8  Pumps, Compressors and Process Components 2012

Pumps

Screw pumps

Abstract

During  the  past  two  decades  multi­phase  pumps  experienced  growing acceptance.  Especially  twin  screw multiphase pumps which cover a wide range of flow rates and pressures have found  many  applications  on  ageing and  matured  oilfields  but  also  on  re­cent  discoveries  with  high  gas  con­tent.  Low  installation  costs  go  along with  small  dimensions  which  makes twin  screw  multiphase  pumps  parti­cularly suitable for the installation on offshore platforms.

Introduction

Since  the  first  discoveries  and  pro­duction  of  abundant  oil  wells  in  the middle  of  the  19th  century,  crude  oil and  petroleum  gas  have  been  funda­mental  sources  of  modern  industrial developments.  As  feed  stock  for  the chemical industry or source for the ge­neration  of  electrical  energy  and  fuel for  the dramatically  increasing moto­rization,  oil  and  gas  have  considerab­ly  contributed  to  improved  wealth  in many countries of the world. The stea­dy rise of world population and the in­dustrial  growth  in  emerging  econo­mies results in increasing demand for energy. 

Powerful multiphase pumps for increased oil & gas production in the Gulf of Mexico

Jörg Narewski, Sven Olson, Hans Jürgen Schöner, Dr. Michael Radke

However,  manhood  is  since  long aware  about  the  fact  that  the  times that  hydrocarbons  which  are  easily available  at  low  costs  are  over.  Most of  the  oil  producing  countries  in  the world have reached their peak produc­tion  years  ago  and  now  face  decrea­sing  hydrocarbon  reserves  which  do not cover their demands. The biggest consumers  of  energy  are  not  always large  producers.  However,  successful exploration  opens  access  to  new  oil and gas fields.

Production  by  region:  Million barrels  daily  world  oil  production increased  by  1.8  million  b/d  in  2010; growth  was  broadly­based,  with  in­creases in all regions except Europe & Eurasia. Moreover, growth was broad­

ly  split  between  OPEC  and  non­OPEC countries.  World  oil  consumption  in­creased by 2.7 million b/d; growth was above average in all regions, although Asia  Pacific  countries  accounted  for the majority (54 %) of global consump­tion growth. (Graphic: source BP)

These new reservoirs, however, are not easy to produce and the oil com­panies  are  faced  with  many  challen­ges.  Most  of  the  remaining  oil  and gas  reserves  of  the  world  are  located close to the arctic circle, in the oceans in  shallow,  deep  or  ultra­deep  water, in deserts and other not easily acces­sible  areas.  Many  of  these  areas  are not  developed  and  a  suitable  infra­structure  must  be  created  to  allow successful  exploration  and  produc­tion of these oil and gas fields. A large growth in new oil and gas field disco­veries  has  been  in  South  and  Central America. Particularly Mexico and Bra­zil  deve loped  assets  which  will  gua­rantee sufficient national reserves for many years.

A large number of today’s oilfields are maturing fields. The reservoir pres­sure has decreased over the years and artificial lift by means of gas or water injection  or  ESP’s  (Electrical  Submer­sible  Pumps)  must  be  used  to  bring the well­flow to the surface and create sufficient pressure to operate conven­tional separation equipment. The use of multiphase pumps with their ability 

Fig. 1: Production by region (Million barrels daily) and consumption by region (Million barrels daily)

Fig. 2: Distribution of proved reserves in 1990, 2000 and 2010 percentage (Graphic: source BP)

Pumps, Compressors and Process Components 2012  9

Pumps

Screw pumps

to operate at very low well head pres­sures  can  economically  increase  the production life of matured wells. 

Many oil producing countries have not  yet  developed  sufficient  infra­structures  to  use  the  associated  gas produced  as  feed  stock,  automotive fuel, power generation or heating pur­poses.  This  gas  is  still  widely  flared or  vented  after  separation  and  con­tributes  significantly  to  the  global warming  by  releasing  CO2

  –  or  even worse CH

4 – emissions to the atmos­

phere.  Most  countries  are  taking  sta­tutory measures to ban all flaring and venting  during  the  next  few  years. Multiphase  pumps  contribute  to  the successful  achievement  of  this  tar­get by transferring the untreated well flow to centralized treatment facilities were the petroleum gas can be econo­mically processed.

