Offshore drilling system for performing subsea wellbore related activities, e.g. drilling a subsea wellbore, wherein the drilling system comprises: - a drilling tower having a top and a base, which drilling tower is to be positioned at or near a moon pool (5, 5a, 5b) of a floating body, e.g. a drilling vessel or platform; - a riser tensioning system (40, 41, 42, 43) adapted to connect a riser extending along the firing line between the subsea wellbore and the floating body; - a tubular string main hoisting device, the tubular string for example being a drill string, said main hoisting device comprising: - a main hoisting winch (20, 21) and main cable connected to said winch; - a crown block and a travelling block suspended from said crown block in a multiple fall arrangement of said main cable, which travelling block is adapted to suspend a tubular string, e.g. a drill string, therefrom along a firing line, e.g. with an intermediate topdrive adapted to provide a rotary drive for a drill string; - a heave compensation system adapted to provide heave compensation of the travelling block, the heave compensation system comprising a main cable heave compensation sheave (30, 31) in the path between said main hoisting winch (20, 21) and the travelling block, a hydraulic sheave compensator (32, 33) connected to said main cable heave compensation sheave (30, 31); - a mobile working deck which is movable with respect to the drilling tower along the firing line within a motion range including a heave compensation motion range; characterised in that - the heave compensation system further adapted to provide heave compensation of the mobile working deck, wherein the heave compensation system further comprises a hydraulic deck compensator, which deck compensator is connected to the floating body and to the mobile working deck to provide a heave compensated motion of the working deck relative to the floating body, wherein the deck compensator is hydraulically connected via a hydraulic conduit (65,66) to the hydraulic sheave compensator (32, 33) of the heave compensation system, such that in operation the deck compensator moves synchronously with the sheave compensator (32, 33) of the heave compensation system. - a drilling tower having a top and a base, which drilling tower is to be positioned at or near a moon pool (5, 5a, 5b) of a floating body, e.g. a drilling vessel or platform; - a riser tensioning system (40, 41, 42, 43) adapted to connect a riser extending along the firing line between the subsea wellbore and the floating body; - a tubular string main hoisting device, the tubular string for example being a drill string, said main hoisting device comprising: - a main hoisting winch (20, 21) and main cable connected to said winch; - a crown block and a travelling block suspended from said crown block in a multiple fall arrangement of said main cable, which travelling block is adapted to suspend a tubular string, e.g. a drill string, therefrom along a firing line, e.g. with an intermediate topdrive adapted to provide a rotary drive for a drill string; - a main hoisting winch (20, 21) and main cable connected to said winch; - a crown block and a travelling block suspended from said crown block in a multiple fall arrangement of said main cable, which travelling block is adapted to suspend a tubular string, e.g. a drill string, therefrom along a firing line, e.g. with an intermediate topdrive adapted to provide a rotary drive for a drill string; - a heave compensation system adapted to provide heave compensation of the travelling block, the heave compensation system comprising a main cable heave compensation sheave (30, 31) in the path between said main hoisting winch (20, 21) and the travelling block, a hydraulic sheave compensator (32, 33) connected to said main cable heave compensation sheave (30, 31); - a mobile working deck which is movable with respect to the drilling tower along the firing line within a motion range including a heave compensation motion range; characterised in that - the heave compensation system further adapted to provide heave compensation of the mobile working deck, wherein the heave compensation system further comprises a hydraulic deck compensator, which deck compensator is connected to the floating body and to the mobile working deck to provide a heave compensated motion of the working deck relative to the floating body, wherein the deck compensator is hydraulically connected via a hydraulic conduit (65,66) to the hydraulic sheave compensator (32, 33) of the heave compensation system, such that in operation the deck compensator moves synchronously with the sheave compensator (32, 33) of the heave compensation system.