Dynamic positioning of remotely operated underwater vehicles

Remotely operated underwater vehicles (ROVs) play an important role in unde rwater technology. ROVs have been used for the past three decades in the of f-shore oil industry. They are essential at depths were human diving is imp ractical. This is the case of the Brazilian Marlim oil field, which has wat er wells deeper than 1500 m. ROVs are used for constructing production prod uction facilities, for inspection, and for investigation. ROVs are linked to a surface vessel by a tether cable that powers the ROV a nd transmits command signals and video images. Human pilots, aboard the sur face vessel, usually operate the ROV through joysticks that command ROV thr usters. Video cameras and other sensors (e.g., depth-meter and compass) are the usual on-board navigation instruments. Acoustic systems (long baseline and short baseline) are often employed as auxiliary navigation systems. In many operations it is required that the ROV keeps its position close to so me underwater structure (station keeping). This is usual in intervention op erations involving ROV manipulators (e.g., opening a valve). Those operatio ns are time consuming, difficult and tedious for the ROV pilot. The multiva riable (six degree-of-freedom) and nonlinear nature of the ROV dynamics mak es manual station keeping difficult. The pilot must continuously correct th e ROV position to counteract the water current and the reaction forces caus ed by the motion of the manipulator. In this article we describe an automatic dynamic positioning system for ROV s (DPSROV) that is based on a mechanical passive arm (PA) measurement syste m. The PA system was selected from a group of candidate measurement systems , including long baseline, short baseline, inertial, speed logs, and taut-w ire. The selection was based on several relevant criteria; namely, precisio n, construction cost, and operational facilities. We describe the DPSROV ha rdware and software; outline the dynamic model of the ROV and the position control system; discuss a conventional P-PI linear controller and the varia ble structure model-reference adaptive controller (VS-MRAC), which were imp lemented in order to show that the DPSROV can execute control algorithms of such diverse complexities and resource requirement; and present some exper imental results obtained with the DPSROV. In a comparative study [1], we ha ve concluded that the PA is the most reliable system for precise ROV dynami c positioning in a short-range workspace (few meters).