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The major problem for offshore oil recovery companies is the formation of barium sulphate (barite) in the tubes. This occurs when the SO4²- rich sea water comes in contact with the Ba2+ contained in the brine to form the insoluble barium sulphate. Scaling occurs when a saturated brine undergoes a temperature or pressure change causing the solubility to decrease, which results in the precipitation of solid crystals. Some of the other common scales that form in the offshore oil recovery process are barium sulphate (barite), calcium carbonate (calcite), strontium sulphate, and magnesium sulphate. Sometimes scaling can develop so rapidly as to cause complete pipe blockage and loss of oil/gas recovery within 24 hours. The removal process for these types of scales is extremely costly and very difficult to do (see Table A2). Table B1 below outlines some of the ways that scaling impacts the profitability of the oil and gas industry:

Scale location Problems caused Costs
Pipelines transferring oil/gas Reduced flow capacity
Increase linear pressure drop
Production shut downs (planned and forced)
Capital costs for increased pump capacity
Chemical inhibition costs
Well bore Loss of communication with reservoir
Loss of reservoir pressure maintenance for injectors
Loss of waterflood pattern injectivity
Increased potential for watered out production wells (reduced secondary recovery)
Workover rig costs
Loss of well and field production potential
Equipment costs to reduce hydrostatic head (pumps)
Chemical inhibition costs
Production shut down (planned  and forced)
Heat transfer equipment on the oil/gas recovery platform Reduced equipment process efficiency
Equipment failure, (can be catastrophic)
Increased thermal energy input costs
Reduced process capacity
Chemical inhibitor costs
Equipment replacement costs
Production shut-downs (planned and forced
Figure A2: Typical Barium Sulphite scaling on a 20cm diameter offshore well pipe
Figure A2: Typical Barium Sulphite scaling on a 20cm diameter offshore well pipe: perhaps the most serious fouling problem arising with offshore oil and gas production (see above table)

When water is injected downhole for reservoir pressure maintenance or disposed of into deep well-accessed saline aquifers, the conditions are perfect for scale formation. Steep pressure and temperature gradients are present - serving as thermodynamic energy wells. These extreme conditions also promote the super saturation of water with the dissolved ions that form the building blocks of scale crystals. Due to the large localized pressure differential, scale forms in the perforated interval of the wellbore where direct communication to the subsurface geological formation of interest exists.

Figure A3: Fouling as illustrated forms on the outside of subsea pipework
Figure A3: Fouling as illustrated forms on the outside of subsea pipework

External pipe fouling caused by contact with sea water

Biological fouling occurs when algae and other living organisms attach onto the outside material forming a biological film layer (protein layer). This first step is referred to as the micro-fouling stage. Once the biological film has been formed larger sea life such as barnacles, mussels, and seaweed can attach, onto the protein layer, forming a biological scale. This second step is referred to as the macro-fouling stage. The build-up of this type of scale can drastically increase the weight of the structure and decrease the hydrodynamics of oil carrying vessels. Whilst this problem is greatest for ships, accounting for up to a 40 % increase in fuel consumption and a 10% speed reduction on their vessels, its effects on stationary oil and gas installations are also serious. Wall thickening through growth at rate of up to 25mm/pa can obviously g can obviously generate dangerous overloading of structural members. More significant is the fact that safety valves in the oil and gas flow pipework can become completely clogged up to cause dangerous pressure build up and thus pipe rupture.

Existing methods of dealing with internal and external fouling

Table A2 presents an overall perspective on and evaluation of these methods

Table A.2 Comparison of methods of removing fouling from pipework on offshore oil /gas installations (the ideal 4 part profile is L L H +L . Cost H+ means cost exceeding ~ £1.5m- £3m per platform pa (5-10% of total OPEX)  L = ~ £227k. 

Cleaning Method Intrusiveness (damage risk) Geometrical constraints Effectiveness Cost
(A) Internal scaling        
Flushing L L L L
Swabbing or PIGging H H M/H M/H
Non-stop production PIGging L/M H H H
Abrasive PIGging H H H M/H
Smart PIGging* M/H H H M/H
Ice PIGging L/M L H L/M
Air Scouring H L H M/H
Chemical cleaning M/H L H H
Chelating agents (prevention) L/M L H H
Chemical inhibitors (ditto) L/M L H H
Jetting H L/M H M/H
Explosives H+ H+ H H
Lining and rehabilitation H M/H H H
HiTClean ultrasonic method L L H + L
(B) External marine biofouling        
Abrasive cleaning H L L H*
Non toxic protective coatings H H L** M/H
HiTClean ultrasonic method L L H + L
*High cost because of need for time intensive use of divers or ROV
** Not effective on static subsea structures like oil/gas rigs because state of the art non-toxic coatings require hydrodynamic drag to remove fouling loosened by coating surface tension effect which is only plausible on ships hulls whilst out of port. Toxic coatings are no longer allowed because of the effects on marine life/food chain