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OPITO

THE OIL & GAS ACADEMY

Produced WaterTreatment

Part of thePetroleum Processing Technology Series

Petroleum Open Learning

Designed, Produced and Published by OPITO Ltd., Petroleum Open Learning, Minerva House, Bruntland Road, Portlethen, Aberdeen AB12 4QL

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OPITO 1993 (rev.2002) ISBN 1 872041 85 X

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Visual Cues training targets for you to

achieve by the end of the unit

test yourself questions to see how much you understand

check yourself answers to let you see if you have been thinking along the right lines

activities for you to apply your new knowledge

summaries for you to recap on the major steps in your progress

Produced Water Treatment(Part of the Petroleum Processing Technology Series)


Contents Page* Training Targets 4

* Introduction 5

* Section 1 - The Problems Associated with Produced Water 6 TheMechanicsofWaterProduction CorrosionProblems ScaleProblems TransportationProblems DisposalProblems

* Section 2 - The Basics of Produced Water Treatment 15 PrimarySeparation GravitySeparation Coalescence ShortDistanceGravitySeparation GasFlotation CentrifugalForceSeparation ChemicalTreatment

1


Visual Cues training targets for you to

achieve by the end of the unit

test yourself questions to see how much you understand

check yourself answers to let you see if you have been thinking along the right lines

activities for you to apply your new knowledge

summaries for you to recap on the major steps in your progress

Produced Water Treatment(Part of the Petroleum Processing Technology Series)


Contents (contd) Page

* Section 3 - Produced Water Cleaning Equipment 23 APISeparators PlateInterceptors(orSeparators) Oil/WaterFiltersCoalescers GasFlotationUnits Hydrocyclones UseofChemicalAdditives

* Section 4 - A Typical Produced Water System 36 TiltingPlateSeparators TheFlotationUnit ChemicalDosingPackage ProducedWaterCaisson

* Test Yourself - Answers 47

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Training Targets

When you have completed this unit on Produced Water Treatment you will be able to :

List the sources of produced water

Describe the mechanics of water production

Explain what problems can arise from the production of water

Explain the basic principles which govern the separation of oil from produced water

Describe the construction and operation of 5 types of oily water clean up facility

Explain the requirement for chemical injection in a produced water treatment system

Describe the flow of water and separated oil through a typical produced water treatment facility

Tick the box when you have met each target.

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In the vast majority of oil fields, water productionbecomes a problem as the field gets older. Towardsthe end of their useful lives some oil wells may beproducing 95% of their total liquid as water. Thisproduced water may be extremely salty and likely tobe of little value to the operator. It is removed fromthe oil stream during primary separation and byother facilities, and has then to be disposed of.However, we are talking of a great deal of water insome cases. How do we dispose of it, and where dowe put it?

Offshore, the obvious place would be into the sea.Dumping this produced water directly fromseparators into the sea, would however, soon havethe operator in trouble with the authorities. Evenafter initial separation the water still contains oil insmall amounts. Serious environmental pollutionwould build up if oil contaminated water were to bedumped directly to the sea.

Onshore, disposal wells may have to be drilled, intowhich the produced water can be injected fordisposal. This also may have its problems. Oil inthe water, or fine solids, could plug the injectionwells in a very short time.

So the water which is produced with, and separatedfrom, the oil in an oilfield must be cleaned prior todisposal. This is what this unit is all about. In the

unit, we will be looking at the produced waterhandling system of an oil production facility. Beforewe examine a typical system, however, I think weshould look at where the water comes from and theproblems it poses in a little more detail. So, I havesplit the unit into four sections as follows:

In Section 1 we will look at the sources of produced water and the problems which may be encountered if we fail to treat it.

Section 2 will cover the basic principles involved in the treatment and clean-up of produced water.

In Section 3 we will examine the construction and operation of produced water clean-up equipment.

Finally, in Section 4, I will take you through a typical produced water handling facility which may be found on an offshore production platform.

Although produced water treatment applies to bothonshore and offshore locations, I will beconcentrating on the offshore situation in this unit.However, most of what I have to say would apply toboth.

Produced Water Treatment Introduction

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Water is present in some form, in most oil reservoirsbefore any production takes place.

There are however, many different types ofreservoir. In one very common one, the oilaccumulates above large volumes of water, which isusually salty. This water is what remains of ancientseas from an earlier period of Earths history.

This body of water is called an aquifer, and thereservoir is known as a water drive reservoir.

In addition, a considerable amount of water may alsobe found as small droplets distributed throughout theoil (and gas) in a reservoir. For reservoirengineering purposes this water is called connatewater or interstitial water. We will just call itformation water.

During production, further injection water may be pumped into the reservoir to assist in pressure maintenance.

Any of these types of water may eventually find theirway into the oil wells and be produced to the surfacealong with the oil. It is all then called producedwater.

Before we look at the problems which can be causedby this produced water, let us first consider how thewater gets into the producing wells,

The Mechanics of Water ProductionLook at Figure 1 which shows a cross section through a typical water drive reservoir.

Produced Water Treatment Section 1 - The Problems Associated with Produced Water

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PERMEABILITYPermeability is a measure of the ability of a fluid to flow through the rock from one pore to another. In order for it to be able to do this, the pores must be interconnected.

Permeability is measured in darcys- named after a French engineer who studied the flow of liquids through filters. He found that the flow increased in proportion to the pressure increase. However he also discovered that the flow was affected by the thickness, or viscosity, of the fluid.

Generally there is a wide spread of permeability in reservoir rocks.

So, the rock properties of porosity and permeabilityallow the oil to flow towards the producing wells. Butwhat causes the oil to flow through the reservoir?

Lets look at that now.

You are probably aware that fluids always flow fromareas of high pressure to areas of low pressure:

The oil producing wells create areas of low pressure in the surrounding reservoir rock as the well is opened at the surface and oil flows into the well

The aquifer is usually at a relatively high pressure. In addition, the injection of water into the aquifer is intended to maintain the reservoir pressure

You can see that the oil lies above the aquifer and thewell is taking oil which is not contaminated with water.The point at which the oil and water touch each other is called the oil water contact.

The oil is able to flow through the reservoir rocktowards the well because the rock is porous andpermeable. These are probably the two most important properties of reservoir rocks.

POROSITYPorostiy is the property of the rock which enables it to hold fluids within itself. The oil, gas and water are contained in tiny holes in the rock called pores.

Sandstone is a common reservoir rock. It is made up of grains of sand which are cemented together at the points where they touch. Between the sand grains are void spaces - the pores.

The ratio of the volume of the pores to total rock volume expressed as a percentage is the rocks porosity.

This means that, if you have a sandstone reservoir with a porosity of 25%, for every 4m3 of reservoir rock, 1m3 consists of holes and 3m3 solid sand grains.

Another common reservoir rock is limestone. This is a rather brittle rock which contains lots of tiny cracks and fissures. These tiny cracks give the limestone its porosity.

The high pressure water in the aquifer,therefore, will tend to displace the oiltowards the low pressure areassurrounding each well bore.

Figure 2 on the next page, shows thesituation with just one producing welland one water injection well.

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If this situation remained constant there would be noproblem. However the situation does not remainconstant. Think about this and try to answer thefollowing Test Yourself question.

Test Yourself 1 As oil is removed from the reservoir what will happen to the position of the oil water contact ?

