Wellbore drilling system comprising: - a drilling tower, - a drill floor having a well center through which a drill string passes along a firing line, - a drilling tubulars storage rack (10, 11) adapted to store multiple drilling tubulars in vertical orientation, preferably multi-jointed tubulars, - a tubular racking device (40, 40') having at least a lower first tubular racker assembly (41, 41'; 141, 141') and at least a second tubular racker assembly (42, 42'; 142, 142') operable at a greater height than the first tubular racker assembly, each tubular racker assembly comprises a base (41b'; 141'b), a motion arm (41b; 142'b) connected to said base, and a tubular gripper member (41t; 141t) connected or connectable to the motion arm and adapted to grip a tubular, wherein the tubular racking device is adapted to grip and retain a drilling tubular by the tubular racker assemblies, and wherein the tubular racking device is adapted to place a tubular in and remove a tubular from the drilling tubulars storage rack (10, 11), and wherein the tubular racking device has a reach at least allowing to transfer a tubular gripped by said first and second tubular racker assemblies between the drilling tubulars storage rack (10, 11) and a position of the tubular aligned with the firing line above the well center so as to allow for building and disassembly of a tubulars string, e.g. a drill string or a casing string, wherein the system further comprises one or more well center tools, each adapted for operation above the well center of the drill floor, e.g. an iron roughneck tool for making up and breaking out of threaded tubular joints, characterized in that the base of the first tubular racker assembly (41, 41'; 141, 141') is vertically mobile between a lower operative position - wherein the corresponding gripper member can place a tubular in and remove a tubular from the drilling tubulars storage rack - and a raised position, and in that the tubular racking device comprises a third tubular racker assembly (43, 43'; 143, 143') comprising a base, a motion arm connected to said base, and a tubular gripper member connected or connectable to the motion arm and adapted to grip a tubular, wherein the base of the third tubular racker assembly (43, 43'; 143, 143') is vertically mobile at least between a well center servicing position, that is below the lower operative position of the base of the first tubular racker assembly (41, 41'; 141, 141'), and said lower operative position of the base of the first tubular racker assembly if said first tubular racker assembly (41, 41'; 141, 141') is moved to a raised position, wherein - with the base of the third tubular racker assembly (43, 43'; 143, 143') in said lower operative position of the base of the first tubular racker assembly and with a tubular gripper member connected to the motion arm - the third tubular racker assembly(43, 43'; 143, 143') is operable for tubular transfer between the firing line and the drilling tubulars storage rack in combination with the second tubular racker assembly, e.g. in case of failure of the first tubular racker assembly, and in that at least one well center tool is adapted to be connected to the motion arm of the third tubular racker assembly (43, 43'; 143, 143'), and in that - with the base of the third tubular racker assembly(43, 43'; 143, 143') in the well center servicing position and a well center tool connected to the motion arm thereof - the well center tool is operable above the drill floor, whilst the first and second tubular racker assemblies are operable in combination for tubular transfer between the firing line and the drilling tubulars storage rack (10, 11). - a drilling tower, - a drill floor having a well center through which a drill string passes along a firing line, - a drilling tubulars storage rack (10, 11) adapted to store multiple drilling tubulars in vertical orientation, preferably multi-jointed tubulars, - a tubular racking device (40, 40') having at least a lower first tubular racker assembly (41, 41'; 141, 141') and at least a second tubular racker assembly (42, 42'; 142, 142') operable at a greater height than the first tubular racker assembly, each tubular racker assembly comprises a base (41b'; 141'b), a motion arm (41b; 142'b) connected to said base, and a tubular gripper member (41t; 141t) connected or connectable to the motion arm and adapted to grip a tubular,