The twin screw pump as multi­phase system

Twin screw multiphase pumps are ro­tary  positive  displacement  pumps  of double volute design. They are hydrau­lically  balanced  with  no  thrust  loads on the pump bearings. After entering through the suction port of the pump the  flow  splits  to  either  side  and  is pressurized within the screw packages with  opposed  thread.  The  screws  are machined from single piece bar stock for  maximum  stiffness  in  order  to maintain  minimum  shaft  deflection under  all  operating  conditions.  They are  installed  in  the  replaceable  cast steel casing insert. The surface of the screws  is  nitrided  for  wear  protec­tion. The torque from the drive screw is transmitted to the idler screw via oil lubricated  timing  gears.  An  external lube  oil  system  provides  well  tempe­rated lube oil to the timing gears and the heavy duty and lifetime optimized tapered  roller  bearings  which  carry the drive screws and the idler screws. The  bearing  span  is  kept  as  short  as possible in order to minimize the shaft deflection.

There is no contact between pump screws and casing insert. To maintain a clearance between the pump screws and the casing insert under all opera­ting conditions is a basic and essential 

Fig. 4: Cut-a-Way of a twin screw multiphase pump

Fig. 3: Comparison between conventional handling and multiphase pumping

10  Pumps, Compressors and Process Components 2012

design  criteria  for  twin  screw  multi­phase pumps.

The  screw  shafts  are  sealed  by double  acting  mechanical  seals  with SiC vs. SiC seal faces on both product and  atmospheric  side.  An  external seal  system  in  accordance  with  the require ments  of  API  Plan  54  provides clean and cool seal oil to each seal. All mechanical  seals  are  installed  on  the suction side of the pumps and hence, exposed to suction pressure only. 

A large multiphase production system in the Gulf of Mexico

The  Gulf  of  Mexico  is  historically  the most  important  area  in  North  Ame­rica for providing oil and gas and raw material in the petrochemical industry to the USA and Mexico. From the start with  oil  and  gas  production  offshore in  the  1960’s  its  importance  is  con­tinuing  to  grow  with  new  larger  hy­drocarbon  deposits  discovered  every year.  Initially,  shallow  water  reser­voirs at water depth up to 100 m were produced  along  the  US  and  Mexi can coastlines.  Development  of  new  ex­ploration  and  prospecting  techno­logies  made  it  possible  to  go  deeper and  deeper. Today  reservoirs  at  a  wa­ter depth of 3000 m can be economi­cally produced. Despite the belief that the  Gulf  would  run  out  of  hydro  car­bon deposits one day, actually more oil and gas are discovered and made eco­nomically possible to produce, thanks to  3­D  seismic  imaging,  directional 

drilling and deep water trees and pro­duction manifolds.

In  the  Mexican  territorial  wa­ters significant findings of oil and gas have  been  done  in  the  last  30  years. The largest reservoir ever found in the western  hemisphere  was  Cantarel, which only recently lost its first place to the pre­salt Tupi field  in Brazil. Be­sides  Cantarel  there  are  other  signifi­

cant fields in the shallow water (up to 100 m) depth in Mexico. The giant Ku­Maloob­Zaap  field  started  to  develop in  the  1980’s  and  1990’s.  It  is  a  large complex of fixed platforms in 60–80 m water  depths.  Some  platforms  are wellhead  platforms  whereas  others have  processing  and  power  genera­tion  equipment.  Also,  an  FPSO  is  an­chored nearby and serves as an exten­ded production and storage facility.

The  oil  is  typically  heavy  with  an API grade of 18 to 22 with significant amounts  of  gas  produced  as  associa­ted gas with the oil. The oil and some 

Pumps

Screw pumps

Fig. 5: Platform locations and power distribution

Fig. 7: Completed multiphase pump skid be-fore dispatch

Fig. 6: Completed multiphase pump skid in 3D

gas  is  exported  to  shore  from pipelines. A lot of the gas is used in power generation with gas tur­bine  driven  pumps  and  compres­sors  and  in  the  processing  of  the crude oil. When the wells started to  flow  initially,  water  injection was  used  to  maintain  reservoir pressure.  As  the  wells  were  age­ing  gas  lift  was  introduced.  With gas  lift  compressed  natural  gas is  sent  down  the  annulus  of  the well  and  through  a  gas  lift  valve, the gas lightens the column of oil in the production tubing to  lift  it to the surface. For some time this method of production is sufficient to  bring  up  the  oil  and  provides enough  pressure  in  the  pipelines to  overcome  line  losses  and  the arriving pressure of the first stage separator. 

Gradually  the  gas  lift  is  not enough to recover the oil in the re­servoir and bottom hole pressure of well reaches a point where the oil  cannot  any  longer  move  from the formation to the well bore.

At  that  moment  there  are some  few  remedies  left  to  save the  reservoir  from  abandonment and one of these is the fast deve­loping  technique  of  multiphase pumping.

In the case of the Ku­Maloob­Zaap  field  the  production  from the wells were estimated to drop very  quickly  and  in  the  next  few years  reach  a  point  where  they would have to be abandoned. Af­ter lengthy studies it was decided 

by the Mexican State Oil Compa­ny that three of the wellhead plat­forms were going to be equipped with multiphase pumps.

The  objective  was  to  be  able to  continue  production  for  many years to come and delay the aban­donment of the reservoir and dra­matically  increase  the  total  re­covery  of  hydrocarbons  from  the formation.

The  multiphase  pumps  pro­positioned was analysed in detail. Production curves were showing a pressure  draw  down  at  the  well­head of 12 bar would have a signi­ficant positive impact on the bot­tom hole pressure and the ability of the wells to continuous produ­cing.  At  this  low  well  head  pres­sure  the  associated  gas  will  ex­pand sig nificantly which means at pump inlet the gas fraction (GVF) can  be  up  to  97 %  with  the  wells sometimes  intermittent  liquid production  (oil  and  water)  it  was necessary  to  provide  the  pumps with a liquid recirculation sys tem to  keep  the  pump  primed  at  all times even with long gas slugs co­ming  from  the  wells.  Each  of  the three  platforms  have  24  produ­cing wells tied to a common suc­tion manifold which in turn is con­nected to either five or six of the multiphase pumps, depending on the platform. The pumps operate in parallel driven by 1900 kW vari­able  frequency  controlled  electri­cal motors. The speed of the mo­tors  which  control  the  pumping 

Fig. 8: Arrangement of the multiphase pumps on the platform deck in 3D

12  Pumps, Compressors and Process Components 2012

capacity  and  thereby  the  pressure  in the  suction  manifold  are  automati­cally set to follow the desired optimal manifold pressure.

All  pumps  are  packaged  in  modules, including  controls  and  instrumenta­tion  per  the  customer  specification. The  packaging  was  done  on  the  U.S. Gulf  coast  and  each  pump  module was transported by barge to the plat­form  for  direct  installation.  In  addi­tion to the pumps (16  in total),  three so­called  E­buildings  were  supplied which contains all the frequency con­verters,  automation,  switchgear  and controls. The E­buildings are air condi­tioned although each platform is spe­cified for unmanned operation.

Pump operating data and design details

The twin screw multiphase pumps in­stalled on each of the three platforms are  designed  for  the  following  para­meters:

phase flow with a minimum efficiency of 97 percent. The separated  liquid  is collected in the reservoir of the liquid management  system.  A  recirculation 

cessary. The volume of the reservoir of the liquid management system is ad­apted to  the maximum expected gas slug length specified by the customer.

Before  dispatch  all  pumps  were subject  to  a  witnessed  hydrostatic test  and  a  witnessed  factory  accep­tance test.

The future

After  completion  and  successful  in­spection of  the skids and all accesso­ries, they have been loaded on barges for transport to their final destinations on the particular platforms. The instal­lation  at  the  three  platforms and  the commissioning is scheduled for 2012.

Fig. 9

The  pump  casing  is  manufactured from fabricated carbon steel with the suction port on side and the discharge port on top. All welds were subject to extensive quality control by means of non­destructive testing.

Both pump screws are made from carbon steel nitrided in order to mini­mize wear. The replaceable casing  in­sert is manufactured from cast carbon steel.

The  construction  materials  for  all wetted  and  pressurized  pump  parts meet  the  requirements  of  the  NACE Standard MR 0175 latest edition.

All pumps are equipped with a  li­quid  management  system  (recircula­tion system). These systems are  loca­ted on the skid downstream the pump discharge  but  within  the  skid  limits. During  the  pump  operation  liquid  is constantly  separated  from  the  multi­

Fig. 11: Bare shaft multiphase pump during testing

Fig. 12: Multiphase pump skids and electric building loaded on a barge

Engine driven multiphase produc­tion systems in the Gulf of Mexico

During  the  past  years  several  twin screw  multiphase  pumps  have  been installed on offshore platforms in the 

Fig. 10: P & ID of the liquid management system

volume  equal  to  approx.  3.0  percent of  the  actual  flow  rate  is  constant­ly  injected  from  the  liquid  reservoir into both suction areas of the pumps. During possible gas slugs this amount of  liquid  is  sufficient  to  ensure  con­tinuous  gas  compression.  The  large surface  of  the  liquid  reservoir  contri­butes  to  the  quick  heat  dissipation and  renders  additional  cooling  unne­

Pumps

Screw pumps

Pumps, Compressors and Process Components 2012  13

Authors: Jörg Narewski (Managing Director Leistritz Pumpen GmbH), Sven Olson President Leistritz Corporation), Hans Jürgen Schöner (Mana-ger Sales Multiphase Pumps & Systems Leistritz Pumpen GmbH), Dr. Michael Radke (Member of Executive Board Leistritz Aktiengesellschaft

US  territorial  of  the  Gulf  of  Me­xico.  Lack  of  electrical  power  on these  platforms  required  gas  en­gine driven equipment. 

fer  a  mixture  of  oil,  water  and gas  with  a  GVF  of  95 %  at  a  flow rate  of  991  m³/h  and  differential pressure of max. 44 bar to a cen­

Fig. 13: twin screw multiphase pump with 630 kW gas engine

Fig. 14: Oil production platform in the Gulf of Mexico

– Smaller footprints– Less equipment– Less weight– Lower capital costs– Quick amortization

Twin screw crude oil export pumps on a platform in the Gulf of Mexico

In 2012 four twin screw pumps as heavy crude oil export pumps will be installed on a production plat­form in the Mexican sector of the Gulf of Mexico. The units are gas turbine driven and pumping hea­vy  crude  oil  with  an  API  gravity of 15 at a flow rate of 1,070 m³/h and  56  bar  differential  pressure. The  oil  contains  a  small  amount of water, traces of H2S as well as chlorides.  Double  acting  mecha­nical  seals  in  the  suction  area  of the  pumps  and  a  seal  oil  supp­ly  system  in  accordance  with  API Plan  54  safeguard  an  operation without  pollution  of  the  atmos­phere.

Conclusion

Twin  screw  multiphase  pumps and  conventional  twin  screw pumps  are  successfully  used  off­

Fig. 15: Gas turbine driven twin screw pump

shore  in  the  Gulf  of  Mexico.  The increasing number of ageing and maturing  fields,  the  life  exten­sion  of  brown  fields  or  the  spe­cial characteristics of green fields will  generate  a  consistent  de­mand for multiphase technology. The  versatile  twin  screw  pumps have  a  large  operating  window and  handle  combinations  of  li­quid  and  gas  in  any  concentra­tion and cover a large range of vis­cosities  at  flow  rates  up  to  5,000 m³/h and differential pressures up to  150  bar.  Capital  and  operatio­nal expenditures are low for twin screw  multiphase  pump  installa­tions  and  they  contribute  to  the protection of our environment by eliminating  the  need  for  flaring and venting.An  independent  operator  in­

stalled an twin screw multiphase pump for a flow rate of 880 m³/h, a  differential  pressure  of  19  bar and a GVF of 97 %. The gas engine is  rated  at  630  kW.  A  liquid  ma­nagement  system  to  safeguard the  operation  during  gas  slugs  is an integrated part of the skid.

A large twin screw multiphase pump was installed by a major in­ternational  operator  on  another platform  in  the  Gulf  of  Mexi­co. The  unit  is  designed  to  trans­

tralized  separation  facility.  With the  liquid  management  system and the 1,500 kW gas engine this is  one  of  the  largest  multiphase pump skids ever built.

The  objectives  to  use  twin screw  multiphase  pumps  versus conventional  equipment  on  both applications were:

– The  reduction  of  the  well  pres­sure to increase the oil recovery

– The reduction of the gas lift re­quirements

Pumps

Screw pumps