Activity that can be carried out by vessels will be where the executing unit is connected to a subsea well or a well on a fixed facility, but does not have primary control of the wells’ block valves. The primary control of the wellstream (christmas tree or well control equipment connected to the well) is handled by a facility (from control room and/or direct operation of check valves) other than the executing unit. Vessel activity can be carried out by a facility with AoC or a vessel without AoC.
Examples of such activities include pumping of various fluids (gas and liquid) into a well through a christmas tree or to a well valve, for fracking, stimulation, cleanup, etc., while well intervention is taking place (the well intervention personnel handle primary control of the wellstream).
The same applies to activities involving maintenance of subsea wells (christmas tree or equipment on the subsea template) or replacement of equipment on subsea wells, where one is not connected to the well, and another facility handles primary control of the wellstream.
Source: Guidelines Regarding the Framework Regulations, Norway, updated December 2012. Regulatory Guidance