An apparatus for performing operations on an offshore well includes a subsea wellhead assembly. A riser extends from the subsea wellhead assembly to a surface vessel. A tool connects to a running string and is lowered through the riser into the wellhead assembly for performing operations at the wellhead assembly. A subsea controller is located adjacent the subsea wellhead assembly. The subsea controller controls the operation of the tool. A surface controller is positioned on the surface vessel, and is in communication with the subsea controller via a control line extending downward from the surface controller to the subsea controller. The control line extends downward from the surface controller along an exterior of the riser.