body. The hoisting of the drill string results in the occurrence over time of tears in the surface which make sealing against a movable sealing body with high differential pressure difficult, and an optional scraping of the pipe wall prior to use of the sealing body will mean an additional running of the drill string in the riser, which is costly and time-consuming. When the drill string and the sealing body are hoisted up internally in the drilling vessel's riser, the fluid that is to be replaced runs out into a suitable place on the drilling vessel and fresh fluid is filled through the externally located smaller pipes using the vessel's high-pressure mud pumps. As the drill string is pulled out of the riser, drill pipes must bedisconnected from the drill string. The drill string is stopped and hung off on slips on the drilling vessel, which take up the weight of the drill string. The high-pressure mud pumps must be stopped before the drill string is hung off in the slips so as to prevent the sealing body from being moved further upwards in the riser because the force of the pressure below the sealing body may exceed the drill string's mass. The vertical heave of the drilling vessel may also result in the same in that the sealing body is moved downwards in the riser but is stopped because the fluid that is below the sealing body and in the externally located smaller pipes cannot be returned quickly enough in relation to the