Subsea Analyzer for in Line Sulphate Measurement in Subsea Seawater Desulphation and Injection Process
Authors: Giolo Riccardo , Albani Yvan , Novello Marco , Smith Lee , Johnson Curtis , Uibel Rory 
Published: October 2023

Abstract
Within the industrialization program of an innovative subsea seawater desulphation technology, the requirement for measuring in-line the sulfates level, prior to treated water injection in the reservoir, was identified.
This allows the measurements to be taken directly in-situ and not requiring dedicated lines to bring the samples to surface, hence allowing removal of another hydraulic line from the umbilicals, which is a key objective to unlock long subsea tie backs.
At the same time, the measure of sulphates can be used as a condition monitoring technique to ensure the correct functioning of the nanomembranes, anticipating through trend analysis preventive maintenance actions.
Various technologies have been initially screened, including the direct measure ones (e.g. colorimetry, Raman) and the indirect ones which measure the total dissolved salts (e.g. through conductivity).
Considering both the performance requirements and the marinization process, the technologies were ranked, and the Raman spectroscopy was selected as the most promising. This solution is based on the scattering effect when the analysed stream is exposed to laser light, which enables the computation of the concentration for the selected molecules.
Raman spectroscopy is a well-established method that has several applications in the topside Oil & Gas industry, such as online monitoring of blending processes, amine gas treating, hydrocarbon groups, sulphates content in water flood injection system.
The marinization and qualification of the subsea technology was undertaken as a joint development with a topside Raman spectrograph provider. A prototype was designed, manufactured and subject to an extensive qualification testing campaign.
In the course of the development, various challenges were identified and overcame, specifically related to the intended environment for operations, i.e. underwater and in particular deepwater. The technology was already qualified and field proven for topside, but the ambient temperature and pressure present in underwater environment led to some adjustments.
As regards the temperature, depending on the water depth, ambient temperature ranges between 15 down to 4 °C. The electronics associated with the laser and camera generate heat and thermal management required an accurate thermal design.
As regards the pressure, the problem was tackled by designing an enclosure which kept the Spectrograph at atmospheric pressure while exposed to the hyperbaric pressure. This was validated through specific test.
Another challenge was posed by the strict requirements dictated by the API 17F, specifically on the shock and vibration tests, which if fact were driving the selection of suitable components and the design of the internal supports for the instrument and electronics.
The subsea version resulting from this development is the first to have the capability of providing permanent inline measurement underwater. It determines the concentration of specific chemical components present in the process stream. This technology is not only limited to desulphation processes but can also measure the presence of CO2 and Oil in Water. The accuracy of this measurement is comparable to top-side laboratory methods.