A protective device for a blow out preventer stack (B.O.P.), and a sidetracking method using same during an oil-field milling or drilling operation requiring a window to be formed in casing to access a formation, the device having a sleeve assembly and a running tool releasably attached to the sleeve assembly, wherein the running tool comprises a running tool housing connected to the sleeve assembly and a separable running tool body connectable within the length of a work string having a BHA with a mill or bit, the running tool housing being shear pinned to the running tool body for running in hole, wherein the running tool housing is configured to shear from the running tool body during deployment into a cased wellbore to sealingly seat the sleeve assembly within a B.O.P. and form a barrier to ingress of swarf into the B.O.P.