For subsea separation and produced water re-injection or discharge to really take hold, technology gaps in subsea water quality measurement must be closed. Progress is being made, says NEL’s Ming Yang.
Marlim Subsea Separation System.
Subsea separation and produced water re-injection (PWRI) or discharge has long been considered as an enabling technology for developing deepwater / ultra-deepwater and marginal fields.
It is an integral part of the subsea processing strategy that brings many benefits for the offshore oil and gas industry, including economical, operational and environmental.
However, to successfully implement subsea separation and PWRI or discharge operations, continuous online subsea water quality measurement devices must be in place. Without such devices, one can only use a remotely operated vehicle (ROV) to extract produced water samples and bring them to the surface for offline analysis. Not only is this extremely expensive, it is also detrimental to the effective control of subsea separation and PWRI or discharge operations.
To date, there is only one instrument that has been developed and deployed for a specific subsea separation and PWRI application. Little information is however available in the public domain in terms of how this instrument was developed and qualified, and how it has performed since its deployment subsea.
The problem of a lack of subsea online continuous monitors for produced water quality measurement subsea has been recognized by the industry, and in the recent years, joint industry projects (JIPs) and other projects have been initiated. Consequently, good progress has been made in terms of developing the technical specifications, identifying the potential technologies and conducting performance evaluation tests in a laboratory environment.
Testing of subsea sensor at NEL. Image from NEL.
Unlike surface operations, subsea separation and PWRI or discharge operations will require the online monitors to operate reliably and accurately at a water depth up to 3000m. The devices will also need to withstand a much higher operating temperature and pressure compared to surface.
For various types of operations, there are also differences in terms of technical requirements. For the subsea discharges, the focus will be on the measurement of oil in water – although there is currently no legislation in place specifically developed for the discharge of subsea produced water. While for re-injection operations, the emphasis will be on the measurement of both solid and oil, in terms of concentration as well particle size.
Below is a set of technical specifications developed for devices respectively for subsea PWRI and subsea separation processing operations as part of a JIP conducted early on by NEL. It must be emphasized that technical specifications are still evolving. Those given in the table should only be used only as a reference.
There are a number of technologies available on the market that offer the potential to be developed for subsea applications. These include:
- Laser induced fluorescence (LIF)
- Light scattering
- Microscopy image analysis
- Ultrasonic acoustic
- Combination of some of the above
All have the functionality to be used for the measurement of oil in water concentration subsea. However, for the measurement of solid concentration and solid particle size, only the microscopy image analysis and possibly ultrasonic acoustic based technologies may be suitable.
Most of the technologies listed above are optical based (with the exception of ultrasonic acoustic). Fouling of optical window has been identified as one of the main issues that must be addressed. While maintenance is relatively easy for surface applications, maintenance subsea is extremely difficult and expensive.
To mitigate fouling, a number of cleaning technologies have been trialed and incorporated into the devices for surface and subsea operations. These technologies include:
- Jetting spray
- Hydro-dynamic (utilizing a high velocity produced water fluid)
While an ultrasonic-based cleaning mechanism may function well within a low pressure environment, for subsea application, in which high pressure is encountered, it may not work terribly well. Therefore, jetting spray and hydro-dynamic arrangements are potentially more suited for subsea applications.
Research and development efforts
In recent years, a significant amount of research and development efforts have been made by operators, manufacturers and independent organizations like NEL.
Among the operators, ExxonMobil, Petrobras and Statoil have been most active. In the case of ExxonMobil, two PWQM (produced water quality monitoring) prototypes based on using J M Canty’s microscopy image analysis were developed and flow loop tested. A jetting spray cleaning mechanism was incorporated into the prototypes. Test results have confirmed the capability of the prototypes developed.
For Petrobras, effort was focused on developing and qualifying a light scattering based oil in water monitor for their Marlim subsea separation system. The system was eventually installed subsea in 2012. The monitor had incorporated a hydro-dynamic mechanism to prevent fouling.
For Statoil, whose ambition is to develop a “Subsea Factory” by 2020, development of a subsea oil in water monitor becomes an important part. Three technologies including microscopy, LIF and ultrasonic acoustic, have been selected for a surface field trial before picking up a particular technology for marinization and further development.
Manufacturers including Advanced Sensors, J M Canty, Jorin, and ProAnalysis have all been working with operators and independent organizations like NEL to further advance their technologies.
NEL has carried out three JIPs in the past six years aimed at accelerating the development these devices. Currently, a surface field trial is being pursued to test one of the technologies. NEL is also heavily involved in a US government sponsored project aimed at developing a subsea produced water discharge sensor.
Closing the technology gap
To widely deploy subsea separation and PWRI or discharge systems, technology gaps in subsea water quality measurement must be closed. A significant amount of research and development effort has been made by the industry in developing such instruments. It is anticipated that real progress will be made in the next few years.
Dr. Ming Yang is the environmental consultancy services manager at NEL, a provider of technical consultancy, research, testing and program management services. Part of the TÜV SÜD Group, NEL is also a global centre of excellence for flow measurement and fluid flow systems and custodian of the UK’s National Flow Measurement Standards.