You will find the answer to Test Yourself 1 on Page 47

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Over a period of time, as the oil water contactapproaches the well intakes, the water will start toflow preferentially to the oil wells. This occursbecause the water is much less viscous than the oiland therefore flows more easily through the rock,bypassing the oil.

The water is said to finger through the oil.

Figure 3 shows water starting to follow thesepreferential routes through the reservoir rock.

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Once water starts to break through to theproducing wells, it tends to be produced in ever increasing amounts.

The ratio of water produced to total production iscalled the water cut and is expressed as apercentage. To make sure you understand this,have a go at the following Test Yourself question.

Test Yourself 2a) If a well produces 3975m3/d of oil and 795m3/d of water, what is the water cut.

b) A well produces a total of 875m3/d liquids and the water cut is 20%. What is the oil production from this well.

c) What is the water cut of a well if the total production is 556m3/d., and the oil production is 397m3/d

You will find the answers to Test Yourself 2 on Page 47

The water cut from a particular well or field dependson a large number of factors. These include:

The geology and porosity of the reservoir rock

The size and, particularly, the vertical thickness of the reservoir

The degree of fracturing of the oil field

The position and depth of the producing wells in relation to the oil water contact

How long the reservoir has been producing oil

Actual water cuts vary tremendously, of course, butcan be as much as 99%. Imagine a field whichproduces a total of 15,900m3 of liquid per day with awater cut of 60 %. This means that 9540m3 of waterare produced every day.

This can pose significant operating problems andthese are what we will look at now.

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Corrosion ProblemsWe said earlier that the aquifer water can be verysalty. Injection water, being in the main sea water, issalty as well. It follows then that the produced waterwill be salty also. In fact the saltiness, or salinity,of produced water is usually considerably more thanthat of normal sea water. To put it in perspective:

fresh water from streams, lakes etc. usually contains less than 0.2 % salt

sea water has an average salt content of 3.5%

produced water can contain up to 15% salt

Pure water in itself is not particularly corrosive.However, up to a point, the more saline it becomesthe more corrosive it is.

If the produced water is allowed to pass through allthe surface processing equipment to the oiltransportation system, it could cause considerablecorrosion damage to pipes, vessels and otherequipment.

In fact, corrosion costs the petroleum industrymillions of pounds annually. It makes sense to try toreduce this expense.

One of the ways of reducing corrosion damage is toseparate the water from the oil at the earliestopportunity and dispose of it. In fact this, togetherwith the separation of gas, is one of the firstprocesses in a production facility. This howevergives rise to another problem - one of disposal. Wewill look at this shortly.

Scale ProblemsSalts are initially dissolved in the water present in areservoir. As conditions change when this water isproduced, the salts may be precipitated as solidsand deposit as scale.

This can reduce pipe diameters, plug vessels andequipment which in turn can lead to lost production.Once again, removal and disposal of produced watercan help prevent the problems of scaling.

Transportation ProblemsThe produced oil may have to be transported from anoffshore location to a shore based refinery or tankerterminal. There are two ways of doing this. If the fieldis large and the economics justify it, the best way is bypipeline to shore. However, some fields are too smallto justify the expense of a pipeline or are too far fromshore. In this case the oil is loaded into a tanker atthe point of production via a tanker loading facility.Either way, water in the oil to be transported cancause problems:

The obvious one we have looked at already, that of corrosion. Salt water in pipelines or tanker loading units can corrode facilities rapidly. I dont think I need to elaborate on that at this time.

If the oil is going down a pipeline, excess water reduces the efficiency of the line, leaving less space for oil.

Water being sent to a refinery with the oil can cause serious upsets in the distillation process. Refinery operators usually limit the amount of salt and water which they will accept.

When loading oil to a tanker there are laid down limits of water in oil which it is permitted to take. If more than, say, 0.5 % of the cargo loaded is water, then the producing company can face severe penalties.

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It would seem from the foregoing that one of thefirst things which we must do on a production facilityis get rid of the water. This is of course whathappens. On most installations water is separatedfrom the produced oil in the first process system.The separated water has no value and has to bedisposed of. But how? This brings us to our finalproblem, that of produced water disposal.

Disposal ProblemsWhen trying to decide how to get rid of the water wemust consider first of all the location of theproduction facility.

Think for a moment and try to decide how youwould dispose of 1590m3 per day of produced waterfrom a site on land.

You may have come up with one of the following:

Dump the water into lakes or rivers

Dump the water into sewers

Both of these solutions would be totallyunacceptable.

In the first case, pollution of the fresh water by theSalts in the produced water would cause damage tothe environment and could destroy wildlife. Drops ofoil in the water would also cause considerableenvironmental pollution.

Sewers are not built for these amounts of water andwould be overloaded, in addition to sufferingpollution problems at the outfalls.

You may have thought of drilling wells and injectingthe water back into a reservoir. This is in fact done.But the water has usually to be treated before it canbe injected. It may have to be filtered and dosedwith chemicals to make it suitable for injection.

If the production facilities are located offshore theproblem of disposal may seem easier. Why not justdump it into the sea? Unfortunately it is not quitethat simple.

After initial separation, the produced water is stilllikely to contain a considerable amount of oil in theform of small droplets. The actual amount will varyfrom installation to installation but could be of theorder of 150 ppm.

(The unit ppm means parts per million. In otherwords, in every million drops of liquid 150 of themwould be oil, the rest water.)

This may not seem very much, but if thosequantities were dumped into the sea, an oil slickwould soon form and pollution would occur.

In most countries the removal of oil from producedwater before dumping it into the sea is a legalrequirement. The quality of produced waterdisposed of in this way is subject to strict control.

For instance, discharging produced water into the sea in the UK sector of the North Sea is subject to compliance with the following conditions at present:

On average, discharged water must not contain more than 30 ppm by weight of oil.

An oil content of up to 100 ppm is allowed in individual monitoring samples

For a normal month of sampling, not more than three samples (4% a month) may exceed the limit of 100 ppm.

Regular monitoring of effluent discharged from each platform to the sea is a stipulated requirement

Samples should be taken at 0700 and 1700 hrs each day

The figures of 30 ppm is the one currently in force and is constantly under review. It is not inconceivable that it could be reduced even further at some future date.

So, bearing in mind that I said that we wouldconcentrate on an offshore location, it would seemthat our biggest problem is getting the oil-in-waterconcentration down to acceptable limits.

This is what we will concentrate on for the rest of thisunit.

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Before going through the summary of this section,try the following Test Yourself question,

Test Yourself 3Are the following statements true or false?

True Falsea) Permeability is a measure of the ability of a rock to allow fluids to pass through it.

b) The oil water contact within a reservoir tends to move down as the production from the field proceeds.

c) Seawater usually has a greater salt content than produced water.

d) Produced water can cause loss of efficiency in pipelines.

e) In the UK sector of the North Sea discharged water must not contain more than 130ppm by weight of oil.


13


Summary of Section 1During this section I have tried to introduce you tothe problems arising from the production of waterwith oil.

We started by looking at the sources of producedwater and you saw that it can be from the aquifer,formation water or the injection water which isused to maintain reservoir pressure.We then looked at the mechanics of waterproduction and considered the rock properties ofporosity and permeability which allow fluids to flowthrough a rock.

You saw that the relatively high pressured waterunderlying the oil pushes the oil towards thewellbores. However the water may eventuallystart to finger through the oil and be produced inever increasing quantities. You discovered thatvery large quantities of water may be producedand I defined the ratios of oil and water productionas the water cut.

