If you need to change or set the surface referenced by all the structures in your pipe network you can do this through the Toolspace.
On the Prospector Tab<Pipe Networks<Networks - expand the network you want to update. Highlight Structures - the list of structures will be displayed in the pane at the bottom of the toolspace. Highlight one of these and press CTRL A to select all. Scroll across until you come to the Reference Surface column. Right click on reference surface column header and select edit. You can then set or change the reference surface for all structures in the network. See screen grab below: