This article presents the development of a remotely operated vehicle (ROV) for the underwater inspection of subsea flexible riser, proposing effective solutions for the associated robotic manipulation system. Novel, underactuated end-effectors have been conceived and developed to firmly hold the robot on the riser during locomotion or operation, without affecting riser's integrity. By using these end-effectors, the robot is able to move along the riser, like an inchworm does, and rotate around the riser at any angle. In principle, the current design could operate up to 2,000 m sea depth with minor modifications. In this study, the service robot carries and deploys digital radiographic equipment for the volumetric inspection of flexible risers. Several preliminary tests, as well as a series of underwater tests, prove the robot's encouragingly positive performance.