We then moved onto the problems of waterproduction and saw that they could beclassified as :

corrosion problems

scale problems

transportation problems

disposal problems

We concentrated on disposal problems offshoreand I indicated that dumping water into the sea isthe easiest option but this is often governed bylegislation. I gave as an example that theaverage oil in water content permitted to bedischarged into the UK sector of the north seamust not exceed 30 ppm.

In the next section we will go on to look at someof the basic theory behind produced watertreatment. In particular we will concentrate on theremoval of oil.

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In this section we are going to look at some of themethods which could be used to treat producedwater. The water can contain dissolved gases andsolids and also some suspended solid particles suchas sand.

However, we are going to concentrate on theremoval of oil from water, so that it can be dumpedto the sea. Lets start with the separation of waterfrom the main oil stream. That is, after all, the firstpart of the treatment programme.

Primary SeparationThe total production from an oil field flows from thewells to the primary separation system. The functionof this system is to separate the production into itsindividual phases of oil, gas and water. The processis carried out in large vessels - the separators. Atypical 3-phase separator is shown in Figure 4.

Produced Water Treatment Section 2 - The Basics of Produced Water Treatment

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The vessel is called a 3-phase separator because itseparates the total flow stream into the threeindividual streams of oil, water and gas. A 2-phasevessel would separate the stream into the liquid andgas streams.

I dont intend to go through the construction andoperation of a separator at this point. A programmeon Oil and Gas Separation is also available in thePetroleum Processing Technology Series.

Briefly, however, the oil, water and gas streamenters the vessel at the inlet and is deflected by theinlet deflector. The gas passes towards the gasoutlet via straightening vanes and mist extractor andthe liquids fall into the liquid accumulation section.

This is where the separation of oil and water takesplace. But how does it occur? Think about it for amoment then answer the following Test Yourselfquestion.

Test Yourself 4If you shake a mixture of oil and water ina beaker and allow it to stand for a periodof time, what will happen to the twosubstances ?

Can you explain your answer?

You will find the answer to Test Yourself 4 onPage 47

What you have read in the answer to Test Yourself 4is exactly what happens in the separator. The waterand oil separate due to a difference in their densities.

Providing the oil and water stay in the vessel for asufficient period of time, the bulk of the water can beseparated from the oil. This water is the producedwater which has now to be disposed of.

Although primary separation is quite efficient, oil mayremain in the water as small droplets. These havealso to be removed. We can now look at some waysof doing that.

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Gravity SeparationThe primary separation we have just looked at is anexample of gravity separation. Oil and waterseparate because of the difference in their density orspecific gravity. Most crude oils are less dense thanwater so they tend to float on top of water.

Even where the amount of oil in the water isminimal, given sufficient time and under the rightconditions, the crude oil will float to the surface of thewater where it can be removed.

There is, however, a theoretical lower limit to thesize of crude oil droplets which will rise freelythrough the water. Oil droplets which have adiameter of less than, say, 5 microns will not risethrough the water, but will stay in suspensionindefinitely. A micron is one millionth of a metre.

In practical terms, the limiting droplet size in an oil/water gravity separator is much higher, and in therange of 50 to 150 microns. This is because of suchfactors as turbulence, limited retention time, andso on.

It would appear then that a certain amount ofproduced water clean up can be done in a simpletank where the water stays long enough for the oildroplets to rise to the surface and be removed.

However, if the smaller droplets will not rise,something else must be done to remove them. Whynot try to combine them into larger drops, which willthen rise? This is indeed done by using varioustypes of coalescer.

CoalescenceTo coalesce simply means to join together or unite.

Entrained oil droplets in the water which are toosmall to rise rapidly by gravity, can be coalesced in anumber of ways. One way is to pass the oily waterthrough a specially developed cartridge. This ismade of a porous plastic medium such aspolypropylene or polyurethane foam. When in usethe oily water flows to the centre of the cartridge andout through the walls, where coalescence takesplace.

The larger oil droplets then rise to the surface of thewater by gravity as before,

Figure 5 shows the cartridge coalescer principle.

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Short Distance Gravity SeparationUnder constant conditions, a drop of oilsuspended in water tends to rise at a fixedrate. This rate is called the terminalvelocity. The time required for separationtherefore depends on the distance the drophas to rise to reach the surface.

In order to speed up this process, varioustypes of equipment have been developedwhich reduce the distance the particleshave to travel.

This type of equipment provides closelystacked parallel plates or tubes throughwhich the water flows withoutturbulence. As the oil droplets risewithin these confined spaces they haveonly a short distance to travel beforereaching a solid surface. where theyconcentrate and coalesce.

Figure 6 shows an end view of a simpleplate pack.

Gas FlotationFlotation is a process which has long beenused for cleaning industrial waste water.The operating principle depends onincreasing the buoyancy of entrained oil orsolid particles, enabling them to rise morefreely through the water. This increase inbuoyancy is achieved by attaching gasbubbles to the suspended particles.

The gas bubbles are generated by :

dissolving gas in the water under pressure and then releasing the pressure prior to entering the cleaning unit

mechanically introducing the gas into the water.

It is the second of these methods which ismore commonly used in the oil industry.

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Figure 7 shows oil droplets with gas bubblesattached rising through water.

In a flotation cell, the oil and gas mixture accumulates on the surface of the wateras a layer of oily froth. This is skimmed from the top of the water to a channel whichdirects the oil to a recovered oil system. The skimming may be over a simpleadjustable weir. Alternatively a system of paddles may be used to sweep the oilyfroth continuously from the surface of the water.

Figure 8 shows a much simplified version of a flotation cell. I will describe this inmuch more detail in Section 3.

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Centrifugal Force SeparationYou are probably familiar with the principle ofcentrifugal force, but lets just remind ourselves of it ina rather simple way.

Imagine a spinning disc, similar say to a record on arecord player. If you dropped a liquid onto the discnear the centre it would be flung to the outer rim of thedisc. The force which causes this to happen iscentrifugal force.

If the liquid was pumped into a container, in such away that it was made to swirl within that container, thecentrifugal force would cause a vortex to be formed.

(A familiar example of a vortex is the cone-shapedwhirlpool which forms above the plug hole when waterruns out of a bath).

If a mixture of two liquids, of different densities, waspumped into the container, centrifugal forces wouldtend to separate the two liquids:

the liquid having the lower density would migrate towards the middle of the vortex

the liquid with the higher density would migrate towards the outside of the vortex

If the two liquids are water and oil, it would be the oilwhich would migrate towards the centre of thecontainer.

This is the principle of a hydrocyclone, anotherpiece of equipment used in the separation of oil fromwater.

Figure 9 illustrates this principle.

The oily water is introduced continuously to the unit.A low pressure outlet is connected to the centre.Through this outlet the oil is continuously removed.Clean water is thrown towards the outside wall of theunit, where it leaves by a separate outlet.

Chemical TreatmentSometimes emulsions form in the produced waterwhich are very difficult to break down.An emulsion is a stable mixture of two or moreimmiscible liquids, one dispersed in another, in theform of very small droplets. There are two distincttypes of emulsion. They are:

Water-in-oil emulsions where a small amount of water is dispersed in a larger amount of oil

Oil-in-water emulsions where a small amount of oil is dispersed in a larger amount of water

The first type is the more common, but oil in wateremulsions can occur and may be a problem in thetreatment of produced water.

In order to break down this type of stable emulsion,chemicals are injected. These chemicals, calleddemulsifiers, help the oil droplets to coalesce andseparate from the water.

