of the pipe (6) is arranged inside the pipe (6) between the lay vessel (1) and the seabed (8), where during normal pipelaying the plug (22) is displaced in the pipe (6) so as to essentially maintain its relative position relative to the pipelay vessel (1), and where in the event of water flowing into the pipe (6), the plug (22) is locked in the pipe (6), whereby water is prevented from filling the rising part/intermediate section (12) of the pipe (6).