To response to the lessons from The Deepwater Horizon oil spill accident off Louisiana in the Gulf of Mexico in 2010, an autonomous underwater robot was proposed and tested for early detection and monitoring system as one technological measure around offshore oil and gas production systems. The mission of the robot is to monitor not only detailed structure of oil and gas plumes in the water columns, but also time-varying structure of transportation of oil droplets in 3-D space. It was found that the proposed system using an autonomous underwater robot is capable of in situ measurement for dissolved substances as well as oceanographic data and water current profiles in water columns which are indispensable to early detection and monitoring of oil spills and gas leaks from offshore oil and gas production platforms. The robot can provide oceanographic simulation with the real-time data to precisely grasp the 3-D structure of oil and gas plumes as a data assimilation scheme between an autonomous underwater robot and oceanographic simulation. (C) 2017 Elsevier Ltd. All rights reserved.