Demulsifiers are usually used in conjunction withsome other form of water clean up facility.

Before summarising Section 2, try Test Yourself 5,which will help you bring together the basicprinciples of water treatment we have covered inthis section,

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Test Yourself 5In the first column of the table below I have listed the following terms: porous medium, plate pack, oil droplets rising,demulsifier, finely dispersed bubbles, vortex. Each is associated with one or more methods of oil removal fromproduced water. Put a tick in the appropriate column (s) to show which one(s).

Short Distance Centrifugal Gravity Coalescence Gravity Gas Force Separation Separation Flotation Separation

porous medium

plate pack

oil droplets rising

demulsifier

finely dispersed bubbles

vortex


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Summary of Section 2In this section we have been looking at some of the basic principles which govern the separationof oil from water in a water clean up facility.

First of all we considered the primary separation of the water from the main oil stream. You sawthat this was a simple gravity separation process. You also saw that gravity separation is thebasis of most produced water treatment facilities.

However, in order to speed up the process or make it more efficient you saw that other types oftreatment could be undertaken.

We considered:

coalescence

short distance gravity separation

gas flotation centrifugal force separation

You also saw that chemicals may have to be injected into the produced water to assist inseparation, particularly if an oil-in-water emulsion has formed.

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In this section we are going to have a look at theconstruction and operation of equipment which canbe used to clean up produced water. A variety ofsuch equipment is available, but we will concentrateon the most common:

API Separators

Plate Interceptors (or separators)

Filter Coalescers

Gas Flotation Units

Hydrocyclones

The simplest of the above pieces of equipment is thefirst on the list so let us start with this.

API SeparatorsAs we have seen, the most common way ofseparating oil and water is by the use of gravityacting on the density difference between the twoliquids.

Time is also required for this process to workeffectively. Each separator is designed to retain theliquid mixture within it until separation has beenaccomplished. This time is known as the residencetime or retention time.

However, the time available on an offshore productionfacility is very limited and residence times areextremely short. As API Separators require relativelylong residence times, there are very few of them foundoffshore. I have included the API Separator becausethey are very common in land installations andcontain features which are found in other devices.

The API Separator is basically a very large open tankor pond which permits a long residence time for the oilto separate from the water.

Figure 10 is an illustration of a typical API Separator.

The produced water enters the unit on the lefthand side. As it enters the chamber it hits a smallstilling plate. The stilling plate distributes theincoming liquids evenly over the width of theseparator and reduces turbulence and mixing.

Underneath the stilling plate is a sludge/debristrap which will catch small solids as they sink. Thesludge outlet is normally designed so that it caneasily be freed if it should become blocked.

Produced Water TreatmentSection 3 - Produced Water Cleaning Equipment

23


The water flows:

from the stilling plate

above and below an intermediate baffle (water with oil in it will rise above the baffle water with no oil in it will fall under the baffle)

under the final baffle

over the outlet weir

out of the separator via the water outlet.

The level in the separator is controlled by adjustingthe height of the outlet weir.

The oil flows:

from the stilling plate above the intermediate baffle

to the surface where it forms a layer of oil on top of the water

into an adjustable oil skimmer

out of the oil skimmer via the oil outlet

The adjustable oil skimmer is normally set atbetween 1/4th to 1/8th of an inch above the level ofthe water.

API Separators require careful adjustment of theskimmer to remove as much of the oil as possible,but without removing any water. Slight changes inflow will raise or lower the height of water falling overthe weir and, if the oil/ water interface is disturbed,water could slop over with the oil.

As I said earlier, API Separators are not suitable foroffshore applications. There, more efficient meansof oil and water separation are necessary. Facilitiesare needed which are designed to reduce theresidence time required for efficient oil/waterseparation to take place.

This reduction in residence time is important. For agiven flow rate of fluid the residence time can onlybe extended by increasing the volume of theseparator. Offshore, where space is at a premium,this is extremely difficult.

One way of reducing the required residence time isto include some form of coalescing device in thewater cleaning unit.

Coalescing devices provide a solid surface whichcan be contacted by small oil droplets. Anaccumulation of these oil droplets creates a thick oilfilm which becomes a source of large drops.Eventually enlarged drops of oil break loose from thesolid surface. These large drops separate from thewater phase much faster than the original smalldroplets.

Coalescing surfaces come in two basic forms:

Plate interceptors

Cylindrical cartridges (called fixed media cartridges)

Lets look at a couple of plate interceptors first.Two main types are in use, the parallel plate andthe corrugated plate interceptor.

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Plate Interceptors (or Separators)Figure 11 shows a typical parallel plate interceptor. Plate interceptors work on theprinciple of short distance gravity separation, which we looked at in Section 2.Have a look at the figure now and try to visualise the flow through the unit.

Now follow the flow with me with reference toFigure 11.

The produced water flows into an inlet chamber which is equipped with a sludge/ debris trap.

Any gas in the produced water stream then rises, and leaves the separator by a vent.

The produced water then flows through the parallel plate pack.

The pack consists of a number of parallel steel orplastic plates connected together with small gapsbetween them. It is set at an angle of about 45 tothe horizontal and the produced water flowsdownwards, towards the right of the illustration.

Flow between the plates is far more streamlined thanin the inlet chamber. In addition, the distancethrough which oil droplets have to rise beforereaching a surface is much smaller.

Oil droplets coalesce on the underside of the platesand slide upwards and backwards against the flow ofthe water. The oil then breaks free and rises to thesurface where it forms a layer on top of the water.The oil is removed from the surface by a skimmer.

The water carries on towards the outlet chamber,then doubles back over the outlet weir whichcontrols the height of the liquids in the separator.

25


The water then flows away, under gravity, out of theseparator.

A parallel plate separator will often reduce the amountof oil in the produced water from 5 000 ppm to :

100 ppm of oil (with a residence time of, say, 10 minutes)

or

50 ppm (With a residence time of 30 minutes)

Their main disadvantage is that frequent cleaning isrequired to remove solids which stick to the plates.

A variation of the parallel plate separator is thecorrugated plate interceptor which we can look atnow.

The plate pack is installed in the same manner as theparallel plate interceptor pack, at an angle of 45, andthe oil and water flows are identical.

Figure 12 illustrates the major difference between thetwo types of separator - in this type the plates arecorrugated. In the tops of the corrugations are slotswhich enhance the oil removal process.

Oil coalesces on the underside of the plates, andaccumulates beneath each corrugation. From there itmigrates backwards and upwards until, eventually, itforms a layer of oil on the surface, as before.

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A corrugated plate separator will often reduce theamount of oil in the produced water from 5 000 ppmto 30 ppm oil with a residence time of 5 to 10minutes.

This type of separator has been adapted for use onoffshore systems and has even been tried in highpressure systems at the wellhead so as to separateproduction water before dissolved gas is released.

The corrugated plates can be made ofpolypropylene, polyvinyl chloride, stainless steel orcarbon steel.

As we have seen, both the parallel plate separatorand the corrugated plate separator have theirplates tilted at an angle of 45. Because of this,both types are often referred to as tilting plateseparators.

When you are satisfied that you understand theconstruction and principle of operation of the plateinterceptors, we can move on to another type ofcoalescer unit.

Oil/Water Filter CoalescersIn an oil / water filter coalescer the oil / waterseparation is achieved by coalescence of dispersedoil droplets within specially designed coalescercartridges.

