Abstract: A modular undersea oil production plant having at least one satellite unit whose product is conveyed by a collector pipe (10) towards a central structure (4) in which operating modules (5,6) are grouped together, said product being removed from said operating modules, wherein said satellite unit has a wellhead (1) linked by a connector (13) to the upper portion of a pipe (44) in a drill hole, an individual auxiliary module (3) which can be raised being associated with said wellhead, said module containing the control and operation components and being connected firstly to said wellhead by a first connector (23) and secondly to one end of said collector pipe (10) by a second connector (24), said wellhead (1) and said auxiliary module (3) being installed on a template (2) in two distinct cavities whose shape is complementary to that of said wellhead and of said auxiliary module which has means for disconnecting it from the collector pipe and from the wellhead and for removing it from the template (2), said disc