4 Seabed Water Treatment and Injection
A partial alternative to the seabed water separation and reinjection is the subsea water treatment and injection. Such process does not impact the production system, but consists in moving the water treatment and injection equipment from surface to seabed.
The technology allows minimizing the distance between the water intake and the injection point(s) by enabling water injection and treatment on the seabed. The purposes are to avoid (1) locating the treatment facilities on the floater, (2) use of water injection risers (decrease the well slot number requirements) and associated flowlines. The incentive is weight and space savings (floater displacement minimization) as well as minimization of flowline cost.
4.1 Subsea raw water injection
Among the 3 applications of Subsea Raw SeaWater injection that have been installed subsea, only one is currently operated; it is the OneSubsea Raw Seawater Injection System installed for Petrobas on Albacora l’Este Field (located in Brazil in 400 m water depth) with raw water injection to 7 wells.
The water is taken from the sea approximately 100 m above the seabed. Then, the flow goes through a filter prior to achieve the inlet of the pump. From this point, the water is conducted by flowlines until the master X-tree, where part of the flowrate is injected, controlled by a choke.
The two other applications are:
the OneSubsea Raw Seawater Injection System installed for Columbia E Field in 2007 @ 145 m water depth. Columbia E application is the first application of this subsea technology and is now inactive.
The Aker Solutions ’LiquidBooster™ Subsea Raw Seawater Injection System (SRSWI) installed for Statoil (located Offshore Norway @ 270 m water depth) on Tyrihans field installed in 2013 and which is now abandoned.
4.2 Subsea water treatment
Such system was developed in particular by Well Processing, jointly owned by Poseidon Group AS and Sørco AS, with the Subsea Water Injection and Treatment (SWIT) system. It is an all-electric unit that treats the seawater from seabed level in four steps:
Inorganic particles greater than 10micron are removed from the raw seawater, utilising a ‘still room’ technique. A trawler protection structure is used (), creating a low water velocity and long pathway to allow particles to drop out before reaching the pump.
A two stages electrochemical treatment unit eliminates bacteria by means of continuous hypochlorite dosing and full stream hydroxyl radical treatment.
Sterilisation (biofilm destruction) is achieved by periodic chemical shock dosing.
The treated water is then pumped into the reservoir through a dedicated injection x-mas tree, using subsea water injection pump. The control and power is provided through a high voltage cable.
The seabed water treatment technology is evolving with many new concepts that address the two following water treatments
Seawater disinfection:
NOV SEABOX unit which is a disinfection and sediment settlement unit. The Seabox unit encompasses three different treatment processes. At the intake, the seawater passes through an electro chlorination grid where sodium hypochlorite is mixed into all of the passing seawater. Inside the Seabox unit, the seawater is allowed to react with the chlorine for more than one hour. At the same time, particles larger than 15 micron will be settled out. At the outlet from the Seabox unit, a second electro chemical process producing hydroxyl radicals is used for final bacteria kill and to ‘decompose’ biological matters. NOV has progressed with the qualification of the SEABOX with the signature of its first contract with Conoco Philips aiming at testing the system during the 2018/2019 winter season in the North Sea at an offshore platform.
Subsea Seawater Desulfation:
NOV, SWIT technology consists of different configurations, where the Seabox unit (see above description and figure for SEABOX) is the cornerstone for providing a fully disinfected water with the bulk part of particles removed. Combined with microfiltration and membranes, it provides with completely particle free water, sulphate reduced of sulphate free water and low salinity water. Water qualities can be adapted to the reservoir-specific needs. The Seabox unit has no moving parts and only the Treatment Unit of the Seabox unit needs to be replaced for maintenance at regular intervals (typically every 4 years).
BHGE Sulphate Removal and Injection System is built on a combination of ultra and nano filtrations technologies
SAIPEM/TOTAL/VEOLIA are developing their respective solutions based on nanofiltration technology, the system is called SRINGSTM.
The SRINGSTM(TM) process architecture is given in figure below. It includes a water intake equipped with a buoyancy strainer located around 100 m above the mud line to avoid sucking suspended solids from the mud.
The water is then pumped to automatic backwashable coarse filters to remove solid particles above 50 microns. The filtered water is then sent to nanofiltration membranes for sulfate removal where only one part of the seawater is able to pass through the membranes, this part (called the permeate) has a sulfate content below 40 mg/l and will be pumped to feed the injection wells.
The other part (called the concentrate) is rejected to the sea via a 100 m long line and a discharge nozzle.
The SPRING unit will require replacing the membrane module regularly (e.g. twice a year) as there is no built-in membrane cleaning system. To avoid membrane biofouling, biocide will be stored in the SRINGSTM module and injected in the SRINGSTM process.
From 2010 to 2013, a series of laboratory tests proved their ability to operate in deep sea conditions (300 bars and 4°C). Then, in 2014, a subsea sulphate removal prototype passed the offshore test during an initial pilot phase carried out in TOTAL operated field, ALIMA, located offshore Republic of Congo. SRINGSTM industrialization started mid-2016. The concept has evolved and consortium is working to get technology ready for first deepwater deployment in 2020.
