1
Implementation of Coordinator
MPC on a Large-Scale Gas Plant
Elvira Marie B. Aske*&, Stig Strand& and Sigurd Skogestad*
*Department of Chemical Engineering
Norwegian University of Science and Technology (NTNU)
Trondheim, Norway
&StatoilHydro R&D, Process Control, Trondheim, Norway
AIChE Annual Meeting, Philadelphia, USA
November, 2008
[email protected]
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Outline
• Introduction and motivation
–
The Kårstø gas plant
• Maximum throughput as optimal operation
• Approach: Coordinator MPC*:
–
–
Maximize flow through linear network
Estimate feasible remaining capacity (R) in units using local MPCs
• Application to Kårstø Gas Plant
– Previous work*: Works well on simulations
– Here: Actual implementation
• Design
• Tuning (plant runs)
• Experiences
• Conclusion
*Aske, E.M.B., S. Strand and S. Skogestad (2008). Coordinator MPC for maximizing plant throughput.
Comput. Chem. Eng. 32(1-2), 195–204.
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Kårstø plant
Control
room
Gas processing
area
4
Snøhvit
Melkøya
North Sea gas network
Norwegian
continental
shelf
• Kårstø plant:
Receives gas from
more than 30
offshore fields
Norne
Åsgard Heidrun
Kristin
TRONDHEIM
Ormen Lange
Statfjord
Tjeldbergodden
Nyhamna
Troll
Frigg
• Limited capacity at
Kårstø may limit
offshore production
(both oil and gas)
Haltenpipe
ÅTS
Kollsnes
Vesterled
Sleipner
Kårstø
Oslo
St Fergus
Europipe II
Ekofisk
UK
Europipe I
Langeled
Zeepipe I
Norpipe
Franpipe
Easington
Emden
Zeebrugge
Dunkerque
GERMANY
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Kårstø plant – 20 years of development
Europipe II
sales gas
Halten/
Nordland rich
gas
Tampen
rich gas
Statpipe
sales gas
Sleipner
condensate
Propane
How manipulate feeds and crossovers?
N-butane
I-butane
Condensate
1985
1993
2003
2000
2005
Naphtha
Ethane
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Maximum throughput
• Often: Economic optimal operation = maximum throughput
– Operate with max feasible flow through bottlenecks
– No remaining unconstrained DOFs (RTO not needed)
“Coordinator MPC”:
• Manipulate TPMs (feed valves and crossovers) presently
used by operators
• Throughput determined at plant-wide level (not by one
single unit)
 coordination required
• Frequent changes  dynamic model for optimization
TPM = Throughput Manipulator
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?
Approach
•
Objective: Max throughput, subject to feasible operation:
– Remaining capacity (R) = Rs = 0 in bottleneck units
– Throughput manipulators (TPMs): Feeds and crossovers
•
Approach: Use Coordinator MPC to optimally adjust TPMs:
– Coordinates the network flows to the local MPC applications
– Decompose the problem (decentralized).
• Assume Local MPCs closed when running Coordinator MPC
– Need flow network model (No need for a detailed model of the entire plant)
• Decoupling: Treat TPMs as DVs in Local MPCs
• Use local MPCs to estimate feasible remaining capacity (R) in each unit
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”Coordinator MPC”: Coordinates network flows, not MPCs
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Remaining capacity (using local MPCs)
•
Feasible remaining feed capacity for unit k:
current feed to unit k
max feed to unit k within feasible operation
•
Obtained by solving “extra” steady-state LP problem in each local
MPC:
subject to already given present state, model equations and constraints
•
Very little extra effort!
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Local MPC applications
• Kårstø: Most local MPC applications are on twoproduct distillation columns:
– CVs: Distillate- and bottom products quality (estimated)
+ differential pressure and other constraints
– MVs: Temperature setpoint (boilup) and reflux flow
– DV (disturbance): Feed flow
• New: Local MPCs estimate their feasible remaining
capacity (R)
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Coordinator MPC
Objective: Maximize plant throughput, subject to
achieving feasible operation
• MVs: TPMs (feeds and crossovers that affect several units)
• CVs: total plant feed + constraints:
– Constraints (R > backoff > 0, etc.) at highest priority level
– Objective function: Total plant feed as CV with high, unreachable set point with lower
priority
• DVs: feed composition changes, disturbance flows
• Model: step-response models obtained from
– Calculated steady-state gains (from feed composition)
– Plant tests (dynamic)
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Export gas
KÅRSTØ
MPC COORDINATOR IMPLEMENTATION (2008)
Rich gas
CV
MV
CV
Export gas
MV
Rich gas
CV
CV
CV
CV
CV
CV
CV
CV
MV
Half of the
plant included:
MV
Condensate
MV
CV
CV
CV
MV
CV
CV
CV
CV
CV
6 MVs
22 CVs
7 DVs
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Step response models in coodinator MPC
Remaining capacity (R) goes down
when feed increases…
+ more…
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Coordinator MPC in closed loop
• Test runs January to April 2008
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Export
gas
TEST 07 FEB
2008
Rich gas
CV
MV
CV
MV1 CV3
Rich gas
CV1
Export gas
CV
CV
CV
CV2
DV
CV
MV2
MV
Condensate
MV
CV
CV
CV
MV
CV
CV
CV
CV
CV
CV
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TEST 07 FEB 2008
MV1
MV2
CV1
CV2
t = 0 min: Turn on
t = 250 - 320 min: Change model gains (tuning)
t = 500 min: Adjust back-off for R in demethanizer
t = 580 – 600 min: Feed composition change (DV)
CV3
DV
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Experiences
• Using local MPCs to estimate feasible remaining capacity leads
to a plant-wide application with “reasonable” size
• The estimate remaining capacity relies on
– accuracy of the steady-state models
– correct and reasonable CV and MV constraints
– use of gain scheduling to cope with larger nonlinearities
→ Crucial to inspect the models and tuning of the local
applications in a systematic manner
• Requires follow-up work and extensive training of operators and
operator managers
– “New way of thinking”
– New operator handle instead of feedrate: Rs (back-off)
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Conclusions
• Frequent changes in feed composition, pipeline
pressures and other disturbances require a dynamic
model for optimization
• Coordinator MPC is promising tool for implementing
maximum throughput at the Kårstø gas plant.
• More focus among operator personnel on
– capacity of each unit
– Plant-wide perspective to decide the plant- and crossover flows
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Acknowledgements
•
•
•
StatoilHydro and Gassco
Kjetil Meyer, Roar Sørensen
Operating managers and personnel at the Statpipe and Sleipner trains.
References
•
•
Aske, E.M.B., S. Strand and S. Skogestad (2008). Coordinator MPC for
maximizing plant throughput. Comput. Chem. Eng. 32(1-2), 195–204.
Full paper: E. Aske, E. Ph.D. thesis, NTNU, Trondheim, Norway, 2009
(Chapter 6). