Copy of GU-504 Gas Flotation Tank Systems Rev2 old.pdf

8/20/2019 Copy of GU-504 Gas Flotation Tank Systems Rev2 old.pdf

1/37

Petroleum Development Oman L.L.C.

RESTRICTED Document ID: GU-504

April 2006 Filing Key:

Guideline for Gas Flotation Tank Systems

Keywords: GFT, MGS, MBF, GLR, microbubble, micro-bubble, IGF, floatation

This document is the property of Petroleum Development Oman, LLC. Neither the whole nor any part ofthis document may be disclosed to others or reproduced, stored in a retrieval system, or transmitted in any

form by any means (electronic, mechanical, reprographic recording or otherwise) without prior written

consent of the owner.


2/37

Guideline for Gas Flotation Tanks Version 2.0

April 2006 2 GU-504

Authorised for Issue:

Signed: ……………………………

Mohammed Mughairy, UEPCFDH – Process Engineering

The following is a brief summary of the most recent revisions to this document.

Version No. Date Author Scope/Remarks

Draft B March

2005

G. Young

UEC81

Issued as “Gas Flotation Tank Selection And

Sizing Guidelines” report UEC/8-03-03-2005

1.0 October

2005

R. Weiter

UEC8

Fully Updated and Changed from Report to

Guideline Format

2.0 April 2006 R. WeiterUEC8

Updated post-Fahud Design Review


3/37


April 2006 3 GU-504

Content

1. INTRODUCTION ....................................................................................................................................................4

1.1. PURPOSE AND SCOPE..............................................................................................................................................4

1.2. COMPLIANCE ..........................................................................................................................................................41.3. BACKGROUND ........................................................................................................................................................4

1.4. FURTHER I NFORMATION.........................................................................................................................................6

2. GENERAL................................................................................................................................................................7

2.1. BASIC PRINCIPLES ..................................................................................................................................................7

2.2. SELECTION OF GFT VS. OTHER DE-OILING METHODS............................................................................................82.3. PERFORMANCE GUARANTEE ..................................................................................................................................9

2.4. LAYOUT ...............................................................................................................................................................10

2.5. PROJECT MANAGEMENT .......................................................................................................................................10

2.6. R EJECT HANDLING ...............................................................................................................................................11

2.7. CHEMICAL I NJECTION...........................................................................................................................................162.8. SAMPLING POINTS................................................................................................................................................16

2.9. ISOLATIONS ..........................................................................................................................................................17

2.10. SKID TYPE SELECTION.....................................................................................................................................17

3. TANK ......................................................................................................................................................................18

3.1. CONSIDERATIONS .................................................................................................................................................18

3.2. MODELLING .........................................................................................................................................................183.3. SIZING ..................................................................................................................................................................19

3.4. LEVELS.................................................................................................................................................................19

3.5. I NLET AND OUTLET NOZZLES...............................................................................................................................213.6. SINGLE CHAMBER ................................................................................................................................................21

3.7. DUAL CHAMBER ...................................................................................................................................................21

3.8. SKIMMERS............................................................................................................................................................22

3.9. BLANKET GAS SYSTEM ........................................................................................................................................233.10. TANK COST......................................................................................................................................................23

4. GLR-TYPE SKIDS ...............................................................................................................................................24

4.1. GENERAL..............................................................................................................................................................244.2. PUMPS ..................................................................................................................................................................24

4.3. CONTROL .............................................................................................................................................................25

5. MULTIPHASE PUMP-TYPE SKIDS .................................................................................................................26

5.1. GENERAL..............................................................................................................................................................265.2. PUMPS ..................................................................................................................................................................265.3. CONTROL .............................................................................................................................................................27

6. REFERENCES......................................................................................................................................................29

APPENDIX A: GLOSSARY OF ABBREVIATIONS ..................................................................................................30

APPENDIX B: MBF VENDOR .......................................................................................................................................31

APPENDIX C: GAS FLOTATION THEORY ...............................................................................................................32

APPENDIX D: EXAMPLE PEFS OF A REACTOR TYPE SKID ..............................................................................33

APPENDIX E: EXAMPLE PEFS OF A MULTIPHASE PUMP-TYPE SKID ...........................................................34

APPENDIX F: EXAMPLE GA OF A DUAL CHAMBER TANK DESIGN ...............................................................35

APPENDIX G: EXAMPLE OF REJECT HANDLING SYSTEM (SLUDGE POND TYPE) ..................................36

APPENDIX H: EXAMPLE OF REJECT HANDLING SYSTEM (OIL EXPORT TYPE) ........................................37


4/37


April 2006 4 GU-504

1. Introduction

1.1. Purpose and Scope

This document gives requirements and recommendations for the selection and design of gas

flotation tanks for new facilities and for retrofits to existing tanks. Its main purpose is to direct thePDO staff and the FED, EMC or EPC contractor (“users”) executing the project.

1.2. Compliance

The use of this guideline is mandatory for users where required by the convention. The following

convention is used in this document.

• The word shall indicates a requirement.

• The word should indicates a recommendation.

• Other expressions is, will be, could etc. are informative.

The use of this guideline is not mandatory for the MBF vendor (“vendor”); as it is this vendor who

supplied much of the requirements information in this document. See Appendix B for furtherdetails.

This guideline does not preclude alternative designs. However users shall obtain written approvalfrom the Corporate Function Discipline Head (CFDH) for Process Engineering for any variance

from requirements (indicated by “shall”) in this guideline, see procedure PR-1247.

Any discrepancies between vendor design and Shell or PDO standards (“the standards”) or this

guideline shall be queried with the vendor and, if unresolved, be addressed by consulting theProcess Engineering CFDH. Vendor design shall be compliant with the standards.

1.3. Background

PDO is by far the largest water producer in the Shell Group accounting for half of the total water

production and 5 times more water than any other single Operating Unit. The Shell group hasextensive experience in treating water to low oil-in-water (OIW) levels for disposal to the surface,

typically in relatively smaller amounts from offshore facilities. Typically total suspended solids(TSS) in produced water are not of interest and get disposed of in the sea.

Outside of PDO there is little Group experience in handling large volumes of water and solids in an

onshore environment. Technology development and reporting has traditionally focused on reducing

space, weight and total offshore installed cost which have significant cost savings or operational benefits in an offshore environment, but are irrelevant to PDO’s operations. The Gas Flotation

Tank (GFT) is one solution to PDO’s unique challenges.

Since early 2004, the GFT technology has been matured by the PDO Water Management Team as

an economic way to treat large volumes of water containing very fine oil droplets, fine solid

particles and combinations of the two present as oil coated solids. The operating principle of the

GFT technology is based on the existing technologies of induced gas flotation (IGF) and dissolvedgas flotation (DGF).

Over the past few years it has been adopted by a number of operating companies, mostly in Canada

where the Micro-Bubble-Flotation (MBF) technology has been developed and where there are a

number of large onshore water producers investing in produced water reuse for both water andsteam floods.


5/37


April 2006 5 GU-504

Flotation systems have been widely used in the upstream E&P industry for decades. These have

been recently (since year 2001) further developed to minimise space, weight and the effects ofvessel motions for the offshore market, resulting in the recent development of the single cell

compact flotation units.

IGF systems such as the one in Figure 1.1 are typically able to generate a gas bubble size in the

100-400 µm range. This is not effective in removing very fine oil droplets or solid particles as only

fine bubbles remove fine droplets and particles.

Figure 1.1: Typical Hydraulic Induced Gas Flotation System

DGF systems (Figure 1.2) have also been used in the E&P industry, although not to the sameextent as IGF, primarily due to the higher cost and requirement for a pressurised gas source with

continuous gas venting. The smaller bubble size of these units typically in the 40-70 µm range

generally improves the separation of fine oil droplets and particularly the fine solid particles.

Figure 1.2: Typical Dissolved Gas Flotation System


6/37


April 2006 6 GU-504

MBF technology makes use of both induced gas bubbles and dissolved gas, hence combining IGF

and DGF. However, the microbubble generation skid (MGS) generates smaller gas bubbles thaneither IGF or DGF, in the range of 5-50 µm. This technology has become available since year

2002.

MBF systems also have the advantage over IGF systems of being able to adjust the bubble size

during commissioning to optimise the system performance by trading off bubble size vs. induced

gas volume.

Further reading on gas flotation is in Appendix C.

1.4. Further Information

For further general reading on de-oiling, waterflooding and solids management see the references

in section 6.

Reference 1 is a PDO guideline for selecting evaluating and monitoring waterfloods. Its use is

mandatory for PDO concept engineers.

Reference 2 has background reading on the oilfield technologies involved in de-oiling of water.

Reference 3 discusses treatment facilities more superficially, it is mostly focussed on the reservoiraspects of waterflooding.

Reference 4 discusses all aspects of solids management, but is focussed to offshore purposes.


7/37


April 2006 7 GU-504

2. General

This section discusses general requirements for gas flotation tank systems. The tanks themselves

are discussed in Section 3. The two different types of MGSs and their requirements are discussed

in Sections 4 and 5.

2.1. Basic Principles

MBF is a technology of saturating the produced water with both dissolved and mechanically

created micro-bubbles of gas, which enhance the separation of oil and solids from water.

The MBF equipment consists of two main elements: Gas Flotation Tank (GFT) and Microbubble

Generation Skid (MGS).

Figure 2.1 shows the basic process flow scheme for the system. The process of flotation consists of

three basic steps: bubble generation in the MGS, attachment of the contaminants to the gas bubble,

and rising of the gas/oil/solids combination to the water surface where contaminants are removed

by skimming off the reject. A reject tank and pumps are also required; see Section 2.6 andAppendix G.

Figure 2.1: Gas Flotation Tank Flow Scheme

The bubble generation occurs in a vendor supplied skid which is installed adjacent to the tank.

The feed to the skid is taken from the clean water outlet, at a rate equal to approximately 25% ofthe net water throughput. The value of 25% is typical, but it can vary between 20 and 30%,

depending on the specific application. The vendor should be contacted for advice.

Gas is then introduced to the skid from the tank or from fuel gas systems. There are two methods

for generating the micro-bubbles by the skid i.e. two fundamentally different MGS types.

Produced

Water

Reject

Clean Water

Gas Flotation Tank

(GFT)

MicrobubbleGeneration

Skid (MGS)

De-oilerInjection

(optional)

Option 1:Fuel gas

Option 2:Blanket Gas(induced by

pump vacuum)

Deposited Coarse Solids

25% Recycle

Static Mixeror Pump orControl Valveto ensurede-oiler ismixed in and

effective

PressureLetdown

ValveFlow=Q

Flow=0.25*Q

Flow=1.25*QFlow=0.038*Q (batch skim, 3%)

Flow=0.060*Q (continuous, 5%)

Blue: scope of vendorequipment supply

Red: scope of vendor

performance guarantee

To Flare/Vapour

Recovery/Vent

Blanket/Flotation

Gas Supply


8/37


April 2006 8 GU-504

• Gas Liquid Reactor (GLR) – a pressure vessel that uses flow, shear, impact and pressure to

create micro bubbles of gas.

• Multiphase Microbubble Flotation (MBF) pump that can accept up to 20% gas (typical use is

10-12% gas) entering the suction and where the hydraulics within the pump create the micro bubbles.

Selection between these methods is illustrated in Figure 2.2 and discussed in sections referred towithin that figure.

The net result in all cases is a stream saturated with micro gas bubbles which creates a rising cloudof bubbles (termed “white water”) inside the GFT.

These systems also allow the gas bubble size and rate to be adjusted during commissioning and

subsequent operation to optimise the system performance. This feature allows different bubble

sizes and flows to be tested to find the most effective combination for a given application.

The recycle stream saturated with hydrocarbon gas is introduced into the main feed stream before itenters the tank. This allows for better mutual dispersion of gas bubbles and oil droplets suspended

in the water in the inlet piping and in the turbulent area at the inlet nozzles.

The use of deoiler chemicals is dictated by desired de-oiling performance and laboratory trials,same as for any form of flotation.

2.2. Selection of GFT vs. Other De-oiling Methods

The selection of the GFT for a specific application is driven by required separation performance,availability of existing skim tanks, operating pressures, requirement for handling of upsets and

available layout space.

Separation Performance

Typical performance achieved using the GFT is removal of 80-98% of oil and removal of 30-75%

of solids, which is same or better than performance of traditional flotation techniques and compact

flotation.

Performance of any flotation based system cannot be predicted from physical properties of the oil,

water and solids. To determine whether any flotation technology is applicable, bench air flotation

and deoiler bottle tests shall be carried out. See Appendix C for further discussion.

The bench flotation tests shall be carried out using a WEMCO™ bench flotation test cell in which

a 4 lt. sample of produced water is aerated for a period of 1 to 5 min. This process is then repeatedwith a range of deoiler chemicals to determine their effect on performance. These results correlate

well against conventional IGF systems. The tests are carried out by a specialist company and

managed by PDO’s Production Chemistry dept. who hold ongoing contracts for this type of work.

Both tests give an indication of how much deoiler chemical will be required.

For the GFT the vendor shall then be given the following information to allow for GFT flotation to

be predicted:

• Results of bench flotation tests and bottle tests

• General arrangement drawings of any skim tanks to be retrofitted

• Process equipment data sheet for the GFT package


9/37


April 2006 9 GU-504

• Any non-compliance with this guideline that would affect the skid performance

Existing Skim Tanks

In production stations where skim tanks are installed MBF technology can be retrofitted to the

existing tank either with or without internals modifications to optimise performance. This providesa large cost saving over a stand alone IGF system. If internals modifications are required the

operations impact of having the skim tank out of service for a period of time should be considered.

Operating Pressure

The GFT can only operate at close to atmospheric pressure and is therefore not applicable toservice in pressurised treatment systems. In such cases hydrocyclones, vessel-type IGF units or in-

line separator (e.g. as at Musallim) should be considered.

Upsets

Like a skim tank, the GFT has a considerable hold-up volume of approx. 1 hour, and hence can be

designed to provide buffer and control volumes in the system allowing the FWKO tanks to be

operated more steadily, which should in turn improve water and oil quality. A conventional IGF

has a 4 minute hold-up volume and hence is neither a buffer against serious oil-in-water process

upsets, nor a means of steadying the level in FWKO tanks.

Cost and Delivery

UEC8 shall be consulted for this information.

Site

The space available on site may restrict the possible locations for a GFT due to the larger foot printcompared to a conventional IGF or hydrocyclone system.

A skid mounted traditional IGF system may provide advantages in terms of project execution andschedule because of minimal onsite construction and commissioning activities.

2.3. Performance Guarantee

The vendor performance must be directly negotiated by each individual project. The following

should be considered:

• Any guarantee is an overhanging liability against the vendor. It can have any number of

potential actions that would be required, from rectifying actions, repair, replacement, partialfinancial penalties to full financial penalties. Hence what guarantee a vendor can provide is a

function of what the potential liability is and how close that guaranteed performance is to the

predicted performance. Although this statement is very general, it is included here to give areminder of how guarantees work in practice.

• In principle, any GFT system, retrofit or new, utilising a single or dual-chamber tank can be

guaranteed for oil removal efficiency.

• Solids removal efficiency may or may not be possible to be guaranteed, same as for IGF or

DGF. This is not readily quantifiable for flotation systems as it depends on the nature and sizeof the solids, the degree of their coating with oil and adherence to gas bubbles. This

information is generally not available up front. A typical efficiency figure is 50%, with a 30-

75% range.


10/37


April 2006 10 GU-504

In general, the finer and lighter the solids are and the more coated with oil, the better they will

be removed by flotation. Hence the GFT should be highly effective for removal of schmoo.Coarse sand will settle out on the bottom of the tank, providing that it is cleaned regularly and

the sand settling voidage is thus available. However coarse sand should normally not enter the

GFT, as the FWKO upstream should remove it. It is recommended that 50% solids removalefficiency should be assumed where there is no schmoo and 70% where most of the solids are

schmoo. The figures are based on previous Shell experience and laboratory tests at Marmul

(Ref.5).

• GFT performance can be broadly predicted as discussed in Section 2.2, but this prediction is

not highly accurate for single chamber GFTs. This is due to significant flow distributionissues, which is unlike IGFs, and due to wall coalescence effects.

• In case of either high performance efficiency being required (typically >90%) or the retrofitted

tank being unable to be modified sufficiently, or the guarantee conditions being very severe,

the vendor may insist on performing their own tests, using their own test skid for a field pilot.

The vendor has a test skid of 120m3/d capacity, footprint 4.5 x 4.5m. It consists only of micro

bubble generation package, with a 4 cell rectangular tank. This is an open system and thereforecannot be used on waters containing hazardous levels of H2S.

• Under all cases, the vendor shall guarantee the bubble size at the MGS outlet. The initial rise

rate of the white water – clear water interface shall be 1mm/s, as measured at a sample point by observation of an open beaker at atmospheric pressure. This corresponds to the rise rate of a

40 µm bubble in water at atmospheric pressure. It shall be noted that this rise rate accelerates

as bubbles coalesce; hence measured over 1 minute the corresponding average rise rate is

10cm/min (equivalent to 50 µm bubble in water).

2.4. Layout

The following requirements shall be taken into account when preparing the site layout. All of these

requirements are aimed at minimising the coalescence of gas bubbles in the tank inlet piping

• The MGS shall be located immediately adjacent to the GFT. For bunded (bermed) tanks this

means that the MGS shall be located inside the bund. If locating the multiphase pumps inside

the bund is not acceptable, the GLR-type skid shall be used, as for this the feed pumps can be

located outside of the bund.

• The pressure letdown valve shall be located immediately adjacent to the tie in of the recycle

line into the main oily water line. This means that a ladder or steps and platform will need to

be provided as this valve will be high above the ground.

• The tie in of the recycle line into the main oily water line shall be located immediately

adjacent to the tank inlet nozzle.

• The number of tees and 90° bends between the MGS and the tank inlet nozzle shall be

minimised as they promote turbulent coalescence of the gas micro bubbles. Long radius bends

shall be used wherever elbows cannot be avoided.

2.5. Project Management

The MGS vendor is normally directly responsible for:

• design and fabrication of the MGS;

• sizing of the tank and design of internals for new purpose built GFTs;


11/37


April 2006 11 GU-504

• design of the tank modifications (troughs, divisions, inlet spreader, outlet collector, nozzle

relocations) of retrofitted skim tanks;

• computer modelling, this is normally required for retrofitted tanks but not for new built GFTs;

• sizing guidelines for piping between MGS and GFS;

•

provision of MGS skid utility requirements; and

• review of layouts and isometrics of relevant connecting piping and general arrangements of the

tank and skid, hence these shall be sent to the vendor.

The vendor should be engaged for commissioning, training, troubleshooting and ongoing support.

The vendor does not manufacture the GFT or any connecting piping between the GFT and MGS.

The vendor needs not be present at HAZOP and HAZID studies and in design reviews, this is at the project group’s discretion.

The pressure letdown valve (Fig.2.1) needs not be supplied by the vendor and should be supplied

by PDO. The valve type is not important. It is the pressure drop that creates additional micro

bubbles, not the geometry or design of the valve. It may be either manual or automatic.

For the GLR-type skid, the feed centrifugal pumps need not be supplied by the vendor and can be

supplied by PDO. Only the flow and discharge pressure specifications are required to be met, thereare no other implications on the performance of the skid.

The vendor has no specific PLC requirements, seal monitoring system requirements, or minimum

flow recycle requirements.

2.6. Reject Handling

The following requirements apply to design of the reject (GFT skimmings) system, see example in

Appendices G and H:

•

The reject stream routing shall be designed so as to prevent closed-loop recycling of the fine

solids removed in the flotation process. Unless these solids are removed they will build up in

the loop until eventually resulting in a zero solids removal efficiency, and can cause verystable emulsions. Since solids will follow the oil path out of the reject tank, full recycle of the

oil separated in the reject tank is not allowed.

• The reject handling system shall not add any oxygen to the recycled water, using blanketing.

Oxygen scavenging by chemical is not reliable enough to be used here. This is due to the

history of mal-operation of such systems in PDO and due to the GFT reject rate, which ismuch higher than any long term average of drainage into any open pit/API separator.

• Centrifugal pumps shall not be used to handle the oil stream from the reject tank, the total

reject stream from the GFT (if any pump is required there) or the recovered oil from sludge

ponds/pits. Due to the high solids content these pumps are considered unsuitable for suchservice. Only the following pump types are acceptable: diaphragm or screw pumps, and

ejectors.


12/37


April 2006 12 GU-504

• Centrifugal pumps may be used to handle the water stream from the reject tank, as the solids

loading here should be lower than in the oil stream. Use of centrifugal pump depends on the

size, load and abrasiveness (sharp edges) of these solids. Also it must be ensured that the rejecttank does not overfill with coarse solids which then slug through the pump, as this would

rapidly wear it out. Any centrifugal pump for this service shall be designed in keeping with the

guidelines in EP2003-5184, Section 12 (Ref.4)

•

If appropriate, the GFT reject treatment system design should be co-ordinated with other slopsand waste handling requirements at the site.

• Unless direct routing of full reject stream to the oil line is possible (e.g. for satellite stations

such as Qaharir), the reject stream shall be routed into a blanketed reject tank, which shall

have minimum residence time either as determined from Figure 2.2 or from sludge hold-upcriteria, next bullet point and Table 2.1 (use the larger value). The basis for Figure 2.2 is

Stokes’ Law, with assumptions listed below:

o 150 micron particle (EP93-1315, Section 8.5.2);

o All oil is locked in emulsion, hence settling out is not of pure oil but of emulsion with

60% entrained water as in Sect.4.3.2 of Dehydration Manual (Ref.6); and

o

Hindered Stokes’ Law settling as in Fig 4.1 of Dehydration Manual (Ref.6), Fs=0.7.This is for 0-3% dispersed phase concentration and covers all PDO GFT reject tanks.

0

1

2

3

4

5

6

7

8

0.75 0.77 0.79 0.81 0.83 0.85 0.87 0.89 0.91 0.93 0.95

Oil Standard Gravity (Water=1)

E f f e c t i v e R e s

i d e n c e T i m e ( h r s )

Water SG=1.00 (fresh water)

Water SG=1.10 (highly saline water)

Figure 2.2: Reject Tank Required Liquid Residence Time


13/37


April 2006 13 GU-504

• As well as satisfying the residence time required for de-oiling, the reject tank shall provide

sufficient voidage in its bottom for sludge settling:

o The reject water outlet line shall be at least 2m above the tank bottom to provide for

solids settling and at least 3m below the inlet line to provide for separation.

o If no detergent is injected into the skimmings, it shall be assumed that 40% of solids

removed in the GFT settle out in the reject tank. In this case the settled sludge shall be assumed to be 15vol% solids and rest oil. This is based on past Fahud sludge test

data.

o If detergent is injected into the skimmings, it shall be assumed that 80% of solids

removed in the GFT settle out in the reject tank. In this case the settled sludge shall

be assumed to be 50vol% solids and rest oil.

o Pure solids density of 3000 kg/m3 can be assumed. This need not be calculated

exactly, given the uncertainties in above two bullet points.

o The required tank diameter can then be calculated as in the examples below:

Case Field A Field A Field B Field B

Detergent Injected Into GFT Reject Stream? No Yes No Yes

Produced Water Rate m3/d 15000 15000 50000 50000

Inlet Solids Loading mg/L 30 30 40 40

Outlet Solids Loading mg/L 10 10 15 15

Reject Tank Height For Sludge m 2 2 2 2

Volume Fraction Of Solids In Sludge 0.15 0.5 0.15 0.5

Fraction Of Removed Solids Which Settles 0.4 0.8 0.4 0.8

Solids Density tonnes/m3 3.0 3.0 3.0 3.0

Oil Density tonnes/m3 0.87 0.87 0.93 0.93

Tank Cleanout Frequency years 1 1 1 1

Pure Solids Settled In Reject Tank tonnes 44 88 183 365

Pure Solids Settled In Reject Tank m3 15 29 61 122

Sludge Settled In Reject Tank m3 97 58 406 243

Tank Diameter Required for Sludge m 8 6 16 12

Table 2.1: Reject Tank Diameter Based on Sludge Deposition

o It is readily apparent that using detergent will minimise the sludge volume, however

the degree to which detergent will work is not known, especially given the low level

of turbulence in the reject stream to mix this detergent in, and hence it will need to be

tested in practice. Hence it is best to size the reject tank assuming no detergent action.If this is effective, it can be seen that it can have the effect of up to doubling the

period between tank cleanouts.

o The above calculation is for the reject tank. Solids build-up rate in the GFT can be

calculated using same assumptions as above (no detergent case), and “FractionRemoved Which Settles” = 0.2.

• The reject tank inlet line shall be 1.5m below the oil-water interface. It was shown by CFD

modelling that lesser separation causes turbulence and re-entrainment.

• Inlet nozzle velocity shall be 0.5m/s maximum (Ref.6, 4.6.2).

• The inlet and outlet nozzles shall be on opposite sides of the tank. Inlet and outlet shall be

positioned so as to avoid short-circuiting.


14/37


April 2006 14 GU-504

• Outlet nozzle velocities shall be 1.0m/s maximum (Ref.6).

• To prevent solids from dropping out in the GFT trough and in piping, velocity for solids-

loaded liquids shall be 1.5m/s or above (EP 2003-5184, section 5.6). This applies to the

following: GFT skimming trough, reject line between the GFT and skim tank, and oilskimmings line from the reject tank.

•

The down slope to horizontal of the GFT skimmings trough should be 5° or greater.

• The reject piping between the GFT and the reject tank shall not contain any solids traps, andany horizontal sections shall slope toward the reject tank with at least a 1:100 slope.

• Cleaning should be manual. The reject tank design shall provide for fast and easy isolation and

cleaning. The tank shall be designed for easy isolation, opening, venting, and vacuum tanker

access. While the reject tank is cleaned, the emulsion pad in the GFT will be allowed to build

up. Reject tank cleaning time of 2 weeks should be allowed for.

• The tank lining shall be highly wear-resistant so as to withstand frequent cleaning. Unless aspare reject tank is provided, tank lining failure and repair is not acceptable. This would lead

to tank being out of service for several months.

•

Valve selection in the solids-contaminated reject system is critically important to avoid jamming and erosion.

Reject from a GFT will average 5 vol% of the total GFT feed for continuous skimming and 3 vol%of the total GFT feed for intermittent skimming. These are experience figures from existing

installations. For example, for 20,000m3/d of treated oily water the intermittent average reject rate

will be 20,000 x 1.25recycle x 0.03reject = 750m3/d.

It should be noted that the reject stream is often named “oily reject” or even “oil”, but this is

misleading. From a simple volume balance, if treated water stream has 200ppmv oil, for a 25%

recycle, and all oil removed in a 3% reject stream, the reject stream liquids will be actually99.5vol% water and 0.5vol% (5000ppmv) oil.

Out of 100% of the reject stream, we can estimate that 97-99% of the liquid will be easily

separable free water and 1-3% may be initially tied up with oil in an estimated 60/40vol% water/oilemulsion with solids entrained. This emulsion is settled out in tank sized as in Figure 2.2.

An emulsion (oil/water/gas) pad thickness of 0.15m should not be exceeded in the GFT under

normal operation, i.e. unless reject tank is being cleaned. A higher thickness may inhibit degassing

and cause foaming. However this is not a firm rule and the vendor shall be consulted if difficulty isexperienced in meeting it. This guideline can also be tested in practice, by allowing the pad to build

up further and checking for any adverse effects on water quality.

It should be noted that with intermittent skimming the instantaneous flow rates will be several

orders of magnitude higher than for continuous skimming and the reject piping shall be designed

accordingly. The actual rate is easily quantified from the interval and duration of skimming.Skimming duration is typically 2-20 minutes. It depends on the exact amount of oil in the inlet

flow, the desired quality of the reject and is typically optimised after commissioning so as to

achieve the best overall system performance.

For intermittent skimming of the GFT, the reject tank oil and free water pump-out operations shall be synchronised with the GFT skimming operation. Pump-out of water shall occur just before a

new batch of intermittent reject flow is dumped into the reject tank from the GFT, thus maximising

the separation time. The free water shall be separated out and recycled back to FWKO or GFTinlet. The FWKO is the better option and should be preferentially used as this avoids a closed loop

for any solids contaminants.


15/37


April 2006 15 GU-504

The reject tank skimmings will be contaminated with solids and some remaining emulsified

water. This layer in the reject tank shall be allowed to build up to a sufficient thickness beforeoverflowing (in the order of 1m) so as to eliminate overflowing free water with it.

Possible options for routing that should be considered are:

• Into a sludge pond (Appendix G). From here the recovered liquids and solids can be removed

to land farm by vacuum tanker. This option is practical where space is available and the flowrate is small as removal on a basis more frequent than once a week is likely to be impractical.

• Into the oil export system (Appendix H); this is common practice in other installations. Therationale is that the associated fine solids which did not gravity-settle in water will not settle

out in the much more viscous oil, but rather will keep moving along with it, and hence the oil

export system will not be fouled. This has the advantage of requiring no solids sludge storageon site.

The solids should be removed by manual cleaning of the gas flotation tank, reject tank and sludge pit, if any. Depending on the nature of solids, and use of continuous vs. batch skimming, it may

also be possible to on-line remove the solids from either the oil stream out of the reject tank or out

of the entire reject stream from the GFT. This can be achieved by the following methods:

•

Injecting surfactant to strip oil from the solids and letting the solids settle in the reject tank.This will greatly increase the solids deposition rate in the reject tank but will also decrease the

sludge volume, as these solids will not be contaminated by oil, which for fine solids accounts

for most of the sludge volume. See Table 2.1

• Injecting acid to alter the pH and dissolve the soluble solids (scales).

• Mechanically removing the solids by centrifuge or desander cyclone.

The coarser solids from the reject will settle out in the reject tank but the fine solids will get carried

out with the oil or water. The reject tank shall be monitored for solids level. The tank shall be

designed for easy isolation, opening, venting, and vacuum tanker access. While the reject tank is

cleaned, the emulsion pad in the GFT will be allowed to build up. Reject tank cleaning time of 2

weeks should be allowed for.

Typical disposal of recovered solids is land farming, landfill, addition to asphalt, incineration and

slurry injection to slurry disposal wells. Land farming is ineffective in Oman, due to high ambient

temperatures. There are landfill facilities at several locations, contact the MSE department.

Treatment and disposal options may be affected by radioactivity of the solids, see Ref.4.

The flow schemes in Appendices G and H are examples of reject handling systems. However anyscheme shall take into consideration special requirements such as nature and quantity of solids,

presence of schmoo and existing treatment and disposal facilities. It is not possible to illustrate and

exhaustively debate all the applicable schemes, neither it is within the scope of this document, see

Ref.4.

Valve selection is critically important on all reject lines, to avoid jamming and erosion. The

designer shall apply Section 4 of Ref.4 to valve selection.

Instrument selection is critically important on all reject lines, plugging. The designer shall apply

Section 12 of Ref.4 to instrument selection.


16/37


April 2006 16 GU-504

2.7. Chemical Injection

An injection point for deoiler chemical injection shall be provided upstream of the MBF skid tie-in point to the water inlet line to the tank. The injection quill shall be designed to meet the

requirements of DEP 31.01.10.10-Gen. This injection point shall be installed whether it is

intended to use it or not, as it is a cheap provision for any future requirements.

For any additional chemical injection points in the system, Production Chemistry shall beconsulted. This requirement will not be driven by oil-water separation duty, but rather by site-

specific issues with corrosion, biological activity or fouling with schmoo.

To ensure effectiveness of the deoiler sufficient mixing must take place downstream of the

injection point. This can be achieved if either a pump, control valve of static mixer is in the

downstream flow path. The deoiler injection location shall therefore be selected upstream of a

pump or control valve where possible or alternatively a static mixer shall be included in the design.

The detrimental effect of any such mixing device to oil removal efficiency in the GFT (because of

oil particle shearing) is negligible. Low differential pressure devices such as these cause negligible

shearing of the already-small oil particles from the FWKO.

2.8. Sampling Points

Pipe sample points shall be provided in these locations:

1. Oily water to GFT (upstream of the deoiler chemical injection point)

2. Immediately downstream of the Pressure Letdown Valve (see Fig.2.1)

3. GFT clean water outlet

4. reject tank inlet

5. water outlet from the reject tank.

6. oil outlet from the reject tank

The vendor shall provide a sample point at skid outlet, for measuring generated bubble size. No on-

line instrumentation is required for this; testing is done by observing bubble rise rate in a beaker

(Section 2.3).

Pipe sample points at locations #1-6 are to be provided with sample quills extending to the pipe

centreline with a curved tip (not a 45° bevelled edge) facing the direction of flow. The wall

thickness of the quill should be as thin as possible and the tip of the quill should be bevelled to asharp edge. Sample points should be located in an accessible position and where possible in a

vertical section of piping with upwards flow.

At locations #4-6 there shall also be provided 2” nipples with isolation valves. This is because

quills may plug up with solids.

Tank sample points should be provided every 1m along GFT and reject tank height and routed toground level.


17/37


April 2006 17 GU-504

2.9. Isolations

Isolations shall be as specified by SP-1125.

Wherever parallel items or trains of equipment are installed on the MGS and the MGS can operate

with one such train/item out of service, it shall be possible to isolate such train/item without

interrupting the operation of the rest of skid.

It shall be possible to carry out maintenance of the MGS or pressure letdown valve while

continuing the operation of the GFT as skim tank (without gas).

2.10. Skid Type Selection

Selection of skid type depends on the availability of pressurised gas supply, layout constraints, skid

capacity and vendor advice/cost.

Figure 2.3 can be used for preliminary guidance; however the vendor shall be consulted in all

cases.

Figure 2.3: Skid-Type Selection Flowchart

Is it possible to place

pumps immediately

adjacent to the GFT?(Section 2.5)

Is pressurised flotation

gas available at

6 barg, (Section 4.1)

GLR reactor type skid

Is the oily water rate

more than approx.

20,000m3/d (Sect.5.2)

MBF pump-type skid

Start here

Yes

Yes

No

No

Yes

Neither type of skid is

suitable.

No


18/37


April 2006 18 GU-504

3. Tank

3.1. Considerations

The design of the GFT nozzles and internals by vendor ensures that:

•

Residence time is not widely distributed, as this would mean that the chance of success ofremoving contaminants would depend on the chance path of the particle through the tank

• Turbulence is minimised be ensuring even flow distribution. Excessive turbulence causes

surface re-entrainment and also high speed localised downdraughts dragging down oil particles.

• Sufficient space is allowed for on the bottom of the tank for accumulation of coarse solids, as

these will not be removed by flotation.

• Fluid streams are not routed past tank walls and other surfaces. This leads to rapid coalescence

of the gas bubbles, hence taking them out of the liquid and rendering flotation less effective.

Hence imparting tangential motion of the liquid in the tank should be avoided.

3.2. Modelling

Historical problems with flow distribution in standard skim tanks can be overcome with the aid of

Computation Fluid Dynamics (CFD) modelling which will highlight potential problem areas for agiven design and allow optimisation of the design.

Hence CFD modelling is also a useful tool for doing retrofits of gas flotation to existing skim tanks

and should be carried out. However caution needs to be applied to using CFD modelling providers

who are not experienced with GFT technology. The requirements of GFTs can be the directopposite of the design requirements of skim tanks, for example see the last point of section 3.1.

While the modelling tool is software, analysis of the results is heavily dependent on previous

modelling and practical experience.

Only providers with demonstrable experience shall be used for CFD. It shall be noted that the GFTvendor usually provides this service and may not be able to guarantee performance otherwise.

CFD modelling currently has the following capabilities and limitations:

• gas, solids and oil particles can be modelled, this is done on basis of simple density and size.

• the separated skimmings layer on top cannot be modelled - the model can currently only track

particles until they reach the surface at which point the model assumes the particles stay at the

surface.

• oil-coated solids can be modelled but in a very simplistic way, by making adjustments to thespecific gravity of the solid without accounting for their affinity for bubble attachment and

surface release

• saline water density and viscosity can be catered for, same for oil

• particle size distributions for both oil droplets and solid particles can be utilised within themodel


19/37


April 2006 19 GU-504

• interactions between oil-gas, solids-gas and oil-solids are not catered for by the model.

Current approach is to either (a) assume gas/oil contact and use rise velocities of gas bubbles

of known size distributions coated with a thin layer of oil or (b) independently modelling the 3 phases. Neither is correct but (a) is closer to reality than (b).

• No use is made of water composition chemistry (i.e. ions) in CFD work.

CFD modelling is not required for new-build tank designs provided as part of a guaranteed package. This is because these designs had been tested in practice already.

3.3. Sizing

Tank sizing for use with MBF should be always carried out by the supplier of the system. It is

based on downward velocity, which takes into account the coalescence of bubbles and location of

the bubbles/bubble-free interface as we do not want bubbles in the underflow.

As a rule of thumb, dual-chamber tanks will require a 60 minutes residence time whereas single

chamber tanks will require 90 minutes residence time, depending on flow patterns and performance

required.

3.4. Levels

The tank height shall consider constraints of the facility into which the tank is to be installed in

addition to the requirements for flotation gas separation.

Control

A flow balance exists between the production system, consisting of facilities upstream of the

FWKO, and the oil export and water injection systems downstream of the FWKO. The systems arenot directly linked by control, i.e. there is no loop that measures and ensures that what is produced

can be always immediately disposed of.

The oil export system is limited by oil ullage available in the FWKO and oil tanks, pump capacity,and by what is happening downstream in the pipeline (e.g. leaks) or in downstream plants (e.g.

stabilisation).

The water injection system is limited by pump capacity and availability, and by the setting of wellchokes. Under normal conditions the free water is dumped from the FWKO or skim tank under

control imposed by throttling the pumps with a level control valve. However for systems operatingnear capacity this valve may be fully open at times, in which case the pump capacity limits as set

by the pump curves and backpressure determined by wellhead chokes.

This means that somewhere in the production system, most often there needs to be a tank that takes

the short-term “swings”. This can be the FWKO, skim tank/GFT, or another surge tank.

It is possible to operate the GFT in one of two configurations: fixed level or variable level.

For fixed level operation the tank (Figure 3.1) is operated nearly full. It is necessary that wateroutflow capacity can at all times match the inflow. Hence the GFT must be able to dump as much

water as is put into it at any instant from the FWKO. If this is impractical because theinstantaneous inflows are too high and cannot be decreased by control, then another tank shall beinstalled downstream of the GFT which shall serve as a surge tank/pump feed tank. This tank can

be much smaller.


20/37


April 2006 20 GU-504

For variable level operation (Figure 3.2) the tank is normally operated at a level below the

overflow trough and the level oscillates around the set point there. The top of oscillation shouldnever reach the overflow trough, unless there is a shutoff valve in the reject line, else the reject tank

will have a lot of water dumped into it which is inefficient, and could in fact exceed the reject

pump-out capacity and overflow that tank, if it is lower.

Skimming shall be initiated automatically, by a timer restricting the outlet LCV by changing the

level set point to an elevation slightly above the weir. Skimming can then be terminated when thereject tank is full or by timer.

Manual skimming of the GFT under operator control shall not be used as this is unreliable and

impractical, as it needs to be done every few hours.

Surface Layer Disturbance

CFD modelling shows significant surface disturbances when liquid level is less than 1.5m above

top of inlet nozzle or spreader. Hence this should be avoided by design and considered to be thelow alarm level, i.e. limit of good performance.

Gas Disengagement

The height required for separation of the flotation gas from the water will be dependant on thegeometry of the internals. A minimum height of approximately 3 m should be used, but the vendorof the MBF package shall be consulted on this.

Figures 3.1 and 3.2 summarise the recommended minimum heights based on DEP

recommendations, past vendor experience and CFD modelling.

Figure 3.1 Tank Level Settings (Fixed Level Design)

Tank Floor

Outlet Nozzle (bottom)2.0 m to allow for sludge

accumulation

Top of Tank Wall

LZHH0.2m (DEP 34.51.01.31-Gen.)

Inlet Nozzle(s)

LAL

NLL

1.5m to prevent disturbance of

surface by inlet turbulence

>3.0m; to be determined for

individual facilities based on tank

diameter, internals selection and

MBF vendor recommendations.

LAH

Overflow skimmer height

higher of 1.0m or 10minutes


21/37


April 2006 21 GU-504

Figure 3.2 Tank Level Settings (Variable Level Design)

Solids Hold-up

A 2m space shall be provided at the bottom of the tank for solids accumulation. For 2 chamber

tanks this does not make the tank 2m higher, as the inter-chamber connection pipe is simply raised

2m of the floor and the tank height remains the same.

Pump Protection

The low level trip LZLL would be normally either at a level 3m above the top of the outlet nozzle

to provide for gas disengagement, or as set by pump NPSH, whichever is higher.

3.5. Inlet and Outlet Nozzles

In all cases, whether new build or retrofit, these will be designed by the vendor, based on previous

experience and CFD modelling.

3.6. Single Chamber

Traditional skim tanks are of single chamber design. The MBF technology was originallyconceived as an enhancement to such traditional skim tanks. Typical required conversions will be

addition of inlet distributor, water outlet collector and fixed overflow weirs.

The separation performance will depend among others on the residence time, where 1 hr would be

considered the minimum, and larger tanks are better. Separation of up to 90% removal efficiency istypically achievable.

3.7. Dual Chamber

If removal efficiencies exceeding 90% are required, it may be required to divide the existing tank

into 2 halves using internal dividers, see Appendix F. The flow is then routed from cell to cell in a

sequential manner, as in a traditional IGF. This gives better control of residence time distribution

and hence better performance than a single cell tank of equivalent size.

Tank Floor

Outlet Nozzle (bottom)2.0 m to allow for sludge

accumulation

Top of Tank Wall

LZHH0.2m (DEP 34.51.01.31-Gen.)

Inlet Nozzle(s)

LAL

NLL

1.5m to prevent disturbance of

surface by inlet turbulence

To be determined for individual

stations based on control philosophy and expected process

operation.

>3.0m; to be determined forindividual facilities based on tank

diameter, internals selection and

MBF vendor recommendations.

LAH

Overflow skimmer hei hthigher of 1.0m or 10minutes (variable

level batch skimming)


22/37


April 2006 22 GU-504

Designs with more than 2 chambers are offered by the vendor but shall not be used in PDO. This is

because further dividing the tank decreases solids hold-up volume, which is critical for PDOapplications. Such design also complicates the operability of the tank.

New built GFTs should always be of dual-chamber design, for any performance requirement. This

is because for the same performance a new built dual chamber tank will always be smaller and thus

cheaper.

The tank is divided into equal chambers by dividing wall. In each chamber there is an inlet box,

which receives the produced water and the recycle flow containing the micro bubbles.

The inlet boxes in each chamber are positioned such that the outlet weir of the box is parallel to theoil collection trough. The oil collection trough runs diametrically across the tank along the line of

one of the baffle walls and collects the skimmed oil from each chamber. For continuous skimming

systems, as the oily froth floating on top of the bubble layer flows over the inlet chamber weir itcontinues across the surface and flows over the weir of the oil collection trough. For batch

skimming systems no continuous flow occurs on the surface.

The weir of the inlet box slopes back towards the tank wall and the bottom of the box is connected

to the tank wall. In the first chamber, which receives the water from the FWKO the inlet box weirextends closer to the tank wall than those of the other boxes then extends vertically downwards for

about 3 meters. This box is open at the bottom to allow any solids in the water to be directed to the bottom of the tank and not fill up the inlet box.

The produced water from the FWKO enters the inlet box in the first chamber where it meets a

recycle stream of cleaned water containing the micro bubbles. The oil extracted from the water by

the bubbles floats across the chamber and flows over the oil trough weir. The cleaner water leavesthe first chamber though an outlet pipe at the bottom of the baffle wall between the first and second

chambers. This pipe is connected to the bottom of the inlet box in the second chamber. This pipe is

internal to the tank.

The water from the first chamber is mixed with a second stream of recycle water containing micro bubbles. The oily froth flows across to the oil collection trough.

The clean water flow leaving the tank from the second chamber splits into two streams; the recycle

stream for the MGS, and the clean water for disposal.

The oil skimming can be done in two ways; the first is to control the level such that the oil floating

on the surface just overflows the weirs, the second is to control the level below the elevation of theskimming weir and periodically overflow as discussed in section 3.4.

The water flows by gravity through the tank and the interconnecting piping between each chamberis sized to minimize the pressure drop through the system. This and the sloping collection trough

together ensure that all compartments can be skimmed despite the implied hydraulic imbalance.

Any such tank shall be designed by the vendor and hydraulics verified independently.

It is important to note that the tank design incorporates water flow patterns to ensure that even

heavy oil in the reject can be removed hydraulically with no requirement for mechanical skimming

devices.

3.8. Skimmers

Floating skimmers shall not be used in GFTs. They suffer from the following problems:

• Limited skim area – the skimmer only skims if there is oil around it, regardless of how muchoil has built up elsewhere in the tank. Excessive oil build-up results in re-entrainment and poor

degassing/frothing of the liquid.


23/37


April 2006 23 GU-504

• The floating skimmer arm provides surfaces for downdraughts – high speed streams of

descending water which tend to transport oil particles to underflow.

• Floating skimmers disturb the flow patterns in the tank, by being an asymmetrical obstacle.

• Floating skimmers can block, sink and get stuck.

Direct draw off nozzles shall not be used either. These create excessive surface turbulence.

Only fixed, trough-type skimmers shall be used.

• For single-chamber tanks the trough should ideally be located at the entire circumference of

the tank, although other arrangements are also acceptable.

• For dual chamber tanks the trough will be in the middle.

• The bottom of the skimmer trough shall slope towards the outlet at a slope of 1:100 or more.

The skimmers should always be designed by vendors.

3.9. Blanket Gas System

The tank blanket system shall be designed as per the requirements of DEP 34.51.01.31-Gen. andAPI 2000.

The flotation gas handling duty of the tank shall be included in sizing of the blanket gas supply and

out breathing valves, as well as for relief and vent sizing.

3.10. Tank Cost

The following information can be used for concept-level estimates:

• A new-build dual-chamber GFT will cost 10-20% more than a single chamber skim tank of thesame size. The range above caters for very large to very small tanks.

•

A retrofit of a single chamber tank to make it a dual-chamber tank will cost 30% of theoriginal tank cost.


24/37


April 2006 24 GU-504

4. GLR-Type Skids

4.1. General

This type of skid uses a Gas Liquid Reactor (GLR) to generate and sort the gas bubbles, see

Appendix D. It achieves a smaller and more consistent mean bubble size (10µm) than the Pump

Type Skid (28µm) but it also induces less gas. Hence the overall flotation efficiency is thought to be equivalent.

The gas consumption rate is normally 0.09 sm3gas/m

3liquid (0.012 scf/gal), which is equivalent to

8% of gas volume in the water at standard conditions.

This will be a continuous gas consumption load which then has to be flared or recovered from the

GFT vapour space.

The gas supply system shall be designed for a maximum of 0.14 sm3gas/m

3liquid (0.019 scf/gal ,

160% of design) to give flexibility.

The gas pressure shall be 0.7 bar higher than the pump discharge pressure given in Section 4.2, i.e.

typically 6 bar.

The reactor is a tall vertical vessel. The largest size that was built by the vendor by 2006 is

4400m3/d throughput (800gpm; GLR800 model; treats 18000m

3/d oily water). Designs are

available for sizes ranging from GLR20 to GLR1800. The GLR has no moving parts, no packing

and no internals subject to plugging. Indicative skid and reactor sizes are below, note that skid size

allows for locating feed pumps on skid:

Net Produced

Water

(m3/d)

Model MGS Rate

(m3/d)

Skid

Length

(m)

Skid

Width

(m)

Skid

Height

(m)

Number

of

Reactors

Reactor

Dia

(m)

Reactor

Height (m)

13,200 GLR600 3300 3.6 2.4 4.1 1 0.8 3.6

60,000-80,000 2 x GLR1800 15,000-20,000 7.3 3.7 4.6 2 1.5 4.1

Eductors shall not be used to induce gas into the water in this service. Although they had been used

in the past, the vendor has unsatisfactory experience with them. Tank level fluctuations causefluctuations in the amount of induced gas which is highly undesirable for the stable operation of

the GLR.

4.2. Pumps

Any centrifugal pump is suitable, as long as it meets the flow and head requirements.

Typical discharge pressure required is 5 bar.

Since centrifugal pumps are highly reliable and available, with a 98% typical availability, they do

not need to be always spared. This depends on reservoir engineering preference, i.e. ability toaccept lower quality injection water for 7 days a year. Skim tanks without gas flotation should in

most cases still achieve 100 ppmv oil in water (but sometimes up to 200ppmv OIW), but only very

small solids removal. Assuming that the tanks upstream are reasonably clean, any coarse solidswill drop out there and any fine solids will not get removed without flotation.

The feed pump does not need to be on skid or be supplied by the MBF vendor. Its location is not

important.


25/37


April 2006 25 GU-504

4.3. Control

The controls for the skid are as illustrated in the PEFS in Appendix D. The backpressure controlvalve is shown in Figure 2.1. The logic can be handled by an on-skid PLC or by the plant control

system, as required by the project.

Once the GLR reactors are running at the correct liquid flow rates (as measured by water flow

transmitters), fuel gas is injected into the discharge stream of the pumps. This water streamcontaining the gas is passed through the GLR vessel in which the micro bubbles are created.

The GLR vessel comes equipped with a differential pressure transmitter (DPT) mounted on theoutside of the unit. This signal is relayed to the transducer which in turn operates the control valve

on the gas line. The signal from the DPT is wired back to the control centre/DCS where a set point

can be assigned which allows the GLR to be run at different liquid levels. Once the set point has

been established the system reacts by either increasing or decreasing the amount of gas beinginjected into the system. If the liquid level in the vessel needs to be raised the system reacts by

restricting the gas control valve therefore allowing less gas into the system and the water column

rises. Inversely to lower the level in the GLR the control valve allows more gas to be introducedinto the system and the liquid level in the vessel drops.

The controls on the vessel make it possible to regulate the total amount of gas being introduced

into the system. Also by running the vessel at different liquid levels it makes it possible to sort thesize of the micro bubbles.

The flow rate of water through the MGS is set by throttling the pumps with the pressure let down

valve (see Figure 2.1). No external recycle ratio flow control is required.

The flow rate through the MGS normally does not vary with the throughput of the treated water.This means that at low feed rates the recycle will be much higher than 25% of the net feed, all the

way to 100% if the net feed is equal to zero.

The GLR reactor itself does have a turn up and turndown range, for example a single GLR1800

reactor can process between 1500 to 2200gpm (8200 to 12000m3/d) of flow.

At least the following information/signals shall be made available in the control room (others as

desired by project):

• Pump suction and discharge pressures.

• Water flow rate

• Gas supply pressure upstream of supply valve

• Gas flow rate

• Differential pressure drop across GLR

• GLR level

•

Pressure at MGS outlet

• Pressure immediately upstream of the pressure letdown valve


26/37


April 2006 26 GU-504

5. Multiphase Pump-Type Skids

5.1. General

This type of skid uses the pump to inspire the gas at the pump suction, using vacuum, see

Appendix E.

Although the generated bubbles are larger, the gas loading possible is higher than for GLR-typeSkid, so performance should be similar, see section 4.1. The vendor has no comparison data

between pump-based and reactor-based designs for oil removal efficiency.

The most common gas consumption rate is 0.11 sm3gas/m

3liquid (0.015 scf/gal), which is

equivalent to 10% of gas volume in the water at standard conditions. Operation with rates of up to

0.14 sm3gas/m

3liquid (0.018 scf/gal), equivalent to 12% of gas in water, is also common.

The gas supply system shall be designed for a maximum of 0.18 sm3gas/m

3liquid (0.024 scf/gal ,

160% of design) to give flexibility.

In order to generate sufficiently small bubbles, the pump is operated with discharge pressure

between 3.4 and 6.8 bar (50-100 psig), as measured at skid outlet.

The GFT vent gas is recycled back to the multiphase pump suction, drawn in by its vacuum, thus

forming a closed loop. Hence the continuous gas consumption load is nil. Imbalances will existonly at start-up and shutdown, when there will be insufficient or excessive gas in the GFT

respectively.

Indicative skid sizes are below:

Net Produced

Water (m3/d)

MGS Rate

(m3/d)

Skid

Length (m)

Skid

Width (m)

Number / Model of

Pumps

15,000 3750 3.3 2.4 3 x MB200

5.2. Pumps

This section provides information on the MB-range of pumps offered by the MGS vendor with

their package. This pump range is new, dating to tests in mid-2005. Prior to this date the MGSvendor used the Edur 200gpm pump. The Edur pump was developed for non-oil industry

applications, and although used there, it does not have the usual features like mechanical seal, end

bearing or support feet. The MGS vendor copied this unpatented pump and added the bearings,

seal, feet and other oil-industry features.

The pump has ANSI fittings and general design however ANSI does not cover multistage pumps

and the pump hence fails compliance based on required dimensions.

This pump is approved for use by PDO rotating equipment department UEC6.

The pumps are available in capacities of 1100, 2200 and 3300 m3/d (models MB200, MB400 and

MB600 respectively, meaning capacity in US gallons per minute).

The MB200 pump has the same volute casting as for the Edur LBU 602 E162L pump from which

it is derived.

The pump is an American 60Hz design. The 60Hz driven pump is a 2 stage unit, 50Hz pumpwould need to be 3 stage. The correct test curves shall be used as there is a difference.


27/37


April 2006 27 GU-504

The pump is available in SS316 as basic material, also in duplex as an option. Duplex lined casing

is not cost effective in this small size and solid 25Cr duplex would be used.

Any skid shall have a spare pump available on site, either in the warehouse or installed. This is

because the flotation efficiency drops steeply even if just 1 pump out of 3 is lost. The loss is not

just a few percent but could be as high as from 90% to 30% oil removal efficiency. Also because of

the new pump design which is not off-the-shelf, this is prudent.

No availability/reliability data available as the pump is too new.

Casting and impellers are made in Columbia, internal parts from USA, assembly and testing in

Venezuela. The pumps would be shipped direct to Oman.

MB200 weighs 75 kg, MB400 weighs 125 kg: larger volute casting and seal ends, same inlet/outletcasting.

The vendor stated that the pump is NACE compliant according to requirements, but is not currently

NACE certified. If PDO requires certification for particular location then that would need to beincluded in that project.

In order to generate sufficiently small bubbles, the pump is operated with discharge pressure

between 3.4 and 6.8 bar (50-100 psig), as measured at skid outlet.

The MB200 pump shutoff differential head is 126m (12.4bar with fresh water).

NPSH required is 4m. The pump operates with up to 0.6 bara at suction.

Figure 5.1 shows a cutaway view of the MB200 pump.

As of April 2006, it is currently considered that MBF pump skids are suitable for oily water

throughput of up to 20,000m3/d, corresponding to the recycle capacity of four MB200 pumps. Alarger number of pumps is considered to pose an excessive maintenance burden.

However, the MB400 and MB600 pumps are new designs and unproven, and hence not considered

suitable for PDO service at this time. This guidance may change with time, contact the Process

CFDH for latest status.

5.3. Control

The controls for the skid are as illustrated in the PEFSs in Appendix E (Fahud design, after Design

Review), the backpressure control valve is shown in Figure 2.1. The logic can be handled by an on-

skid PLC or by the plant control system, as required by project.

The water flow is measured by individual magnetic flow meters at suction to each pump. The flow

rate of water through the MGS is controlled by throttling the pumps with the pressure let downvalve (see Figure 2.1).

The total gas flow is measured by a flow meter on the common gas supply to skid. At

commissioning, the gas flow to each pump is manually set using a manual DN15 needle valve and

the common flow meter, no further adjustment is required.

There is a throttling valve in the pump suction. It’s function is to drop the pump suction pressurefar enough so that gas from the GFT head space can be injected. To do that, a vacuum is created at

the pump suction. Vacuum down to 0.6 bara (6m absolute water head) is acceptable, no more, or

the ability of the pump to induce tank gas will be reduced correspondingly.


28/37


April 2006 28 GU-504

The recycle flow rate is constant and does not vary with the throughput of the treated water. This

means that at low feed rates the recycle will be much higher than 25% of the net feed, all the wayto 100% if the net feed is equal to zero.

At least the following information/signals shall be made available in the control room (others as by

project):

•

Multiphase pumps suction temperature

• Pump suction and discharge pressure. The multiphase pump suction pressure transmitter needs

to cover 0-1bar of vacuum

• Water flow rate

• Gas supply pressure upstream of supply valve

• Gas flow rate

• Pressure at MGS outlet

• Pressure immediately upstream of the pressure letdown valve

Fig.5.1 Cutaway view of the MB200 pump (60Hz)


29/37


April 2006 29 GU-504

6. References

The information in this guideline was developed from information supplied by GLR Solutions, Shell

literature in the Reference list and PDO’s own analysis.

All references are available from this web site:

http://sww9.pdo.shell.om/funct-disp/UEP/water/Design%20Documents/Forms/Ross1.htm

1. PDO Waterflood Scheme And Water Quality Selection, Evaluation And Monitoring.

2. EP93-1315, De-oiling Manual.3. 2003 T&OE Waterflood Manual, Section 4.

4. EP2003-5184, Shell Expro Sand Management Guide.

5. MMPS Bench Air Flotation and Bottle Test Work 16–19 July 2005, OPES/Baker Petrolite.

6. Dehydration Manual 1.0, Chapter 4, Shell, 1999.


30/37


April 2006 30 GU-504

Appendix A: Glossary of Abbreviations

API American petroleum institute

CFDH Corporate function discipline headCFD Computational fluid dynamics

CS Carbon steelDEP Design engineering practiceDGF Dissolved gas flotation

DPT Differential pressure transmitter

EMC Engineering and maintenance contractE&P Exploration and production

EPC Engineering, procurement and construction

FED Front End Design

FIC Flow indicator and controller

FWKO Free water knock outGA General arrangement

GFT Gas flotation tank

GLR Gas-liquid reactorHAZID Hazards identification

HAZOP Hazard and operabilityIGF Induced gas flotation

LCV Level control valveMBF Micro bubble flotation

MGS Micro bubble generation skid

MOL Main oil line NA Not available

NPSH Net positive suction head

OIW Oil in water

OU Operating unit

PCV Pressure control valvePEFS Process engineering flow scheme (others call these P&IDs)

P&ID Process and instrumentation drawingsPFS Process flow scheme

PLC Programmable logic controllerSG Standard gravity

SIEP Shell international exploration and production

TSS Total suspended solidsUSD United states dollars

Units of Measure:

barg bar gauge

gpm United States gallons per minute

µm micron or micrometer (10-6

meter)

mg/L milligrams of solids per litre of liquid ppmv parts per million by volume (millilitres of liquid1 per cubic meter of liquid2)


31/37


April 2006 31 GU-504

Appendix B: MBF Vendor

GLR Solutions

Currently only one vendor is identified with the capability to provide proven tank flotation technology:

GLR Solutions of Calgary, Canada. The vendor is a small local company built on the basis of the GLR

patented technology.

The vendor provides both the GLR-type skids and multiphase pump-type skids. In 2004 and 2005 the

vendor was a reseller of the Edur multiphase pump. The vendor has now developed its own multiphase

pumps. The pumps are available as of mid-2005.

This vendor has a track record of 14 installations using GLR reactor-type skids (since 2002), and 8

installations using multiphase pump-type skid (since 2004). Most are located in Canada, 2 in Cuba, 3 inVenezuela, and upcoming projects in Libya and Iran.

The vendor has in-house CFD modelling expertise in modelling of gas flotation tanks.

Tender board justification should be based on comparison of cost of the GFT system against the cost of

alternative treatment methods, as per the hydrocyclone and IGF 3-year price agreement contracts.

Other Vendors

No oil industry vendors were identified with a track record of successfully fitting gas flotation systems to

tanks.

No vendors were identified with experience of CFD modelling of such systems. There are many CFD

modelling companies; however none of them is known to have an understanding of the performance

requirements behind this technology.

It needs to be noted that it is not true that simply good fluid distribution will lead to successful tankflotation, as experience comes with implementation and calibration of models, also see section 3.2.

Shell

The only experience with tank flotation in Shell had been past Shell Group trials of sparging of gas directly

into tanks/vessels. These were unsuccessful as the spargers plugged and the gas-oil contact was poor due to

the rapid growth of bubbles.


32/37


April 2006 32 GU-504

Appendix C: Gas Flotation Theory

The performance of gas flotation in terms of oil and solid removal efficiency is increased by increasing

collision and attachment efficiencies, and gas/liquid contact time. The collision efficiency is increased by

increasing oil-drop size, gas concentration, and decreasing gas-bubble size.

A range of factors affect the attachment efficiency including;

• Water chemistry (anions, cations, pH, viscosity)

• Relative surface tensions

• Oil properties (viscosity, surface tension, droplet size)

• Emulsion stability

• Residual chemicals (corrosion inhibitors, demulsifiers)

• Solids type and size

• Oil wetted solids

• Temperature

No correlation has been found between basic system properties and attachment efficiency. This means that performance can only be predicted through laboratory or field trials which have generally been accepted in

the E&P industry as giving a good indication of flotation performance.

To increase the collision efficiency it is often not practical to increase the oil droplet size as coalescingelements add to system maintenance with frequent rapid fouling. This is particularly true within PDO where

the nature of gathering systems generally involve the use of a number of chemicals many of with act as

surfactants causing solids to be come oil wetted. Therefore to maximise the collision efficiency, the gas

concentration should be maximised with the minimum allowable bubble size.

The minimum allowable bubble size is determined by the system geometry in that the gas bubble must have

a net upward rise velocity. The MBF systems generate bubbles in the range of 5-50 µm. These start bubblescoalescing as soon as they are created, and especially after they are introduced into the tank where they

reside the longest.

The gas/liquid contact time is determined by the residence time and flow distribution in the flotation system,increasing the residence time will therefore increase the potential gas/liquid contact time.

Some of these important parameters are determined by the system design, whereas others are characteristics

of the feed. Therefore, a system which works in one facility will not give the same result in another and anychanges to the feed stream will result in a change in performance. For this reason it is important to assess

the impact of chemicals used in the upstream on the flotation performance for example when changing

emulsifier chemicals types and dosing rates. Reference 1 shall be consulted by PDO Concept Engineers for

guidance on this topic.


33/37

G u i d e l i n e f o r G a s F l o t a t i o n T a n k s

V e

r s i o n 2 . 0

A p r i l 2 0 0 6

3 3

G U - 5 0 4

A p p e n d i x D : E x a m p l e P E F S o f a R e a c t o r T y p e S k i d


34/37


V e

r s i o n 2 . 0

A p r i l 2 0 0 6

3 4

G U - 5 0 4

A p p e n d i x E : E x a m p l e P E F S o f a M u l t i p h a s e P u m p - T y p e S k i d


35/37


V e

r s i o n 2 . 0

A p r i l 2 0 0 6

3 5

G U - 5 0 4

A p p e n d i x F : E x a m p l e G A o f a D u a l C h a m

b e r T a n k D e s i g n


36/37


V e