PETRONAS TECHNICAL STANDARDS DESIGN AND ENGINEERING PRACTICE

MANUAL (SM)

GUIDELINES FOR HYDRAULIC

DESIGN OF MULTIPLE PIPE SLUG

CATCHERS

PTS 20.056

DECEMBER 1984

PREFACE

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Code 500.20.800 (Production) - IN 73186

Code 500.30.720 (Natural Gas) - IN 73210

GUIDELINES FOR THE HYDRAULIC DESIGN OF MULTIPLE-

PIPE SLUG CATCHERS

(January 1982 - December 1984)

by A. BosApproved by: R.V.A. Oliemans

SUMMARY

Guidelines are presented for the hydraulic design of multiple-pipe slug catchers. These criteria are basedon engineering practice with existing slug catchers and on laboratory model studies carried out at KSLAwith a proper two-liquid system to simulate high-pressure gas and condensate.

January 1985

CONTENTS

1. INTRODUCTION

2. SLUG CATCHERS GENERAL

2.1. Necessity of a slug catcher

2.2. Types of slug catcher

3. EXPERIENCE WITH MULTIPLE-PIPE SLUG CATCHERS

3.1. Den Helder slug catcher

3.2. St. Fergus slug catcher

3.3. Bintulu slug catcher

3.4. Bacton slug catcher

3.5. Eemshaven slug catcher

3.6. Summary of lessons learned from field experience and model studies

4. DESIGN GUIDELINES FOR MULTIPLE-PIPE SLUG CATCHERS

4.1. Slug catcher size

4.2. Slug catcher geometry

4.2.1. Inlet section

4.2.2. Bottle section

4.2.2.1. Choice of primary and secondary bottles4.2.2.2. Length of the entrance section of the primary bottles required for

settling of small droplets4.2.2.4. The dual-slope approach4.2.2.5. Equalizer system4.2.2.6. Bottom header

4.2.3. Gas outlet section

4.2.3.1. Gas risers4.2.3.2. Gas outlet header(s) and gas outlet(s) 16

5. FINAL REMARK

1 TABLE

13 FIGURES

APPENDIX I : The two-liquid test facility for the simulation of slug catchers (1 Table)

APPENDIX II : The liquid storage capacity and height of a multiple-pipe slug catcher 1 Figure)

APPENDIX III : Settling of mist in the separation section of the primary bottles (1 Figure)

APPENDIX IV : Simulation of bottle choking (Kelvin-Helmholtz instability) in the AmsterdamTray Test Column multiple- pipe slug catcher (1 Figure)

LIST OF SYMBOLS

REFERENCES

GUIDELINES FOR THE HYDRAULIC DESIGN OF MULTIPLE-PIPE SLUG CATCHERS

(January 1982- December 1984)

1. INTRODUCTION

At the end of gas pipelines operating in two-phase flow mode normally a gas-liquid separator,known as "slug catcher", is installed. The function of this facility is to separate the liquid from thegas before the gas enters the gas-treating facilities downstream of the pipe and to store theliquid. A widely applied slug catcher configuration is the multiple pipe concept.

Throughout the years the Group has accumulated experience with multiple-pipe slug catchersbased on:

1. engineering practice with and measurements on existing slug catcher,

2. model studies carried out in the Laboratory (KSLA).

This has resulted in the formulation of a number of guidelines for the design of a multiple-pipeslug catcher which are presented in this report. Here only the hydrodynamic aspects areconsidered. Based on this report a more general PETRONAS Technical Standard manual will beprepared by PETRONAS, Kuala Lumpur. The recommended design codes and material selectionprocedures will be subject of a separate PTS manual at present in preparation by PETRONAS.

2. SLUG CATCHERS GENERAL

2.1. Necessity of a slug catcher

Trunk lines transporting natural gas often show the phenomenon of liquid formation, mainly dueto retrograde condensation. Due to a slip in velocity generally occurring between the gas andliquid phases the amount of liquid in the pipeline steadily accumulates to an equilibrium level.The total amount of liquid present at any time in a long two-phase pipeline operating in steadystate condition can be significant. When operating conditions are changed large volumes of liquidmay emerge from the pipeline. This can be caused by a change in volume flow i.e. in velocity, orby running a sphere (or pig) through the line. The largest slug that can ever occur is the onecaused by sphering. These occasionally very large volumes of (live) liquids must be handled andstored on- shore as they emerge from the line preferably without reducing velocity the flow in thepipeline. For this reason a slug catcher is always connected to a two-phase pipeline. A slugcatcher consists essentially of two parts:

- a separator part, separating the liquid from the mixed stream arriving under normal (steady)flow conditions;

- the storage part, receiving and storing the incoming liquid slug created by upset conditions(which also includes running a sphere through the pipeline).

When a more or less continuous slug of liquid arrives, the liquid displaces the gas present in theslug catcher thus guaranteeing a continuous supply of gas to the downstream facilities(compressor, treating plant, LNG plant). Gas lines operate generally at velocities of up to 12 m/sand large slugs will take only a matter of minutes to arrive. The holding capacity of the slugcatcher therefore must be essentially as much as the volume of the largest slug. Although liquidcarry-over must be limited, a slug catcher is not meant to replace a high-efficiency separator.

2.2. Types of slug catcher

Slug catchers can be broadly classified into the three following categories:

1. vessel type,

2. multiple-pipe type,

3. parking loop type.

The geometry of the vessel type slug catcher could range from a simple knock-out vessel to amore sophisticated lay-out as has been designed by Stone & Webster for a non-Shellsubmarine pipeline (see Fig. 1). Since a vessel-type slug catcher is relatively short for a givenvolume this type is preferred in the case of limited plot sizes (e.g. offshore platforms).

When larger liquid volumes have to be accommodated, say more than 1000 m3 either themultiple-pipe or the parking loop type is preferred. In Fig. 2 a typical example of a multiple-pipeslug catcher is shown (existing slug catcher in Den Helder at the end of a 36" gas pipeline fromthe K-14 field in the North Sea). It consists of an entrance section where liquid/gas separationtakes place and an array of parallel down-sloping bottles (of standard line pipe size) for liquidstorage. An incoming liquid slug flows via the splitter into the inlet manifold and then viadowncomers into and down the sloping bottles. As a consequence the gas present in the bottlesis displaced and flows through the gas outlet risers mounted on the bottles to the gas header andthe gas-treating plant. The liquid/gas exchange that occurs in the bottles guarantees anuninterrupted gas supply to the downstream facilities during liquid slug arrival, providedexcessive liquid carry-over can be avoided.

