An automatic modular pipe welding device comprising: a support carriage (1; 701) adapted to be removably connected to the pipe and further adapted to move relative to said pipe; a processing rig (2; 702) removably connected to said support carriage (1; 701) at a location between first and second opposite ends, wherein said processing rig is detachable from said support carriage; and being characterised by the following features: a moving slide attached to said processing rig (2; 702) at the first opposite end; a motor located at the second opposite end of the processing rig at a distance from the moving slide, the motor adapted to drive the moving slide; sensing means attached to an adjustable mechanism 3) on one side, wherein said sensing means are optical sensing means, wherein said sensing means are adapted to identify at least one feature upon the pipe and adapted to track said feature and provide a command to the motor to drive said moving slide to adjust position of said moving slide relative to said seam; a welding tip attached to another opposite end of said moving slide at a distance from said sensing means; and said adjustable mechanism attached to said moving slide on an opposite end of said moving slide, for enabling fine adjustment of the distance between the welding tip and the sensing means, wherein said support carriage (1; 701) has drive means adapted to move said processing rig (2; 702) in response to the relative position between said processing rig (2; 702) and said feature, whereby automatic tracking can be accomplished without obstruction of the features by welding debris. a support carriage (1; 701) adapted to be removably connected to the pipe and further adapted to move relative to said pipe; a processing rig (2; 702) removably connected to said support carriage (1; 701) at a location between first and second opposite ends, wherein said processing rig is detachable from said support carriage; and being characterised by the following features: a moving slide attached to said processing rig (2; 702) at the first opposite end; a motor located at the second opposite end of the processing rig at a distance from the moving slide, the motor adapted to drive the moving slide; sensing means attached to an adjustable mechanism 3) on one side, wherein said sensing means are optical sensing means, wherein said sensing means are adapted to identify at least one feature upon the pipe and adapted to track said feature and provide a command to the motor to drive said moving slide to adjust position of said moving slide relative to said seam; a welding tip attached to another opposite end of said moving slide at a distance from said sensing means; and said adjustable mechanism attached to said moving slide on an opposite end of said moving slide, for enabling fine adjustment of the distance between the welding tip and the sensing means, wherein said support carriage (1; 701) has drive means adapted to move said processing rig (2; 702) in response to the relative position between said processing rig (2; 702) and said feature, whereby automatic tracking can be accomplished without obstruction of the features by welding debris. a moving slide attached to said processing rig (2; 702) at the first opposite end; a motor located at the second opposite end of the processing rig at a distance from the moving slide, the motor adapted to drive the moving slide; sensing means attached to an adjustable mechanism 3) on one side, wherein said sensing means are optical sensing means, wherein said sensing means are adapted to identify at least one feature upon the pipe and adapted to track said feature and provide a command to the motor to drive said moving slide to adjust position of said moving slide relative to said seam; a welding tip attached to another opposite end of said moving slide at a distance from said sensing means; and said adjustable mechanism attached to said moving slide on an opposite end of said moving slide, for enabling fine adjustment of the distance between the welding tip and the sensing means, wherein said support carriage (1; 701) has drive means adapted to move said processing rig (2; 702) in response to the relative position between said processing rig (2; 702) and said feature, whereby automatic tracking can be accomplished without obstruction of the features by welding debris.