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NOV takes water treatment to the seabed

NOV has delivered the world’s first full-scale subsea water treatment system, providing a cost-effective solution for expanding offshore water treatment capacity.

International oilfield services provider NOV received the Offshore Technology Conference’s Spotlight on New Technology award earlier this month for the subsea SWIT, a low-sulfate/low-salinity treatment system. SWIT is an extension of Seabox, a subsea disinfection and sedimentation module for which the company won the same award in 2018. SWIT can be arranged in different ways to achieve any water quality required for enhanced and improved oil recovery operations.

The first treatment step is performed in the Seabox module, where disinfection is done via electrochlorination and particles are settled out through gravitational separation, resulting in a water quality suitable for waterflooding. Higher water quality can be achieved with several different configurations of SWIT membrane filtration units. The Seabox together with the ultrafiltration module can remove suspended solids as small as 0.1 micron. Adding nanofiltration as a third step typically results in sulfate content below 40 mg/l. The Seabox-ultrafiltration-reverse osmosis setup can produce water with sulfate content lower than 2 mg/l, as well as total dissolved solids below 5,000 ppm, depending on operator specs.

The Seabox alone can process 40,000 bbl/d of seawater, while a SWIT system accompanying one Seabox can generate 20,000 bbl/d of treated effluent due to volumes lost through crossflow membrane filtration. These nominal capacities can easily be scaled depending on the prospect and deployment location, NOV Completion & Production Solutions sales manager Eirik Dirdal told Water in Oil.

The remotely operated Seabox and SWIT technologies are qualified for installation at depths up to 3,000 meters (~9,840 feet), far below the UV zone. “There aren’t a lot of particles or organic growth down there in the first place, so the starting point of water quality is a lot better than when pulling water from or near the UV zone,” Dirdal said while explaining some the advantages the subsea system has over traditional topside treatment packages.


SWIT encompasses Seabox with either an ultrafiltration module, ultrafiltration and nanofiltration modules, or ultrafiltration and reverse osmosis modules.

Treatment moduleMain functionOutcomeReservoir impactIntervention interval
SeaboxDisinfection through two-phase electrolysis process; particle sedimentationBacteria control; suspended particle reduction down to 10 micronsReduce souring potential; injectivity control4-5 years
UltrafiltrationAbsolute filtrationSuspended particle removal down to 0.1 micronsInjectivity control2-2.5 years
NanofiltrationSulfate removalDissolved solids reduction – low-sulfate waterAvoid scale formation; further reduce souring potential
Reverse osmosisSalt removalDissolved solids reduction – low-salinty waterEnhanced oil recovery through altered wettability properties

As both Seabox and SWIT have been commercialized just within the past year, the new technologies have not been widely adopted. Seabox was successfully piloted in the North Sea at ConocoPhillips’ Ekofisk field, but SWIT has yet to be installed at a commercial project.

Dirdal acknowledged that it may be a challenge to gain new Seabox and SWIT customers at a time when operators around the globe are slashing capital budgets. Nevertheless, he is confident that even in this low oil price environment, operators will understand the benefits of these subsea solutions.

The technologies can bring savings to brownfield developments such as mature assets with increasing water cuts and fields adding production from nearby tiebacks. This is particularly appealing in areas with much existing infrastructure and where water treatment capacity is limited by original project designs, such as the North Sea and Gulf of Mexico. As subsea technologies, Seabox and SWIT are decoupled from a topside host, making it easy to add water treatment capacity and attain the necessary water quality while avoiding large modifications on existing infrastructure.

“Normally when operators consider new technology, they increase the contingency to capture the perceived risk of using new technology,” Dirdal explained. “If one assumes the same contingency for this technology as for conventional topside solutions, then it’s around a 40% reduction in capex for a typical 20-kilometer [~12.5-mile] tieback case.”

Additionally, operators can easily add the Seabox module alone as a pretreatment step to optimize existing topside systems. Thorough disinfection would improve the uptime of the topside treatment package by minimizing issues such as membrane fouling and clogging. Thus, with Seabox, operators would cost savings from reduced maintenance and chemical use.

Dirdal told Water in Oil that despite low oil prices, many operators may consider the technology for new developments. “One of the downsides of lower oil prices is that a lot of greenfield projects are being put on hold, but some projects could potentially move forward if they have the right technology to improve their economics,” he concluded.

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