The advantage of a multiple-pipe slug catcher is that it is easy to operate since no flow controlmeasures are required. A disadvantage is, however, the counter-current gas-liquid flow in thebottles which will promote liquid carry-over.

The problem of counter current flow is avoided in the parking loop type slug catcher. Thisconcept has been introduced by Texas Eastern2 and is shown in Fig. 3. In this novel concept theseparating and storage parts are virtually disconnected: it consists basically of a large separatorwith the liquid outlet connected to a long single. The incoming gas/liquid stream is separated inthe vessel. When the liquid level rises rapidly, indicating that a slug rather than a gas-liquidmixture is arriving, the gas flow from the vessel is restricted forcing the liquid to flow into thepipeloop. In this loop a sphere is present to separate the liquid entering from the gas present.W ith the other end of the loop now opened to the downstream facilities the gas is driven out in adirect co-current manner. The stored (parked) liquid can be discharged as a single slug by usinghigh pressure gas in case the location is at a booster compressor station as shown in Fig. 3 orthe liquid can be discharged gradually to a downstream treating plant. This type slug catcher isspecifically suited for offshore application where the separator can be placed on the platform andthe loop on the seabottom. All valves and controls will be on the platform. But also in onshoreapplications space will be saved, particularly if the pipeloop is laid parallel to and in the sametrench with the incoming pipeline. This concept, however, requires more sophisticated control, incontrast with the multiple-pipe slug catcher and may also require additional facilities to effect agradual discharge of the liquid to a downstream treating plant. For a further development of thistype of slug catcher far more studies are required.

In the following sections of this report, of the types of slug catchers mentioned so far only themultiple-pipe type will be discussed in more detail.

3. EXPERIENCE WITH MULTIPLE-PIPE SLUG CATCHERS

A great number of multiple-pipe slug catchers are in operation throughout the world. They varywidely in geometry, mainly because of the many degrees of freedom possible in the design ofthis type of slug catcher. Up to now a unique design procedure is still lacking.

In Table I a few representative slug catchers operated by Shell or in which Shellhas an interest are listed, together with relevant information. For the respective layouts see theFigs. 2, 4, 5, 6 and 7. To the list is also added the slug catcher projected at the end of the F-3pipeline (in Eemshaven). It has not been built as yet. However, relevant information is obtainedfrom model experiments.

The performance of these slug catchers and their specific features will be discussed in thefollowing paragraphs.

3.1. Den Helder slug catcher

The Den Helder slug catcher in operation since 1975 has already been briefly discussed inSection 2.2 of this report. Its geometry and discussions are shown in Fig. 2. The slug catchercontains eight primary bottles (slope 1.5 %). Primary bottles have a dual function:

1. gas/liquid separation,

2. liquid storage.

The slug catcher was designed as a symmetrical slug catcher with 4 bottles located between thetwo inlets to the inlet header (2-4-2 configuration). However, this was changed into a 3-3-2configuration in the construction phase. In its first 4 years of operation the slug catcher performedsatisfactorily. However, in 1979 when the gas flow rate in the pipe was increased above 20million Nm3/d liquid carry-over started to occur. Since ultimately the gas flow rate in the pipelinehad to be increased up to 30 million Nm3/d the slug catcher performance had to be improved.

From neutron backscattering (NBS) measurements carried out on the slug catcher it was foundthat the liquid carry-over was caused by a maldistribution of the slug over the inlet header,leading to an uneven distribution of the liquid slug over the slug catcher bottles and hence tooverloading of several bottles. Subsequent tests in a two-liquid perspex model of the Den Helderslug catcher 4 indicated that the maldistribution and hence the carry-over could be remedied byinstalling constrictions in the downcomers of the slug catcher. The set-up of the model studieswith the two-liquid perspex model is described briefly in Appendix I.

In view of the results of the model studies NAM installed in the slug catcher an insert whichnarrowed the portholes of the down-comers from 24 to 15 inch, This modification improved theperformance of the slug catcher substantially. A second series of NBS measurements carried outon the Den Helder slug catcher revealed an even distribution of the slug over the inlet headerand bottles. Furthermore the onset-of-carry-over flow rate shifted from 20 to 34 million Nm3/d5.These values were in good agreement with the ones predicted by the model studies.

Summarising, the experience gained with the Den Helder slug catcher highlights the followingpoints:

1. Avoid maldistribution of the slug over the bottles by a proper design of the inlet section(e.g. avoid asymmetry and select a proper diameter of the inlet header and downcomerportholes.

2. The two-liquid model slug catcher is representative of practice and is therefore a usefulaid for slug catcher design.

3.2. St. Fergus slug catcher

The St. Fergus slug catcher (Fig. 4) in operation since 1982 is located at the end of the 36"FLAGS pipeline which transports natural gas from the Brent field to Scotland. Fig. 4 shows thatthe slug catcher consists of 13 bottles (slope 0.4 %). The bottles 1 to 9 inclusive are primarybottles into which the liquid slug flows directly from the inlet header. The bottles 10 to 13inclusive are secondary bottles. Contrary to the primary bottles the secondary bottles have astorage function only and are filled from the lower end via the primary bottles and bottom header.

Other features of the slug catcher geometry are the double riser and equalizer system. Thephilosophy of using two risers rather than one riser per bottle is to lower the flow rate of theexiting gas in the risers and therefore the chance on liquid entrainment. The equalizer system isintended to equalize the pressure in the bottles.

The slug catcher is designed for gas flow rates up to 30 million Nm3/d. However, up to now(1984) the gas flow rate did not exceed 20 million Nm3/d. In most cases hardly any liquid wasremoved from the pipeline by pigging since the pipeline pressure was too high to allow thedevelopment of two-phase flow. Up to now, for those occasions that liquid slugs were present,there have not been any liquid carry-over problems.

It was feared that similar to the Den Helder slug catcher the slug distribution over the primary.bottles was not even, which could give rise to liquid carry-over at higher gas flow rates. To verifythis, laboratory studies were carried out with a two-liquid model of the slug catcher 6. The modelstudies led to the following main conclusions.

1. The slug catcher in its present state can handle slugs up to a size of 1700 m3 at gas flowrates up to 40 million Nm3/d. This is well above the maximum flow rate of 30 millionNm3/d anticipated for the FLAGS pipeline.

2. The incoming slug is unevenly distributed over the primary bottles in particular at low gasflow rates.

3. If carry-over takes place it originates from the secondary bottles after the slug hasentered the slug catcher. Improvement of the slug distribution over the primary bottlesdid not have a significant effect on the onset-of-carry-over-flow rate.

4. After the slug had been received the condensate level was not at the same height: thecloser the bottle to the (eccentrically located) gas outlet the higher the condensate level.In particular in the secondary bottles. This "manometer" effect is caused by the pressuredrop in the equalizer and riser headers due to the gas stream through these headers.This effect is promoted by the eccentric location of the outlet and could give rise to arelatively early onset of liquid carry-over.

The experience gained with the St. Fergus slug catcher shows:

1. Use of secondary bottles is advantageous. Because a part of the incoming slugwill flow into the secondary bottles, less gas is displaced countercurrently in theprimary bottles and the carry-over originating from the primary bottles issuppressed.

2. A central location of the gas outlet is recommended so as to minimize themanometer effect.

3.3. Bintulu slug catcher

The Bintulu slug catcher (Fig. 5) in operation since 1983 is located at the end of the 36" pipelinetransporting natural gas from the Luconia fields offshore of Sarawak to the MLNG plant atBintulu. A second parallel pipeline will be connected to the slug catcher in about 1990 when thegas production of the Luconia fields is at its maximum of about 46 million Nm3/d.

Up to now the gas flow rate has been lower than 10 million Nm3/d and only very small slugshave been received in the slug catcher. No liquid carry-over has been observed.

Fig. 5 shows that this slug catcher consists of 10 primary bottles at a slope of 0.75 %. In order topromote stratified flow, the downcomers are at an angle of 45 . Furthermore, the slug catcher isequipped with a triple riser system and a double equalizer system. The slug catcher has beendesigned by SIPM and in its conceptual stage model tests at KSLA have been performedmaking use of the two-liquid test facility. In the model studies the conditions of the 1990 scenariowere simulated:

Two pipelines are in operation. Maximum flow rate through each pipe is 23 million Nm3/d. Onepipeline only at a time is sphered7.

The model studies showed:

1. In the original conceptual design (in which no constrictions were used, neither in thedowncomers nor in the gas outlet header exits) it was observed that the slug wasunevenly distributed over the bottles and also a maldistribution of the gas flow in the gasoutlet headers was observed, which was very sensitive to the location of the gas outlet.Due to this maldistribution liquid carry-over took place at a flow rate per pipelineequivalent to 14.5 million Nm3/d, which is far below the maximum flow rate of 23 millionNm3/d per pipeline expected in 1990.

2. By the installation of properly sized constrictions in the downcomers and in the gas outletheader exits this maldistribution was suppressed and no longer liquid carry-over wasobserved, not even at a flow rate per pipeline equivalent to 35 million Nm3/d.

Lessons to be learned from these model studies are:

1. Avoid maldistribution of the slug over the bottles by a proper design of the inlet section.

2. Avoid complex riser systems since these can lead easily to gas flow maldistribution.

3. The position of the gas outlet has a significant effect on the slug catcher performance.

3.4. Bacton slug catcher

The Bacton slug catcher in operation since 1968 is located at the end of the two 30" gaspipelines which transport gas from the Leman Bank Fields in the North Sea to England. Up to1973 only one pipeline was in operation. The original version of the slug catcher (designed byEagleton Eng.) is shown in Fig. 6a. It is seen that the original slug catcher consisted of 6 bottles(2 primary and 4 secondary bottles at a slope of 0.75 %). The bottle part of the slug catcher isburied. A special feature of this slug catcher is the use of vertical tee gas/liquid separators in thedowncomers. The philosophy of the use of these devices was to achieve an early gas/liquidseparation in the downcomers. Gas should escape through the horizontal branch (see Fig. 6). Inthe period 1968-1973 the gas flow rate was below 10 million Nm3/d and no liquid carry-over ofthe slug catcher was observed.

From 1973 onwards the slug catcher serves two pipelines. For that purpose it was split into twoslug catchers with 3 bottles each (one secondary and two primary bottles) (see Fig. 6b). Inpractice, in particular slug catcher 2 in the split-up version had an unsatisfactory performance (50vol % liquid carry-over at a flow rate of about 13 million Nm3/d per pipeline!).

It was suspected that the use of a vertical tee separator had a negative influence on the slugcatcher performance. This has been confirmed by laboratory studies carried out with a perspexmodel and using as test media water and air 8,9.These studies showed the vertical tee is a poorgas/liquid separator. At high gas flow rates and a high liquid loading (>80 vol %) it even acts as agas/liquid mixer by sucking gas from the horizontal branch into the vertical stream.

In slug catcher 2 the effect of the vertical tee separators on the slug catcher performance hasbeen eliminated by the installation of a blind plate in the separation header which isolates thisheader from the gas outlet. This modification gave some improvement of the slug catcherperformance. To avoid liquid carry-over during the arrival of a pigged slug now the strategy hasadopted to decrease the gas flow rate several hours before the expected slug arrival.

In summary, both laboratory studies and field experience suggest that the use of a vertical teeseparator as a gas/liquid separator should be avoided.

3.5. The Eemshaven slug catcher

In the near future (1986): a 24" two-phase pipeline will be built to transport the gas/liquidhydrocarbons produced in the F-3 block (North Sea) to Eemshaven. A slug catcher will beinstalled at the end of this pipeline. Fig. 7 shows SIPM's conceptual design of the slugcatcher. It has a symmetrical lay-out consisting of four primary bottles and two sets of foursecondary bottles. The downcomers make an angle of 45 with the horizontal and their port holesare equipped with constrictions to suppress an uneven distribution of the slug over the primarybottles. The total liquid storage concept is 3600m3 of which 1000m3 is intended for permanentliquid storage serving as a buffer for the gas/liquid treating facilities located downstream of theslug catcher. The bottle part of this slug catcher will be buried.

