An offshore drilling system for performing subsea wellbore related activities, e.g. drilling a subsea wellbore, wherein the drilling system comprises a drilling vessel, the drilling vessel comprising: - a floating hull subjected to heave motion, the hull comprising a moonpool (5,5a,5b), - a drilling tower at or near the moonpool, - a tubular string main hoisting device, the tubular string for example being a drill string, the main hoisting device comprising: o a main hoisting winch (20, 21) and main cable connected to said winch, o 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 sting, 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 passive and/or active heave motion compensator device (32,33) connected to said main cable heave compensation cable sheave (30,31), - a riser tensioning system (40,41,42,43) adapted to connect to a riser extending along the firing line between the subsea wellbore and the vessel, the riser tensioning system comprising a tension ring and tensioner members connected to said tension ring, wherein the drilling system further comprises: - a slip joint having a lower outer slip joint barrel and an upper inner slip joint barrel, wherein the outer barrel is adapted to be connected to a fixed length section of the riser extending to the subsea wellbore, and wherein the slip joint is provided with a locking mechanism adapted to lock the slip joint in a collapsed position, wherein the tension ring of the riser tensioning system is adapted to be connected to the outer barrel of the slip joint, characterized in that the vessel is further provided with a cable connection system, which cable connection system comprises: - an inner slip joint barrel connector that is adapted to be secured to the inner slip joint barrel, - a connection cable winch (61,62) and connection cable, - a top sheave assembly supported in stationary operative position on the hull of the vessel in or above the moonpool, and a travelling bottom sheave assembly secured to the inner slip joint barrel connector, wherein the connection cable extends between the top sheave assembly and the bottom sheave assembly in a multiple fall arrangement, - a connection cable heave compensation sheave (66,67) in the path between the connection cable winch (61,62) and the top sheave assembly, wherein the main cable heave compensation cable sheave (30,31) and the connection cable heave compensation sheave (66,67) are mechanically interconnected so as to allow for synchronous motion thereof. - a floating hull subjected to heave motion, the hull comprising a moonpool (5,5a,5b), - a drilling tower at or near the moonpool, - a tubular string main hoisting device, the tubular string for example being a drill string, the main hoisting device comprising: o a main hoisting winch (20, 21) and main cable connected to said winch, o 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 sting, 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, o a main hoisting winch (20, 21) and main cable connected to said winch, o 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 sting, 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 passive and/or active heave motion compensator device (32,33) connected to said main cable heave compensation cable sheave (30,31), - a riser tensioning system (40,41,42,43) adapted to connect to a riser extending along the firing line between the subsea wellbore and the vessel, the riser tensioning system comprising a tension ring and tensioner members connected to said tension ring, - a slip joint having a lower outer slip joint barrel and an upper inner slip joint barrel, wherein the outer barrel is adapted to be connected to a fixed length section of the riser extending to the subsea wellbore, and wherein the slip joint is provided with a locking mechanism adapted to lock the slip joint in a collapsed position, - an inner slip joint barrel connector that is adapted to be secured to the inner slip joint barrel, - a connection cable winch (61,62) and connection cable, - a top sheave assembly supported in stationary operative position on the hull of the vessel in or above the moonpool, and a travelling bottom sheave assembly secured to the inner slip joint barrel connector, - a connection cable heave compensation sheave (66,67) in the path between the connection cable winch (61,62) and the top sheave assembly,