An apparatus (10) for pipeline inspection includes groups of ultrasonic sensor units (14). An upstream group is axially offset from a downstream group. In each group, the sensor units (14) are spaced circumferentially from one another. The sensor units in the downstream group are arranged out of phase with the sensor units in the upstream group. The downstream group provides inspection coverage for the circumferential spacing between the sensor units (14) in the upstream group, and the upstream group provides inspection coverage for the circumferential spacing between the sensor units (14) in the downstream group. Each sensor unit (14) includes an array of ultrasonic sensors (16) defining an arc of a circle extending in a circumferential direction with respect to a longitudinal axis (X) of the apparatus (10). Each array is mounted on a carrier (30) and is biased locally in a radial direction. Each carrier is movable from a first radial position to a second radial position in response to changes in pipe diameter.