By introduction of a new design feature, the dual slope concept, the slug catcher could be mademore efficient from a liquid storage point of view and therefore less expensive. The separationpart of the primary bottles is at a slope of 2.5%, while their storage part and also the secondarybottles are at a slope of 0.4%. Compared to a single-slope slug catcher (bottle slope 1.5%) thebottles are shorter by some 30m but the storage capacity is still the same. This is shown in Fig.8. An additional advantage of the dual-slope concept is, that the height of the slug catcher isreduced by 2.7m. A point of concern, however, is that this concept may have hydrodynamicconsequences. For instance, a hydraulic instability (hydraulic jump) could occur at ordownstream of the point where the slope changes from 2.5% to 0.4%. Such instability couldpromote liquid carry-over during filling of the slug catcher, in particular when the slug catcher isnearly full.

To investigate this, laboratory studies have been carried out with a Perspex model, using thetwo- liquid test facilities. These studies showed that the change in bottle slope did not introduce ahydrodynamic jump, but merely a gradual increase in liquid hold-up. The liquid carry-overstarted at a flow rate equivalent to about 40 million Nm3 /d, which is well beyond the maximum of10 million Nm3/d which will probably be reached in 1993.

Further tests showed that a reduction of the bottom header diameter from a diameter equivalentto 48 to 36 inches did not have a significant effect on the flow rate at which the onset of carry-over is observed.

The major outcome of these studies is that the dual-slope concept is feasible.

3.6. Summary of lessons learned from field experience and model studies

1. Avoid asymmetry in the slug catcher design. For instance the inlet manifold should besymmetrical and if one gas-outlet is used it should be at a central location.

2. The geometry should be as simple as possible. For instance multiple riser systemsshould be avoided.

3. Avoid maldistribution of the slug over the bottles by a proper design of the inlet manifold.

4. Application of secondary bottles is useful in that they relieve the flow conditions in theprimary bottles.

5. A vertical tee separator in the downcomers should be avoided.

6. The dual slope concept is feasible.

7. The two-liquid model approach is representative of actual conditions and is therefore auseful aid for slug catcher design.

4. DESIGN GUIDELINES FOR MULTIPLE-PIPE SLUG CATCHERS

Based on the experience obtained with multiple-pipe slug catchers (encompassing engineeringpractice, field tests and model studies) a number of guidelines for the design of multiple-pipe slugcatchers are formulated.

4.1. Slug catcher size

The size of the slug catcher is directly related to the maximum of liquid volume, Volsc , it has tohold. The slug catcher should be able to intercept the maximum possible slug size emerging fromthe two-phase pipeline at any moment (Volint). If required it should also contain a buffer volume ofcondensate, Volbuffer, in order to guarantee the buffer liquid supply to liquid-treating facilitiesdownstream of the slug catcher

Volsc = Volint + Volbuffer (1)

As far as the intercepting capacity of the slug catcher is concerned, it has been pointed out in theIntroduction that the largest slug which could occur is a slug generated by sphering. This slugsize is estimated with the computer program TWOPHASEII developed by KSLA. With thiscomputer program it is possible to calculate, among other things, the two-phase flow regime, theliquid hold- up in and pressure drop over a two-phase pipeline under steady-state conditions. Inprinciple the size of the sphered (or pigged) slug and therefore Volint is calculated from thefollowing formula:

Volint = Lpipe ( )HL - l (2)

where L denotes length, HL is the liquid hold-up averaged over the pipe length and l the liquidvolume fraction flowing in the pipe. Note that eq. (2) gives the maximum slug size since ingeneral between two subsequent sphering runs not sufficient time is lapsing to allow the liquidhold-up in the pipeline to build up to its equilibrium level. Also the pig or sphere will not have a100% efficiency.

No general rule can be given for the determination of Volbuffer since it is largely determined by thecharacteristics of the liquid-treating facilities.

The slug catcher is considered to be full (contains Volsc) when the liquid level in the primarybottles just reaches the bottle section immediately underneath the risers (assuming the sameliquid level in all bottles). See also Fig.II.1 in Appendix II. Under these conditions there is noliquid hold-up underneath the risers of the primary bottles and the risk of liquid entrainment in thegas stream exiting the primary bottles is minimal. In Appendix II the relations are given tocalculate Volsc as a function of bottle slope, length and diameter and number of primary andsecondary bottles both for single and dual-slope slug catchers.

4.2. Slug catcher geometry

In general a multiple-pipe slug catcher could be divided into three sections.

- Inlet section- Bottle section- Gas outlet section.

The guidelines for the slug catcher design are given below by reviewing the slug catchersectionwise.

4.2.1. Inlet section

In this section the distribution of the incoming liquid over the bottles takes place. Also here a startis made with the gas/liquid separation by promoting the occurrence of stratified two-phase flow.This section entails:

- end of the pipeline- splitter(s)- inlet header- downcomers- expanders.

The geometry of the inlet system should be symmetrical. If there are fewer than 4 downcomersno splitter is required. To avoid maldistribution of the slug over the bottles more than 8downcomers should be avoided. Based on the experience with the Den Helder slug catcher theportholes of the downcomers should be at most 40% of the inner diameter of the inlet header toguarantee an even liquid distribution. A smaller porthole cross- section is achieved by either aconstriction in the portholes or simply by a smaller downcomer diameter. If constrictions are usedthey should be located eccentrically, close to the wall of the downcomer (see Fig. 9). In this waya jetting effect which could lead to excessive mist/foam generation is suppressed because theliquid is guided along the wall. Also dirt accumulation upstream of the constriction is avoided. Theoption of using a constriction rather than decreasing the downcomer diameter is slightly preferredsince because of the expansion downstream of the constriction the segregation of gas and liquidis promoted. This segregation could be even more promoted by the selection of a downcomerslope of 1 rather than using vertical downcomers. From the open literature it is known that an angle of 45 with the horizontal plane is optimal for the development of stratified flow 12

In existing slug catchers the diameter of the downcomers is smaller than that of the bottle (ratherarbitrarily: Ddowncomer 2/3 Dbottle). The downcomer is connected to the bottle through an eccentricconical expander either with the flat side up (Den Helder) or the flat side down (St. Fergus,Bintulu). Due to the expansion a further gas/liquid separation will take place. There is a slightpreference for the flat side down option because in that case no discontinuity in bottle slopetakes place, which could upset the development of stratified liquid flow.

4.2.2. Bottle section

This section encompasses:

- primary bottles

- secondary bottles

- equalizer system

- bottom header

In the primary bottles the gas/liquid separation is completed and the liquid is stored. Thesecondary bottles have a storage function only. The equalizer system is meant to equalize thepressure of the bottles.

4.2.2.1. Choice of primary and secondary bottles

The choice of the number of primary, npb, and secondary bottles, nsb , to be used in the multiple-pipe slug catcher depends on several factors.

1. Gas flow rate in the pipeline

2. Required liquid storage volume of the slug catcher

3. Plot size available (length available for bottles)

4. Diameter of the bottles to be used

5. Slope of the bottles

6. Single or dual slope concept slug catcher?

It is attractive to keep the number of primary bottles as low as possible because of the followingreasons:

1. The slug catcher becomes less expensive because fewer fittings are used in the inletand bottom header.

2. There is less chance of a slug maldistribution over the primary bottles.

On the other hand the higher the gas flow rate in the pipe (which determines the velocity of aslug generated by pigging), the more primary bottles are required so as to avoid overloading ofthe bottles and therefore liquid carry-over. The maximum flow rate a primary bottle can acceptwithout liquid carry-over is a function of the physical properties of the gas and liquid, the bottlediameter and slope and the amount of liquid which can flow from the lower end of the primarybottle to other bottles. For more information on this flow rate see Section 4.2.2.3.

For design purposes the worst case should be considered, i.e. no liquid flow to other bottles fromthe lower end. Also a certain degree of maldistribution of the slug over the bottles should betaken into account. It is recommended to assume that the most heavily loaded bottle receives20% more than in the case of an even distribution (120/npb %). Furthermore (if npb > 1) npb shouldbe even from a symmetry point of view.

Once npb is determined, the total storage volume of the primary bottles is calculated with theequations presented in Appendix II. By subtracting this value from the required slug catcherstorage volume, the volume of the secondary bottles is found. Subsequently, the number ofsecondary bottles is calculated.

4.2.2.2. Length of the entrance section of the primary bottles required for settling the small droplets

Under steady-state operating conditions when no sphering is carried out a continuous gasstream with some liquid continually enters the slug catcher. It is divided via the inlet section overthe bottles. Due to this a reduction of the gas velocity takes place and consequently smalldroplets entrained in the gas phase could settle. The first part of the primary bottles (between theconical expander and the riser system) should be long enough for a sufficient completion of thisprocess. In Appendix III it has been made plausible that in a full-scale slug catcher with 36 inchbottles the time needed to settle droplets larger than 0.5 mm is in the order of 4 s. Since it ishighly unlikely that the gas velocity in the first part of the bottle will exceed say 2 m/s, a length ofsay 8m in the case of a 36 inch bottle, or more general an entrance length of about 10 bottlediameters should be adequate for most settling purposes.

4.2.2.3. Effect of bottle slope on choking

The bottles of the multiple-pipe slug catcher should slope downwardly. This will facilitate theliquid filling of the primary bottles by gravity and the flow of gas displaced by the incoming liquidto the gas outlet system. It is essential that during the slug catching operation the liquid in theprimary bottles flows down as a stratified layer so as not to impede the ascending gas stream.Beyond a given liquid flow rate into the bottle which is among other things a function of the bottleslope, stratified flow can no longer be maintained and choking of the bottle will take place. One ofthe two following mechanisms could be responsible for this:

1. More liquid flows into the bottle than can be transported as a satisfied layer in the bottleitself. This will be the case when the bottle slope is very small and therefore the gravitydrain is insufficient. In Appendix I of Ref. 3 a calculation model has been presented topredict the onset of choking according to this mechanism.

2. Kelvin-Helmholtz instability 13. When the relative velocity between the descending liquidstream and the ascending gas stream in the primary bottle exceeds a critical value, theliquid/gas interface in the bottle becomes unstable and excessive wave formation willtake place. In bottles equipped with a riser this will occur predominantly near to the riser(see also Ref. 3). The Kelvin-Helmholtz instability will take place when the bottle slope isrelatively steep.

Based on laboratory studies carried out at KSLA (simulation of a slug catcher bottle using theTray Test Column facilities) a model has been developed to predict this type of instability (Ref. 14and Appendix IV of this report).

The two models have been combined to a general choking criterion.

Both for Den Helder and St. Fergus slug catcher conditions (pressure 70 and 110 bar, resp.) theonset-of-choking flow rate is calculated as a function of the bottle slope. It is seen from Fig. 10that in the case of the Den Helder slug catcher the bottle slope (1.5 %) is about the optimalslope. Furthermore, the predicted onset flow (0.67 m3/h) is in good agreement with the 0.70m3/hmeasured in practice 5. As far as the St. Fergus slug catcher is concerned, its slope (0.4%) iswell below the optimal value as predicted by the choking criterion (2.5 %).

Based on the choking criterion and realizing that the optimum is rather flat it is recommended totake 1% as a minimum value for the bottle slope in multiple-pipe slug catchers. Fig. 10 suggeststhat from a choking point of view slopes up to 3% would be acceptable. However, large bottleslopes could lead to an unacceptably high elevation of the slug catcher.

4.2.2.4. The dual-slope approach

The experience with the model of the Eemshaven slug catcher has shown that the dual-slopeconcept is feasible. The advantage of this approach is that a more efficient use is made of theliquid storage capacity of the bottles. A drawback is that the liquid hold-up underneath the risersof the secondary bottles is relatively high. In the Eemshaven case the slope of the storage part ofthe slug catcher is 0.4%. From a choking point of view this value is too low (see Fig. 10) but stillacceptable for the Eemshaven slug catcher because this slug catcher will not operate undersevere flow conditions (see Section 3.5). In general it is recommended to take as minimum slopefor the secondary bottles and the storage part of the primary bottle a value of 1%. Fig.11 showsthe effect of the slope of the separation section on the liquid storage capacity of the primary andsecondary bottles when the slope of the secondary bottles and the storage part of the primarybottles is either 0.4 or 1%. It is seen that particularly in the case of a 0.4% slope a substantialincrease of the storage capacity is obtained, a slope of the separation section larger than say2.5% gives only a minor further improvement. Therefore a value of maximal 2.5% isrecommended for the slope of the separation section.

4.2.2.5. Equalizer system

Both in the St. Fergus and in the Bintulu slug catcher an equalizer system is used. It was found inthe model tests that the effect of an equalizer system on the slug catcher performance is verysensitive to the slug catcher geometry and is not always beneficial. In the model studies carriedout for the St. Fergus slug catcher, it was found that the equalizer system had a negative effecton the slug catcher performance. Gas flowing from the primary to the secondary bottles throughthe equalizer system caused liquid carry-over of the secondary bottles 6 . It is thereforerecommended not to use an equalizer system.

4.2.2.6. Bottom header

It is common practice to design the bottom header with the same diameter as the bottles. Themodel studies carried out for the Eemshaven slug catcher showed, however, that a smallerdiameter (down to 75% of the bottle diameter) did not affect the slug catcher performance. Afurther reduction is not recommended because of the risk of blockage of the bottom header bysludge or dirt always present in the lower part of the slug catcher. In relation to possible dirtaccumulation one should keep the header accessible for cleaning. To prevent gas carry-underduring liquid drainage of the slug catcher it is recommended to have the bottom header below thelower end of the bottle. A possible geometry is given in Fig. 12, where the bottle slopesdownwardly at an angle of 45 into the bottom header.

4.2.3. Gas outlet section

This section entails:

- risers

- outlet header(s)

- gas outlet(s).

4.2.3.1. Gas risers

The gas risers must act as vertical separators. The lower the gas velocity the lower the loadfactor l defined as

lr

r r=

-G

L GSGv

and the smaller the liquid droplets which remain entrained in the riser gas stream. It is proposedto take for slug catcher application l 0.20. It can be derived with the equation of Appendix IIIthat droplets with a diameter larger than 2mm will settle from the stream through the riser(assumed liquid fraction = 0.1). This is acceptable if one takes into account that the slug catcheris not meant to be a high-efficiency separator for small liquid droplets. This criterion willdetermine the maximum value for vSG. In the worst case we have the following scenario:

1. The riser(s) is (are) located at the primary bottle which receives (120/npb)% share of theincoming slug (in case of an even slug distribution over the bottles the share would havebeen (120/npb)%).

2. The slug is generated by pigging or sphering.

3. There is no liquid flow from the heavily loaded bottle to the other bottles. This means amaximum gas flow through the riser(s). If the number of risers per bottle is denoted by m,the gas velocity in the riser should obey to the following relationship:

vSG =

12

4

022

.

.n

Q

m D

pbpipeline

riser

L G

Gpr r

r

-

To keep the gas outlet as simple as possible and to avoid maldistribution of the gas flow in theriser system, preferably only one riser should be used per bottle. This could be achieved bytaking npb and/or Driser as large as possible with as practical upper limit Driser =Db .

The riser should have a minimum height to allow liquid entrained in the riser gas stream to settle.It is recommended to take the riser height at least equal to 5Driser.

4.2.3.2. Gas outlet header(s) and gas outlet(s)

The diameter of the gas outlet header(s) and gas outlet(s) should not be taken too small. In thecase of small diameters the pressure drop over the gas outlet system could become significant.The consequence of this is that after the slug has been received the liquid level in the bottlesclosest to the gas outlet will rise relative to the level in the other bottles as has been observedwith the model of the St. Fergus slug catcher (manometer effect, see Section 3.2 of this report). Itis advised to take

Doutlet = Dgoh DriserThe gas outlet should not be located eccentrically. In Fig. 13 a few recommended gas outletconfigurations are given. The lay-out should be as nearly symmetrical as possible.

5. FINAL REMARK

This report contains the current state of knowledge on multiple-pipe slug catchers at KSLA. It isintended to be a document which has to be updated regularly. Readers and users of this reportare encouraged to make comments suggestions and to supply additional experience which couldlead to the upgrading of the design criteria outlined here.

TABLE I : REVIEW OF REPRESENTATIVE MULTIPLE-PIPE SLUG CATCHERS

* Original 6-bottle slug catcher divided into to serve two pipelines.

** Dual-slope concept (end of 1983).

*** Between brackets the capacity of the full-scale slug cacther following from field measurements.

+ Related to one slug catcher of the twin-concept

FIG. 1 : VESSEL SLUG CATCHER WITH SEPARATE SURGE DRUMS

FIG. 2 : THE GEOMETRY OF THE DEN HELDER SLUG CATCHER

FIG. 3 : PARKING LOOP TYPE SLUG CATCHER FOR LOCATION AT A BOOSTER COMPRESSORSTATION (ONLY VALVE REQUIRED FOR OPERATION OF THE SLUG CATCHER ARE INDICATED)

FIG. 4 : THE St. FERGUS SLUG CATCHER

FIG. 5 : THE BINTULU (SARAWAK) SLUG CATCHER (LENGTHS IN METRES, DIAMETERS ININCHES)

FIG. 6 : THE BACTON SLUG CATCHER

FIG. 7 : CONCEPTUAL DESIGN OF THE EEMSHAVEN SLUG CATCHER

FIG. 8 : EFFECT OF INTRODUCTION OF DUAL-SLOPE CONCEPT FOR THE EEMSHAVEN SLUG-CATCHER ON LIQUID STORAGE CAPACITY AND HEIGHT OF THE RISER FOOT OF THE PRIMARYBOTTLES

FIG. 9 : THE LOCATION OF THE DOWNCOMER RESTRICTIONS

FIG. 10 : THE ONSET-OF-CHOKING FLOW RATE FOR A 36 OD BOTTLE AS A FUNCTION OF THEBOTTLE SLOPE (NO LIQUID FLOW TO OTHER BOTTLES)

FIG. 11 : THE STORAGE CAPACITY OF THE PRIMARY AND SECONDARY BOTTLES IN A DUAL SLOPE SLUG CATCHER

BOTTLE IN A DUAL-SLOPE SLUG CATCHERDISTANCE BOTTOM HEADER RISER 300 mSLOPE OF STORAGE SECTION EITHER 0.4 % OR 1 %

FIG. 12 : THE RECOMMENDED LOCATION OF THE BOTTOM HEADER RELATIVE TO THE BOTTLE

FIG. 13 : RECOMMENDED GAS OUTLET CONFIGURATIONS (VIEW FROM ABOVE)

APPENDIX I

THE TWO-LIQUID TEST FACILITY FOR THE SIMULATION OF SLUG CATCHERS

For the assessment of the performance of existing and conceptual slug catchers a two-liquid test facilityhas been in use at KSLA since 1980. Through a perspex model of the slug catcher under study,kerosene is circulated representing the gas phase in practice. A condensate slug is simulated bydiverting the kerosine stream through a tank containing a 55%w ZnCl2 concentrate in water, thus forcingthe concentrate into the slug catcher.

The reason for the choice of a kerosene/ZnCl2 concentrate system rather than air/water for instance isthat, as far as the density ratio is concerned, a much better similarity is obtained (see Table I.1). Themismatch as regards the viscosities is less important because both in the full-scale and model slugcatcher (if the scale is not too small) all fluids flow turbulently.

In the simulation studies Froude scaling is applied, i.e. the flow conditions in the model slug catcher arethought to be representative for those in the full scale slug catcher characterized by the same Froudenumber

Fr = vD gSG

G

L G pipe

rr r( )-

where vSG is the superficial velocity of the light phase in the pipeline upstream of the slug catcher.

For the 1:20 perspex model of the Den Helder slug catcher this implies for instance:

vfull-scale = 11.6 * vmodelThe experience with the Den Helder slug catcher has shown that the concept of Froude scaling is correctand also that the two-liquid modelling approach is justified.

TABLE I-1 : COMPARISON OF PHYSICAL PROPERTIES

APPENDIX II

THE LIQUID STORAGE CAPACITY AND HEIGHT OF A MULTIPLE-PIPE SLUG CATCHER

1. Storage capacity

The following assumption are made:

(1) The slug catcher is filled to its maximum capacity when the liquid level in the primarybottles is at the level of the bottle slope transition (in the case of a dual-slope slugcatcher) and just reaches the bottle section immediately underneath the riser(s) (see Fig.II.1)

(2) The bottle slope(s) is (are) smaller than 5 %. Then cos q 1 and sin q tgq

(3) There are npb primary and nsb secondary bottles.

(4) For the bottle part between the bottom header and risers a length of L is available.

Volsc = Volbh + n1 Volpb +n2 Volsb

Volbh = p4

2D Lbh bh

Single-slope slug catcher

Volpb = Volsb = p

q4 22D L

Dtgbb-

Dual-slope slug catcher

primary bottle : Volpb = p

q4 22

2

D LDtgbb-

secondary bottle : Volsb = ( )p q4 2 12

2

D LDtg

Hbb

L rf- -

,

Htg tgtgL,rf

-q qq

2 1

2

If q 2 >>q1, HL, rf 1 and Volsb p4

2D Lb

2. Height difference between lower end of bottle and riser foot

Single-slope slug catcher

Dhpb = Dhsb = L tg q

Dual-slope slug catcher

primary bottle : Dhpb = L tg q1 + Db 1 12

-

tgtg

qq

secondary bottle : Dhsb = L tg q1

FIG. II-1 : THE FILLING DEGREE OF THE BOTTLES IN A MULTIPLE-PIPE SLUG CATCHER FILLED TO ITS MAXIMUM CAPACITY

APPENDIX III

SETTLING OF SMALL DROPLETS IN THE ENTRANCE SECTION OF THE PRIMARY BOTTLES

For the calculation of the time required to settle small droplets in the entrance section of the primarybottles the following assumptions are made.

(1) The mist droplets are spherical and have all the same diameter.

(2) Effect of turbulence on settling velocity is neglected. In other words, the vertical velocityfluctuations in the turbulent gas flow compensate each other. Then the settling velocity is equalto that in stagnant medium.

According to Ref. 16 the following formulae apply for the settling velocity of an isolated mist droplet

If Re = v ds G

G

rh

for Re < 1

vs = ( )r r

hL G

G

gd- 2

18

for 1 < Re < 103

vs follows from

C Red g

nwG L G

G

2 3

2

43

=-r r r( )

and Fig. II-41 in Ref. 15

for 103 < Re < 105

vgd

sL G

G

=-

176.( )r r

r

For the calculation of a droplet in a swarm the relationship of Richardson and Zaki holds (See Also Ref.16)

(vs) swarm = vs (1 H)n

where n ranges from 4.56 to 2.39 for 0.1 Re 500. Furthermore the conservative assumption is madethat the mist droplet has to travel a distance D, so

ts = D

vs swarm( )

Fig. III.1 shows for the Den Helder slug catcher tsettle as a function of the droplet diameter for severalvalues of Hmist. Rather arbitrarily it is assumed that droplets larger than 0.5 mm should settle. FromFig.III.1 it is seen that this is achieved within about 4 s even when H is as high as 0.2.

FIG. III-1 : THE TIME REQUIRED TO SETTLE SMALL LIQ. DROPLETS AS A FUNCTION OF THEDROPLET DIAMETER IN THE FIRST PART OF A PRIMARY BOTTLE

APPENDIX IV

SIMULATION OF BOTTLE CHOKING (KELVIN-HELMHOLTZ INSTABILITY) IN THE AMSTERDAMTRAY TEST COLUMN

In 1977 at KSLA a test installation was built at the Amsterdam Tray Test Column (TTC) to study the two-phase flow phenomena occurring during filling of a slug catcher bottle. Tests were performed in a modelslug catcher bottle with an inner diameter of 30 cm and an inclination of 1.5 % using amongst othersbutane up to a pressure of 13 bar. In contrast to the real situation the test bottle in the TTC experimentshad no riser. The liquid phase flowed into the bottle from the upper end and the gas from the lower end.

It was observed that if the velocity of gas relative to the descending liquid stream exceeded a criticalvalue, excessive wave formation took place and eventually "choking" of the pipe occurred. This "choking"effect is known as "slugging" or Kelvin-Helmholtz instability in the open literature (Wallis and Dobson 13).For more details see Ref.14.

Based on the TTC data supplemented with information from the literature 13,15

, Darton et al. 14 arrived

at the following criterion for choking.

