Fremgangsmåte og anordning i forbindelse med stigerör

The riser system includes a small diameter riser that can be disconnected from the subsea BOP/wellhead assembly to obtain access to the wellbore for large diameter casing while continuously maintaining a hydrostatic head to control the well. The riser system further includes a large diameter riser joint having one end connected to the BOP/wellhead assembly and one or more hydraulic conduits extending from the BOP/wellhead assembly to the small diameter riser which extends to the surface. The small diameter riser has a first position where the small diameter riser is aligned and connected to the other end of the riser joint and a second position where the small diameter riser is non-aligned and unconnected with the large diameter riser joint. The riser system further includes a shifter which moves the small diameter riser from the first position to the second position. Downhole operations are conducted through the small diameter riser in the first position. When it is necessary to have a larger diameter access to the well, the shifter is actuated to move the small diameter riser to the second position allowing access to the well through the large diameter riser joint and BOP/wellhead assembly.