In our example in Figure 13, a set of cartridgeswould be mounted on a deck-plate within a verticalpressure vessel. Note that, for simplicity, I have onlyshown one cartridge in the vessel, and omitted thedeck-plate.

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The flow through the unit is as follows:

oily water enters the base of the vessel

is distributed to the cartridges

flows radially outwards, through each cartridge into the main part of the vessel

As the fluid passes through the cartridges, the oildroplets are forced into close contact with each other.They coalesce within the cartridge wall and rise fromthe outer cartridge surface to the top of the vessel.These droplets then form an oil layer which isdischarged through the oil outlet. (Have another lookat Figure 5 on Page 13.)

An oil/water interface level is maintained in thevessel by a level controller (LC), which controls flowfrom the oil outlet.

An oil/water filter coalescer will often reduce theamount of oil in the produced water from 5 000 ppmto between 2 and 15 ppm.

Under severe operating conditions, the cartridgesmay not last very long.

However cartridge life can be improved by :

pre-filtering the oily water

steady state operation

scale inhibition, where required

Before moving on to the next part of this section,have a go at the following Test Yourself question.

Test Yourself 6Fill in the missing word or words from the following sentences.

a) In an A.P.I. separator the produced water enters the unit and hits a small ................... ....................... which distributes the incoming liquids.

b) In a plate interceptor the inlet chamber is equipped with a ...............or ...................trap.

c) The outlet ..................... controls the height of the liquids in the unit.

d) The pack consists of a number of ......................plates.

e) Tilting plate separators may have .....................or .......................plates.

f) The oily water flows radially outwards through the ..................... where the oil droplets .......................

You will find the answer to Test Yourself 6 on Page 49

We can now take a look at a produced water clean-up facility which has asomewhat different operating mechanism. This is the gas flotation unit.

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Gas Flotation Units You will remember from Section 2 that the principle of operation of a gas flotation unit (also called a depurator)is that the oil droplets, and any suspended solids, are assisted to the surface by small gas bubbles. The oil andsuspended solids are then skimmed off as a froth.

Part of a typical flotation unit is illustrated in Figure 14. Study the illustration carefully.

Let us first of all take a look at the flowof produced water through the unit.

The produced water:

enters the inlet chamber

flows out of the inlet chamber, under a baffle, and enters the first flotation cell where most of the oil is removed from the water

flows over the top of an internal baffle and enters the second flotation cell where the rest of the oil is removed from the water

flows out of the second flotation cell, under a baffle, and enters the exit chamber as clean water

flows out of the exit chamber into the suction of the recirculation pump

leaves the recirculation pump and goes to the water outlet or back into the two flotation cells

Make sure you can follow the flow of thewater before proceeding.

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We will now look at how the water is aerated.

You can see in Figure 14 that the recirculation pumptakes water from the exit chamber. The water cango in one of two directions:

water outlet

back to the flotation unit

The level in the exit chamber is controlled by a levelcontroller (LC) which opens and closes the valve inthe water outlet.

If the level rises the valve will open and allow waterto leave the system. If the level falls, the valve willclose and retain water in the system.

The bulk of the water (up to 70% of the designthroughput) is recirculated back to the two flotationcells.

As the water enters each cell it passes through aventuri. This is a device which uses the flow ofwater to create a low pressure area. Gas, from thearea above the water, is sucked into the venturi andmixes with the water. The gas and water mixture isthen discharged at the outlet at the bottom of eachcell.

The gas drawn into the venturi results in millions oftiny bubbles being released at the bottom of theflotation cell. These bubbles attach themselves to oildroplets in the water, and carry them to the surface.

The oil and gas then form a layer of foam on top ofthe water.

In the type of unit we have been looking at, a venturiwas used to create the gas bubbles. As you saw inSection 2, other methods of creating the bubblesmay be used.But what happens to the oil ?

Take a look at Figure 15. This shows a side view ofthe flotation cells. You can see the layer of oily foamon top of the water.

To the side of the unit is a collecting box called alaunder. The oily foam spills over a weir into thelaunder where it separates into oil and gas.

The gas flows back into the flotation cells and the oilcollects at the bottom of the launder. The oil level iscontrolled by a level controller (LC) which opensand closes a valve on the oil outlet line.

30


In general:

In some large units there maybe as many as ten flotation cells

The residence time in a flotation unit may be as low as three minutes

A flotation unit may reduce the amount of oil in the produced water to around 10 ppm

Flotation units may also remove most of the solids suspended in the produced water

A problem associated with flotation units is that theyare difficult to control. The size of the bubblesaffects the efficiency of the unit and it may takemany hours to set up each venturi to give theoptimum operating conditions.

If you are happy with the construction and operationof flotation units, try the following Test Yourselfquestion.

Test Yourself 7

Correct the following sentences, which all refer to a flotation unit.

a) Oil spills over a weir into the exit chamber.

b) Water flows under the middle (internal) baffle, in a two cell unit

c) The recirculation pump takes its suction from the launder.

d) Gas is introduced to the top of the cells as finely dispersed bubbles.

e) Gas from the area above the water is sucked into a venturi and mixed with the oil.

f) In the launder, oil and water separate and the oil level is controlled by a level controller.


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HydrocyclonesThe most important development in oil / water treatment in recent years is thehydrocyclone. This is a unit which uses centrifugal force to separate oil and water.

Figure 16 is an illustration of a hydrocyclone.

The hydrocyclone has a cone shaped liner within apressure shell. The cone shaped liner of the unit isfabricated as a thin walled vessel.

Oily water enters the unit through an inlet into theliner. The inlet is designed to spin the water as itenters the swirl chamber. Inside the swirl chamberthe flow forms a vortex which passes along thelength of the liner.

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The water / oil mixture is accelerated to high velocityand strong centrifugal forces develop. Thispromotes oil / water separation. The more densewater phase moves to the wall of the liner anddisplaces the lighter oil phase towards the centre ofthe vortex.

The water continues to flow to the water outlet alongthe sides of the liner. The oil flows in the oppositedirection, via the low pressure central core, to beremoved at the oil outlet. The oil stream is called thereject oil stream.

Total residence time of the liquid in the hydrocycloneis about two seconds. Hence the equipment canbe compact.

The capacity of a hydrocyclone is dependent on thepressure drop between inlet pressure and reject oilstream pressure.

Hydrocyclones are usually mounted as groups inparallel to increase capacity. Each unit can beopened up or closed in. This enables the operator tomaintain optimum conditions during varying flowrates.

In addition, units can be installed in series toincrease oil removal. The water leaving onehydrocyclone enters the next, and so on. Using thissystem oil concentrations as low as 5 ppm can oftenbe achieved.

The major factors influencing the performance of ahydrocyclone are:

The specific gravity difference between the oil and water. The greater the difference, the greater the potential for rapid separation.

The oil droplet size. Larger droplets move more rapidly towards the central core.

Temperature. This affects both density and viscosity. Higher temperatures increase the potential for easy separation and therefore hydrocyclones are most often installed upstream of any produced water coolers.

Higher flow rates. These increase the intensity of the centrifugal separation forces.

The reject oil stream will not be 100% oil. It will be amixture of oil and water. This mixture is then fedback into the main process, where the oil isrecovered.

Hydrocyclones have a weight / efficiency ratio whichmakes them attractive for use offshore.

Use of Chemical AdditivesChemicals are increasingly used in produced watertreatment facilities. I dont intend to go into thechemistry of the way that they work. However, Ithink that you should be aware of the main types ofchemical used and what their function is.