Choking takes place, if

( )( )

H vg D

H H

L LL

L G

L L

r

r r

q

--

+-

>1

036 634

1

0

2 2 3.6

. .

where

fr

r=

-

H v

H vL L L

L G G( )1

LIST OF SYMBOLS

Cw drag coefficient

d droplet diameter m

Fr Froude number =-

v

D gSGG

L G

rr r( )

H hold-up

g gravity constant m/s2

L length m

m number of risers per bottle

n number of bottles or exponent in Richardson-Zakiequation (Appendix III)

v velocity m/s

Vol volume m3

Re Reynolds number associated with particle settling

Re=

v dS GG

rh

Greek symbols

n dynamic viscosity Ns/m2

l load factor =-

vSG

G

L G

rr r

m/s

or

volumetric fraction of liquid flow intwo-phase flow

q angle between bottle and horizontal plane

r density kg/m3

f flow parameter =-

H v

H vL L L

L G G

r

r( )1

Subscripts

b bottle

bh bottom header

G gas

int intercept

L liquid

goh gas outlet header

pb primary bottle

rf foot of the riser

s settle

S superficial

sb secondary bottle

1 referring to separation section of the slug catcher in the dual-slope concept

2 referring to storage section of the slug catcher inthe dual-slope concept

Superscript

- average

REFERENCES

1. A.R. Huntley and R.S. Silvester, Hydrodynamic analysis aids slug catcher design, Oil and Gas J.81(1983)95, Sept. 19.

2. A.E. Martin, Handling liquids in offshore gaslines gets new approach. Oil and Gas J. 79 (1981)143-148.

3. A. Bos, J.A. van Klaveren and R. Meerhoff, "Liquid carry-over at the NAM slug catcher in DenHelder". I. Problem analysis using the neutron back-scattering technique", AMGR.82.288.

4. A. Bos and J.G. du Chatinier, "Liquid carry-over at the NAM slug catcher in Den Helder. II. Modelstudies", AMGR.82.335.

5. A. Bos, C.A. Kok and J.G. du Chatinier, "Liquid carry-over at the NAM slug catcher in DenHelder. III. Solving of the problem", AMGR.83.266.

6. A.Bos and J.G du Chatinier, Analogue modelling of the St.Fergus slug catcher, AMRG.84.134.

7. A. Bos and J.G. du Chatinier, "Improvement of the Bintulu (Sarawak) slug catcher",AMGR.83.354.

8. C.M. Verheul, "A model study of existing and new slug catcher configurations for natural gaspipelines from off-shore platforms", AMGR.0208.71.

9. A. Hortulanus and P.E.M. Duyvesteyn, "Model study for the modification of the Bacton slugcatcher", AMOR.0001.73.

10. J.G. du Chatinier and A. Bos, "Engineering studies on liquid entrainment in slug catchers. Modeltests for the design concept of NAM's F-3 slug catcher", AMRS.84.07, PR-1.

11. N. Trompe, R.V.A. Oliemans and J.A. ten Hagen, "TWOPHASE", A computer program for thehydraulic design of horizontal and inclined pipelines with two-phase gas/liquid flow. User guide",AMGR.79.391.

12. H.D. Beggs and J.P. Brill, A study of two- phase flow in inclined pipes, J. Petr. Tech. 25 (1973)607.

13. G.B. Wallis and J.E. Dobson, The onset of slugging in horizontal stratified air -water flow, Int. J.Multiphase Flow 1 (1973)173.

14. R.G. Darton and G. Lentz, "Amsterdam tray test column. Countercurrent two-phase flow in anear-horizontal pipe. Investigation of conditions in a simulated slug catcher storage bottle duringfilling", Amsterdam Tray Test Column Test Report 79, Layout 28,AMGR.83.265.

15. E. Kordyban and T. Ranov, "Mechanism of slug formation in horizontal two-phase flow", J. BasicEngng. 92(1970)857.

16. W.J. Beek and K.M.K. Muttzall, "Transport Phenomena", John Wiley & Sons Ltd.,London, 1975,p. 101 ff.

TITLEPREFACECONTENTSINTRODUCTIONSLUG CATCHERS GENERALNecessity of a slug catcherTypes of slug catcher

EXPERIENCE WITH MULTIPLE-PIPE SLUG CATCHERSDen Helder slug catcherSt. Fergus slug catcherBintulu slug catcherBacton slug catcherThe Eemshaven slug catcherSummary of lessons learned from field experience and model studies

DESIGN GUIDELINES FOR MULTIPLE-PIPE SLUG CATCHERSSlug catcher sizeSlug catcher geometryInlet sectionBottle sectionChoice of primary and secondary bottlesLength of the entrance section of the primary bottles required for settling the small dropletsEffect of bottle slope on chokingThe dual-slope approachEqualizer systemBottom header

Gas outlet sectionGas risersGas outlet header(s) and gas outlet(s)

FINAL REMARKTABLE I : REVIEW OF REPRESENTATIVE MULTIPLE-PIPE SLUG CATHERSFIGURES1 : VESSEL SLUG CATCHER WITH SEPARATE SURGE DRUMS2 : THE GEOMETRY OF THE DEN HELDER SLUG CATCHER3 : PARKING LOOP TYPE SLUG CATCHER FOR LOCATION AT A BOOSTER COMPRESSOR STATION 4 : THE St. FERGUS SLUG CATCHER5 : THE BINTULU (SARAWAK) SLUG CATCHER (LENGTHS IN METRES, DIAMETERS IN INCHES6 : THE BACTON SLUG CATCHER7 : CONCEPTUAL DESIGN OF THE EEMSHAVEN SLUG CATCHER8 : EFFECT OF INTRODUCTION OF DUAL-SLOPE CONCEPT FOR THE EEMSHAVEN SLUG-CATCHER ON LIQUID STORAGE CAPACITY AND HEIGHT9 : THE LOCATION OF THE DOWNCOMER RESTRICTIONS10 : THE ONSET-OF-CHOKING FLOW RATE FOR A 36 OD BOTTLE AS A FUNCTION OF THE BOTTLE SLOPE ( NO LIQUID FLOW TO OTHER BOTTLES )11 : THE STORAGE CAPACITY OF THE PRIMARY AND SECONDARY BOTTLES IN A DUAL SLOPE SLUG CATHER12 : THE RECOMMENDED LOCATION OF THE BOTTOM HEADER RELATIVE TO THE BOTTLE13 : RECOMMENDED GAS OUTLET CONFIGURATIONS (VIEW FROM ABOVE)APPENDICESI:THE TWO-LIQUID TEST FACILITY FOR THE SIMULATION OF SLUG CATCHERSII : THE LIQUID STORAGE CAPACITY AND HEIGHT OF A MULTIPLE-PIPE SLUG CATCHERIII : SETTLING OF SMALL DROPLETS IN THE ENTRANCE SECTION OF THE PRIMARY BOTTLESIV : SIMULATION OF BOTTLE CHOKING (KELVIN-HELMHOLTZ INSTABILITY) IN THE AMSTERDAM TRAY TEST COLUMN

LIST OF SYMBOLSREFERENCES