SRB Treatment

What are SRB's?

Many species can survive in oxygen rich (aerobic) seawater, or in the oxygen starved (anaerobic) environments.  Aerobic bacteria can convert iron from the ferrous (Fe2+ ) to the ferric (Fe3+) state, and produce ferric hydroxide (Fe [OH]3), which is highly insoluble and causes formation damage.  Iron oxidizing bacteria can be slime forming species that form mats of high-density slime that cover surfaces. If allowed into the well, they shield corrosion forming bacteria colonies from chemical bactericides and plug the pores of the matrix holding the oil.  

Anaerobic species such as Sulphate Reducing Bacteria, (SRB's) can convert sulfate or sulphite that is naturally occurring, or present in a variety of drilling muds into hydrogen sulphite (H2S).  When combined with iron, iron sulphide is formed which is a scale that is very costly to remove.  In addition, SRB species can cause pitting corrosion of steel, and elevated H2S increases the corrosiveness of the water, which increases the possibility of hydrogen blistering, sulfide stress cracking and can lead to costly sweetening of sour oil.

To protect against BRB's, UV systems are usually installed following filtration of the seawater to remove suspended solids.  As filters can often be a source of microbial growth, UV equipment is best located post to filtration.

UV light has no residual effect, and so periodic conventional disinfection using sodium hypochlorite or other biocides is still required. The advantages of UV being added as a process stage include reduced chemical use, space saving, and improved operator safety.

How does UV work?

UV light is a physical, non intrusive process that has broad industrial and municipal uses. Systems comprise of 316L, Duplex steel or Super Duplex steel chambers that contain high powered UV lamps.  Wiping systems keep the quartz sleeve free from fouling, whilst a UV monitoring probe is used to ensure optimum disinfection.  Systems are usually supplied as duty/standby configuration.  For offshore use, modular skid mounted packages are usually built and tested before installation.

UV light between 200nm and 300nm can pass through the water, and is absorbed by the DNA contained in the nucleus of all living organisms. When the UV light is absorbed, the DNA becomes so damaged that the organism is instantaneously rendered non viable. Normal cell function ceases; the organism cannot replicate, respirate nor assimilate food. Once viability ceases, the colony quickly dies.

No organism is capable of surviving UV light. Many species are now increasingly tolerant to chlorine, and there are now 17 confimed pathogens such as Cryptosporidium and Giardia that demonstrate a high resistence to tradditional chlorine disinfection. Additionally UV light has been demonstrated to be very effective against SRB species, and is non selective; any micro-organisms present in the seawater will be deactivated and killed.

UV systems are sized based on three main factors: Flow rate, water quality and the challenge organism. Computational Fluid Dynamics (CFD) models are used extensively to design and correctly size UV systems.

Proprietary CFD software simulates both the flow and radiation profiles. Once the 3d model of the chamber is built, it is populated with a grid or mesh that comprises of thousands of small cubes. Points of interest, such as at a bend, near a sleeve surface, or close to the wiper mechanism use a higher resolution mesh, whilst other areas within the reactor use a coarse mesh. Once the mesh is produced, hundreds of thousands of virtual particles are fired thru the chamber. Each particle has variables of interest associated with it, and the particles are harvested after they exit the reactor. Discrete phase modeling produces delivered dose, headloss, and other chamber specific parameters.

The CFD models used are referenced against physical tests, particularly validation tests undertaken for drinking water projects in order to determine the accuracy of the model developed by CFD.