The photo above shows the Island Wellserver underway. This ship is classified as a well intervention vessel meaning it performs subsea services on offshore oil wells. Statoil explains:
Occasionally wells have to be re-entered for maintenance and technical purposes (e.g. replacing and re-perforating tubing; replacing flow control devices; isolating production zones; monitoring production behaviour; and logging pressure, flow and temperature).
The procedure, which is known as well intervention, is difficult enough to do from conventional surface platforms let alone when wells are installed on the seafloor.
The normal subsea procedure is carried out from an anchored semi-submersible rig attached to the wells by risers (gas/fluid conduits linking subsea equipment to surface platforms).