Complex sentence: Only personnel with confined-spaces training are permitted to enter the tunnel system, comprising 2.76 miles of inaccessible tunnel and 11.07 miles of piping; 3.78 miles of direct-bury piping in the ground; and 2.2 miles of accessible tunnel and 12.27 miles of piping.
Split into simpler sentences: Only personnel with confined-spaces training are permitted to enter the tunnel system. Comprising 2.76 miles of inaccessible tunnel and 11.07 miles of piping; 3.78 miles of direct-bury piping in the ground; and 2.2 miles of accessible tunnel and 12.27 miles of piping.