Mobility analysis

A drillstring anchor for reacting torque from a downhole portion of a drillstring to a wellbore, the drillstring anchor comprising

• multiple segments disposed along the drillstring anchor, each segment comprising a respective gripper and a respective actuator capable of being driven to cause the respective gripper to adopt at least one of (a) a first state in which it is urged outwardly for gripping the wellbore and (b) a second, passive state, the gripper of at least one segment being actuable independently of the gripper of at least one other segment, the multiple segments comprising a first segment and a second segment coupled to each other so as to permit relative longitudinal motion therebetween,
• wherein the drillstring anchor is configured to allow relative axial movement of the downhole portion of the drillstring and the drillstring anchor when the respective gripper of one or more of the multiple segments is gripping the wellbore.