Automation of the Dewatering Operation for Crude Oil Storage Tanks

This paper describes the implementation of an Automatic Dewatering System in one of ADNOC Offshore's crude oil storage tanks, carried out as a pilot project. The system automates tank dewatering, reducing Health, Safety & Environmental (HSE) hazards and removing the need for standby operators during the process. Automated dewatering in crude oil storage tanks is achieved by installing two oil-water interface detector probes in each draw-off sump, resulting in a total of eight probes distributed across four sumps. These probes operate in conjunction with both the Local Control Panel and the Distributed Control System (DCS) to manage the dewatering process automatically. The primary probe, designated as the Control Probe, is situated at the upper section of the draw-off sump and governs the actuation of the water draining motor-operated valve (MOV) based on the detected oil-water interface level. The secondary probe, referred to as the Alarm Probe, is installed at the lower portion of the sump and serves as a safety feature by triggering an alarm and ensuring closure of the MOV should the Control Probe malfunction. This arrangement significantly reduces the risk of oil contamination in the water drain line. As a result, the system reliably discharges only clean, oil-free water from the tank via the dewatering lines connected to the sumps. Automatic dewatering technology was successfully integrated into an existing Crude Storage Tank, with installation and commissioning completed without interruption to ongoing operations. Comprehensive testing during the Site Acceptance Test (SAT) encompassed multiple operational scenarios, all of which yielded satisfactory results and confirmed effective system integration. The technology substantially reduced oil carryover, consistently achieving the process guarantee of 50 ppm oil in produced water. The system has operated efficiently for over nine months, reducing the need for full-time field operators and lowering operating costs and HSE risks during dewatering. Its successful implementation in one of ADNOC Offshore's crude oil storage tanks automated the dewatering process, eliminated standby operator requirements, and paved the way for adopting similar technologies in other tanks.