An Intervention-AUV (or I-AUV), is a hover capable AUV whose primary role is direct contact with subsea structures for measurement or physical manipulation of components. The aim of the ALIVE project is to develop an Intervention-AUV capable of docking to a subsea structure which has not been specifically modified for AUV use. This paper describes the modular structure of the ALIVE AUV, including its distributed software architecture and in particular the ADS (Autonomous Docking System). It then outlines the sonar and video sensor processing techniques used for real-time control of the AUV to perform tracking and 3D pose reconstruction. In addition, details of the system tests and practical trials used in the development process are described.