Emulsion breakers or demulsifiers. Thesechemicals assist in separating oil/water emulsions.They break down the mechanisms which cause theemulsion to form.

Flocculation and flotation agents. Thesechemicals act as seeds around which small solidparticles or oil droplets may collect. The increasedsize assists in the separation process.

Corrosion Inhibitors. These chemicals helpprevent corrosion of the vessels and pipework of theproduced water system.

Biocides. These chemicals kill bacteria in thewater. They are used to prevent the formation ofslimes.

Oxygen scavengers. These chemicals are used toremove residual oxygen in water. This also helps toprevent corrosion.

Scale inhibitors. These chemicals preventdissolved solids coming out of solution and beingdeposited on pipework, etc., as scale.

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In all instances, great care is needed to ensure that:

the chemicals used are harmless to the environment

one chemical will not counteract the effects of another chemical used in the process

the chemicals used will not affect downstream processing

In all cases chemicals should be introduced to thesystem at the correct dosage rates. Too high adosage rate is often worse than no dosage at all.

Summary of Section 3In this section we have looked at different types of oil/waterseparators which may be found on an oil production facility.

We have looked at the construction and operation of :

API separators

parallel plate separators

corrugated plate separators

oil / water filter coalescers

gas flotation units

hydrocyclones

We have also taken a brief look at chemicals which may be used in a Produced Water System.

In the final section of this unit we will consider a typical produced water handling system.Before you move on to that however have a go at the following Test Yourself question.

34


Test Yourself 8State whether the components listed on the right are part of :

a) An A.P.I. separator

b) A tilting plate separator

c) An oil / water filter coalescer

d) A flotation unit

e) A hydrocyclone

Note: some of the components are found in more than one type of unit.


List of components

1. swirl chamber

2. adjustable oil skimmer

3. baffle

4. weir

5. cartridge

6. level controller

7. sludge trap

8. cone shaped liner

9. venturi

10. recirculation pump

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In this final section, we are going to look at a completeproduced water system.

The system I will use as an illustration includes twotilting plate separators, a flotation unit and a producedwater caisson. It is fairly typical of produced waterhandling systems which you might find offshore.

In our example, the function of the system is to remove:

oil

entrained gas

fine solids

from the produced water streams. This is achieved byshort distance gravity separation and flotation.

Take a look at Figures 17. This is a simple blockdiagram of the overall system. Study this for a momentand get a general idea of the relationship between thevarious subsystems.

As we work through the system in detail, however, wewill need a more complex drawing. This is Figure 18,entitled A Typical Produced Water System. In order toallow easy reference to this drawing, I have included it as a separate sheet in your pack.

Take this out now and study it for a few minutes.

In order to help you, I have also included a symbol key,which includes those symbols used in the drawing.

Produced Water TreatmentSection 4 - A Typical Produced Water System

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Look at Figure 18. You should be able to identifyfour separate parts of the system. They are:

the two tilting plate separators (TP-01 and TP02) in the upper left hand corner of the figure

the flotation unit (DP-01) in the upper right hand corner

the chemical dosing system in the bottom left hand corner

the circulation and recycle pumps in the bottom right hand corner

We will look at each of these sections in turn as Iguide you through the flow diagram. Because thepumps are part of the flotation unit equipment we willstudy these two sections together.

Not shown in Figure 18 is the produced watercaisson where the treated water enters the sea.We will look at that later as a separate item.

Tilting Plate SeparatorsThe produced water enters the system from themain oil, gas and water separation facilities. Findthe entry point in Figure 18 and follow the flow.

The first thing you will see is that a chemical isinjected at this point. The chemical being injected isa demulsifier which assists in the separation of theoil and water. You will remember we discussed theuse of demulsifiers in Section 2.

The diagram indicates that the chemical enters theproduced water line via an injection quill.

Figure 19 is an illustration of an injection quill. It isdesigned to ensure that the chemicals are efficientlymixed with the water flow.

37


The chemical injection line is protected from anyback flow by a non-return valve (Figure 18).

Downstream of the injection quill, oily water from theplatform drains system joins with the producedwater.

Just after this connection is a sample point. Inorder to obtain a true sample, this connectionis placed after the point at which the twostreams combine.

Beyond the sample point another line enters theproduced water line. It is protected from back-flowby a non-return valve. This line is the discharge linefrom the float recycle pumps P-02A and P-02B.We will be looking at these two pumps later.

The produced water then splits into two individuallines, one to each of the two tilting plateseparators. Each line is fitted with a butterfly valvewhich is indicated as being LO. This means that thevalves are normally locked open to make sure thatTP-01 and TP-02 are not accidentally isolated.

The tilting plate separators are identical, so we willjust look at TP-01.

In our example, TP-01 has three sets of corrugatedtilted plates. The produced water enters the side ofTP-01 via three separate connections. This ensuresthat turbulence is reduced at these points byreducing the produced water flow rate.

Any gas which is released from the produced wateris vented off to the vent header. Note that in ourexample, there is no valve on the vent line. Thereare no pressure relief valves on TP-01 so, to preventan accidental over-pressure situation, the vent line isfitted without an isolation valve.

TP-01 is also fitted with a nitrogen purgeconnection. If TP-01 is shut down, the vessel can bepurged with nitrogen. The nitrogen ensures that anair/gas flammable mixture cannot occur, bysweeping out any flammable gases before thevessel is opened for maintenance.

TP-01 is fitted with a level gauge (LG-01) so thatthe operator can check the liquid level in thisseparator. Connected to the level gauge is a levelswitch high (LSH-01). If LSH-01 is activated it will :

sound an alarm in the Control Room via level alarm high (LAH-01)

cause a shutdown of the produced water system via the E.S.D. system

At this point. let me say a few words aboutshutdown systems.

All offshore production facilities are protectedby a safety shutdown system. This comprisesdedicated sensors, actuators, valves,pipework, etc. They are installed to enable asafe and effective shutdown of plant andequipment in a controlled manner. The wholesystem is called an emergency shutdownsystem (E.S.D. system).

Levels of shutdown may be designated.These depend on the degree of hazard 10personnel, plant and the environment. Lessserious hazards may only require theshutdown of individual items of plant orequipment. Severe hazards, however, maynecessitate a total platform shutdown.

If our platform had four designated levels ofshutdown, with level 1 being the most severehazard level, then the produced watershutdown would probably be a level 3. This isindicated in Figure 18.

You will appreciate that E.S.D. systems arevery complex and I do not intend to talk aboutthem any further here. Other units in thePetroleum Processing Technology Serieswill describe these systems in much moredetail.

38


Now back to the tilting plate separators. You willnotice that a water jetting hose connection isfitted. This allows a high pressure water hose to beconnected into the system for washing out anysludge which collects in the sludge trap of TP-01.Each trap is also fitted with a connection to allow theliquidised sludge to drain away to disposal.

You will remember from Sections 2 and 3 thatseparation of oil from the produced water takesplace in the tilting plate separator. These two liquidphases are discharged via separate lines.

The recovered oil flows from TP-01 and joins withthe oil line from TP-02. The combined oil streamthen flows to a slop oil tank. The oil from the slopoil tank will be pumped back into the primaryseparation system.

The water from TP-01 passes through a butterflyvalve before joining with the water line from TP-02.The combined water line then takes the watertowards the flotation unit DP-01

That completes our look at the tilting plateseparators. Before you move on to look at theflotation unit however, have a go at the followingTest Yourself question.

