Development of Oil-In-Water Sensors for Subsea Produced Water Discharge and Re-Injection

Abstract

Real time subsea sensors to measure oil content in produced water is a key technology gap in subsea produced water treatment and disposal, an emerging enabling technology for deepwater development. A two-year project to develop subsea produced water discharge sensors was initiated in 2014 to develop the sensor technology to TRL 3 (API 17N). The project focused on sensors to measure oil content in produced water for discharge permit compliance monitoring and process monitoring. The performance requirements for these sensors are also applicable to reinjection applications where water quality is often highly specified. The technical requirements for the sensors were developed with input from industry subject matter experts and discussions with regulatory agencies. A technology gap analysis on existing surface sensors was conducted, the sensors were ranked and three of them were selected for further development in the project. Key technical requirements for the sensors are accuracy in correlating to Oil and Grease by EPA Method 1664, design and service conditions for deepwater installations, service life, reliability and integration with subsea systems. Main technology gaps for the sensors are measurement accuracy and reliability. A proof of concept of a new sensor technology based on Confocal Laser Fluorescence Microscopy (CLFM) was also conducted. Four subsea produced water sensor prototypes were designed, constructed and bench scale tested. The technologies utilized by the sensors are light scattering, microscopic imaging, laser induced fluorescence and confocal laser fluorescence microscopy. The performance of the sensors for oil concentration measurements were evaluated from June to September 2016 in a 3-inch diameter, one-pass flow loop using filtered seawater and various additives for simulating the anticipated characteristics of produced water in subsea treatment and discharge systems. The bench-scale testing results showed that deviations in oil-in-water content of less than 15% compared to hexane extracted materials were achievable in some conditions, but none of the sensors was able to achieve this target for the full range of water composition variations that were studied. The sensor measurements of oil content were affected by multiple parameters to varying degrees (compositional variations included amongst other parameters: oil content and API gravity, temperature, salinity, solids content, gas bubbles and chemical content). All sensors were able to achieve oil-in-water measurements within 50% deviation of the EPA Equivalents for some parameters, but each sensor had two or more parameters for which it had much larger than 50% deviation. Most of the observed parametric effects were consistent with expectations related to the measurement principles. The bench-scale testing results showed that the tested existing sensor technologies were robust, had good or acceptable accuracies under test conditions similar to those at which the instruments were calibrated, and had well-defined trends in respect of the changing compositional effect. The tests also confirmed the feasibility of CLFM technology and its potential for further development into a robust and accurate subsea produced water quality sensor. On fouling mitigation, Digitrol's hydrodynamic approach seemed to have worked well with both manual fouling induced by oil soaking and that by grease brush-painted on the optical window. All the other sensors coped well with the fouling by oil soaking, but not by grease brush-painted. For further development of the tested sensors, improvements were identified for achieving higher accuracy, reducing the impact of parameter effects, and meeting the conditions of subsea applications. It was also identified that the criteria for evaluating and accepting online sensors as a tool for regulatory compliance is an important aspect to be addressed ahead of or during further development and commercialization.