The drilling fluid(mud) is both inside the drill pipe and in the annulus between the pipe and the rock. After the drilling is complete, cement grout is pumped

The drilling fluid(mud) is both inside the drill pipe and in the annulus between the pipe and the rock. After the drilling is complete, cement grout is pumped into the annular space, replacing the mud. then the mud in the pipe will be replaced with sea water. Then they will pull the fiber optic cable through the pipe. Drill pipe comes in various sizes, probably about 4-8 inch diameter for this job.