Test Yourself 9With reference to the tilting plate separatorsin our typical system, see if you can answerthe following questions.

a) What is the function of an injection quill ?

b) Why is the sample point downstream of the connection for the oily drains system?

c) What system is used to ensure that the tilting plate separators are not accidentally isolated?

d) What is the nitrogen purge connection used for?

You will find the answers toTest Yourself 9 on Page 50

The Flotation UnitThe water which leaves the tilting plate separatorsshould have a fairly low oil content - say 60-80 ppm.We now have to polish the water to reduce the oilcontent to less than 30 ppm (the standard for oursystem). This is done in the flotation unit.

You should remember the principle of operation ofsuch a unit from Section 3. If you need to refreshyour memory, do that now before continuing.

Referring back to Figure 18, you will see that, fromthe tilting plate separators the water can be routedeither:

to flotation unit (DP-01)

to a by-pass round DP-01

The reason for the by-pass is to allow the flotationunit to be taken out of service for maintenance etc.

If you look at the system you can see that there aretwo tilting plate separators, but only one flotationunit. Normally both separators are in use.

We can, however, keep one separator on line whilstwe clean and maintain the other one. Any loss ofefficiency would be taken care of by the flotation unit.

39


But what happens if we have to clean or maintain theflotation unit?

We have already seen that corrugated plateseparators, (The type we have selected), can reducethe oil content of produced water to 30 ppm. So, ifboth tilting plate separators are on line, it would bepossible to by-pass the flotation unit, temporarily, sothat it, too, could be maintained.

Under normal conditions the water from theseparators flows through a butterfly valve towardsthe flotation unit. Downstream of the butterfly valveis a second chemical injection point wheredemulsifier can be injected from the chemicaldosing package.

The chemically treated water enters the inletchamber of the flotation unit and from there into eachaeration cell in turn. Our unit has four separateaeration cells. Within each cell oil foam accumulateson top of the water. A motor (M) is indicated on theleft hand side of the unit. This motor drives a set ofpaddles which skim the oil foam from the surface ofeach cell into the launder.

The launder is fitted with a level gauge (LG-03) toallow the operator to check the level of oil in thelaunder. Connected to the gauge is :

a Level Switch High (LSH-03)

a Level Switch Low (LSL-03)

If LSH-03 is activated it will start float recycle pumpP-02A or P-02B. The pump running light (XL-03 orXL-04) will light in the control room to alert theoperator that the pump is running.

The oil is pumped back into the inlet line to the tiltingplate separators. From there it is recovered againand discharged to the slop oil tank.

If LSL-03 is activated it will stop P-02A or P-02B.The pump running light (XL-03 or XL-04) willextinguish in the control room to alert the operatorthat the pump has stopped.

The oil leaving the launder is filtered before it entersthe suction of the float recycle pumps. Each pump isfitted with a discharge pressure relief valve(PSV-01 and PSV-02). The PSV is fitted on thepump side of the discharge Isolation valve.

If the discharge pressure exceeds a pre-set value,say 30 psi, then PSV-01 or PSV-02 will lift. This willrelieve the discharge pressure by circulating oil backto the suction of the pump.

The oil from the float re-cycle pumps is fed into theproduced water line downstream of the chemicalinjection point and downstream of the sample point.If it was fed into the line upstream of :

the chemical injection point - it would get a second dose of chemicals

the sample point - it would affect the amount of oil being measured as entering the system for treatment

Neither of these conditions are desirable.

The exit chamber of the flotation unit is fitted with alevel gauge (LG-04) to allow the operator to checkthe level of water in the exit chamber. Connected tothe level gauge is :

a Level Switch High (LSH-04)

a Level Switch Low (LSL-04)

If LSH-04 is activated it will :

sound an alarm in the Control Room via Level Alarm High (LAH-04)

will cause a level 3 shutdown

A level 3 shutdown generated by LSH-04 would shutdown the flow of produced water which is leaving theprimary separation system.

40


If LSL-04 is activated it will stop the hydrauliccirculation pumps (P-01 A or P-01 B). The pumprunning light (XL-01 or XL-02) will extinguish in theControl Room to alert the operator that the pump hasstopped.

The exit chamber of the flotation unit is also fittedwith a level transmitter (LT-05) which feeds asignal to a level controller (LC-05). The levelcontroller opens and closes a level control valve(LV-05) to maintain a constant level in the exitchamber.

The water which flows through LV-05 is the treatedwater leaving the system. There is a sample pointjust downstream of LV-05. This is the position where the final water quality is checked.

At this point I think that you should go back overwhat we have looked at up to now in this section,then have a go at Test Yourself 10.

Test Yourself 10Which of the following components are not part of the flotation unit.

skimmer motor

float recycle pumps

plate pack

launder

pump running light

filter cartridge

hydraulic circulation pumps

level gauge

discharge pressure relief valve

exit chamber level switch low

chemical injection point

inner liner


41


Lets now continue with the flotation unit by lookingat the gas aeration of the water.

Water is fed into the gas aeration side of the flotationunit by either hydraulic circulation pump P-01A orP-01B. These pumps take their water supply fromthe exit chamber of DP-01, and the water is thenfiltered before being recirculated to DP-01.

The combined discharge of P-01A and P-01B splitsinto four separate lines, one for each of the flotationcells. The operator can balance the flow of water toeach cell, by adjusting the opening or closing of theindividual inlet butterfly valves.

A fuel gas line provides the gas required foraeration via PCV-01. PCV-01 is a forwardpressure control valve. This means that it controlsthe pressure downstream of where it is installed.PCV-01 maintains a blanket of gas on the flotationunit with a constant pressure of a few inches watergauge.

The water flowing through a venturi causes the gasto be sucked into the water. It is then released astiny bubbles. (If you are having difficulty visualisingthis, go back to the description of flotation units inSection 3 and refresh your memory.)

If there is a problem in the fuel gas system thepressure of the gas blanket could fall and affect theoperation of the produced water handling system.

However, tied into the fuel gas line, downstream ofPCV-01 is an automatic nitrogen back-up system.The nitrogen back-up facility is activatedautomatically if the fuel gas pressure fails belowa pre-set value.

The system comprises:

flow orifice (FO-01) - a flow orifice is a small plate with a precision drilled hole in the centre -the size of the hole determines the amount of flow through the orifice

emergency shutdown valve (XV-01) - this has an FO indication underneath. This means that the valve will fall open if the instrument air pressure is lost.

solenoid valve (XV-01) - a solenoid valve is a valve which is opened or closed by an electro-magnet

limit switches (ZSH-01 and ZSL-01)

Solenoid valve XY-01 is fitted into the instrument airsupply to XV-01. This valve is a three-way valve.

In normal operation:

the electrical signal to the solenoid of XY-01 is live

the air flows through XY-01 and maintains pressure on the actuator of XV-01

the valve stays closed

limit switch ZSL-01 is activated and the signal valve closed is indicated in the Control Room.

If the low fuel gas pressure condition is activatedthen the electrical signal to the solenoid is madedead.

When this occurs:

XY-01 changes position

the air supply to XV-01 is cut off

the air supply to XY-01 is vented to atmosphere

The result of these actions is that XV-01 will open.When this occurs the movement of the valve willactivate ZSH-01 which will signal to the control roomthat the valve has opened.

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Nitrogen will then be supplied to DP-01. The amountof nitrogen will be regulated by the orifice size ofFO-01.

