A method of assembling well casing piping or tubing, from individual units of tubing (20, 25), by means of lifting, gripping, and mechanical handling means that allow operation outside in proximity of said well by comprising the following consecutive steps: a) descent inside said well of a part of the first tubing unit, the remaining part of said first tubing unit remaining outside said well being supported by said lifting, gripping and mechanical handling means (4/S1); b) assembly of an additional unit of tubing (4/S2) and verification of assembly quality (4/S3), said assembly operations and verification of assembly quality being performed outside said well; c) descent inside said well of part of said further tubing unit (4/S4, 4/S5, 4/S6); d) repetition of phases b) - c) up to the last tubing unit; e) descent of the remaining part of said last tubing unit inside said well, wherein - said assembly operations of step b) comprise welding of consecutive pipes (12,13); - step a) is preceded by a step of preparing individual tubing units (20, 25) in which mechanical couplings are applied to said tubing units (20, 25) in order to prepare them for descent operations, for example steel rings or plates characterised in that: - said steps b) and c) are realized by means of a sequence (4/S1, 4/S2, 4/S3, 4/S4, 4/S5, 4/S6) of opening and closing of a system of clamps (2,4,23,24) in which are distinguished: a lower clamp (2,23) that closes on the pipe (12,13) to support a part of it outside the well during phase b) and an upper clamp (4,24) that closes on the pipe accompanying its descent into the well during phase c). a) descent inside said well of a part of the first tubing unit, the remaining part of said first tubing unit remaining outside said well being supported by said lifting, gripping and mechanical handling means (4/S1); b) assembly of an additional unit of tubing (4/S2) and verification of assembly quality (4/S3), said assembly operations and verification of assembly quality being performed outside said well; c) descent inside said well of part of said further tubing unit (4/S4, 4/S5, 4/S6); d) repetition of phases b) - c) up to the last tubing unit; e) descent of the remaining part of said last tubing unit inside said well, - said assembly operations of step b) comprise welding of consecutive pipes (12,13); - step a) is preceded by a step of preparing individual tubing units (20, 25) in which mechanical couplings are applied to said tubing units (20, 25) in order to prepare them for descent operations, for example steel rings or plates characterised in that: - said steps b) and c) are realized by means of a sequence (4/S1, 4/S2, 4/S3, 4/S4, 4/S5, 4/S6) of opening and closing of a system of clamps (2,4,23,24) in which are distinguished: a lower clamp (2,23) that closes on the pipe (12,13) to support a part of it outside the well during phase b) and an upper clamp (4,24) that closes on the pipe accompanying its descent into the well during phase c). - said steps b) and c) are realized by means of a sequence (4/S1, 4/S2, 4/S3, 4/S4, 4/S5, 4/S6) of opening and closing of a system of clamps (2,4,23,24) in which are distinguished: a lower clamp (2,23) that closes on the pipe (12,13) to support a part of it outside the well during phase b) and an upper clamp (4,24) that closes on the pipe accompanying its descent into the well during phase c).