There is another connection to DP-01 which wehave not yet mentioned. This is a third chemicalinjection point positioned on the top of the unit. Itenables demulsifier to be injected into the main bodyof DP-01.

We have completed our look at the flotation unit.By the time the water leaves the unit it should havean oil content down to specification. To achieve this,we have injected demulsifier into:

the tilting plate separators

the flotation unit

We will now take a look at the chemical injectiondosing package, which is the final part of Figure 18.

Chemical Dosing PackageThis package consists of :

demulsifier drum D-01

chemical dosing pumps P-03A and P-03B

Demulsifying chemicals are pumped into D-01 fromdrums with a small hand pump via the hoseconnection. A level indicator (LI-06) allows theoperator to stop filling the drum when the correctlevel is reached and to monitor the level ofdemulsifier in the drum during normal operations.

If the operator fails to re-fill the drum when a lowlevel is reached, a level switch low (LSL-07) willactivate.

This will ;

activate a low level alarm (LAL-07) in the control room to warn the operator

shut down the chemical dosing pumps

The chemical dosing pumps are reciprocating /positive displacement / variable stroke pumps. (In areciprocating pump a piston moves backwards andforwards inside a cylinder. In a variable strokereciprocating pump the amount of liquid pumped iscontrolled by changing the length of the stroke of thepump cylinder.)

These pumps are fitted with discharge pressurerelief valves PSV-03 and PSV-04. The PSVs arefitted on the pump side of the discharge isolationvalves.

If the discharge pressure exceeds say, 30 psi, thenPSY-03 or PSY-04 will lift. This will relieve thedischarge pressure by circulating chemical back tothe demulsifier drum.

Chemical dosing pump P-03A supplies thedemulsifier to the tilting plate separators. PumpP-03B supplies the demulsifier to the flotation unit.

The flow of demulsifier leaving the pumps ismonitored by a sight glass (SG) in each of the lines.

That completes our look at the chemical dosingpackage.

The treated water has finally to be disposed of to thesea. This brings us to the final part of this section,the produced water caisson.

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Produced Water CaissonBasically, the produced water caisson is a longresidence time vertical separator. It is designed toremove any traces of oil which may be left in theproduced water when it leaves the flotation unit.

Figure 20 is an illustration of the produced watercaisson.

44


The caisson is a pipe of about 1.2m diameter whichhangs from the lower deck level of the platform intothe sea. The bottom of the caisson may be 30m orso below sea level. The actual length of the caissonwill depend on water depth and the height of thelower deck above sea level,

The bottom of the caisson is open to the sea andthe level of water inside the caisson will go up anddown with the rise and fall of the tide.

The produced water line from the flotation unitenters the top of the caisson at deck level. This lineextends down into the caisson to a point where itsend is under the level of the water, even at low tide.The produced water discharges into the caisson atthis point.

The produced water may stay in the caisson forupwards of 30 minutes, This is a much longerresidence time than anywhere else in the systemand traces of oil will be able to float to the surface,where they can accumulate.

Inside the caisson is a small sump with a weir setat a level which is just above the highest high tidelevel. The oil which has accumulated on top of thewater can then spill over the weir into the sump.

When the oil in the sump reaches a pre-set highlevel it will activate a level switch high (LSH-09).When activated LSH-09 will start the oil sumppump. The oil is pumped to the slop oil system,from where it will be returned to the primaryseparation system.

When the oil in the sump reaches a pre-set lowlevel it will activate a level switch low (LSL-09).When activated, LSL-09 will stop the oil sumppump.

You have now completed the section on a typicalproduced water system. Have a go at the final TestYourself question before going through the sectionsummary.

Test Yourself 11With reference to the system youhave been following in Section 4, answer thefollowing questions,

a) In the automatic nitrogen purge system on the flotation unit, what controls the flow of gas?

b) What is used to skim the oil from the surface of the water into the launder?

c) Where does the separated oil from the tilting plate separators go to?

d) Why is there a by-pass around the flotation unit?

e) Where does the oil accumulate in the produced water caisson?

You will find the answers to Test Yourself 11on Page 50.

45


Summary of Section 4In this section we have looked at how a typical produced water system operates,

I have described a system which includes:

tilting plate separators

flotation unit

chemical dosing

produced water caisson

and which combine to treat produced water for dumping to the sea.

As you worked through the section you followed the main flow lines andtraced the path of the water and oil. I pointed out the points where chemicalis injected into the system and where sampling takes place.

You also discovered the function and operation of the instrumentationassociated with such a system and the safety features involved.

Although the system I described is similar to many you would comeacross offshore, it is a hypothetical one. You must remember that, ifyou are working on a produced water system you should become familiarwith the layout and operating procedures of that particular system.

Now go back to the training targets for this unit and make sure that youhave met those targets,

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Check Yourself 1The water underlying the oil pushes the oil towards theproducing wells. The aquifer expands to fill the space left by theoil which has been removed. The oil water contact therefore willrise up the reservoir towards the producing well intakes.

Check Yourself 2a) total production = 4770m3/d (3975+ 795)

Therefore water cut = 795 x 100% = 16.67% 4770

b) oil production is 80% ( 100 - 20) 875m3/d Therefore oil production = 875 x 80 = 700m3/d 100

c) water production = 159m3/d (556 - 3971) Therefore water cut = 159 x 100% = 28.6% 556

Check Yourself 3a) True

b) False

c) False (produced water can contain up to 5 times the amount of salt present in sea water.)

d) True

e) False (the current figure is 30 ppm)

Check Yourself 4Oil and water have different densities. Water is the mostdense, and would sink, allowing the oil to float on top.

The two liquids would separate from each other and the oilwould tend to float on top of the water.

Check Yourself - Answers

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Check Yourself 5Your answer should look like the following:

Short Distance Centrifugal Gravity Coalescence Gravity Gas Force Separation Separation Flotation Separation

porous medium

plate pack

oil droplets rising

demulsifier

finely dispersed bubbles

vortex

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Check Yourself 6a) stilling plate

b) sludge debris

c) weir

d) parallel

e) flat or corrugated

f) cartridge, coalesce

Check Yourself 7a) Oil spills over a weir into the launder.

b) Water flows over the middle (internal) baffle in a two cell unit.

c) The recirculation pump takes its suction from the exit chamber.

d) Gas is introduced to the bottom of the cellsas finely dispersed bubbles.

e) Gas from the area above the water is sucked into a venturi and mixed with the water.

f) In the launder, oil and gas separate and the oil level is controlled by a level controller.

Check Yourself 81. e

2. a

3. a - d

4. a - b - d

5. c

6. c - d

7. a - b

8. e

9. d

10. d

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Check Yourself 9a) The injection quill is designed to ensure that the chemicals are efficiently mixed with the main water flow.

b) In order to ensure that a true sample is obtained, i.e. after the two streams have mixed together.

c) The inlet valves have a lock open facility.

d) In order that nitrogen can be introduced to sweep out any flammable gases before the vessels are opened for maintenance. This ensures that an explosive air gas mixture cannot form.

Check Yourself 10plate pack

filter cartridge

inner liner

Check Yourself 11a) The size of the hole in the flow orifice.

b) A motor driven set of paddles.

c) To a slop oil tank and from there to the main separation system.

d) To allow the unit to be taken out of service for maintenance whilst the plate separators are still working.

e) On top of the sea water from where it spills over a weir into a sump.

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Produced Water Treatment.pdfProduced WaterProduced Water Treatment

PWT Figure 18

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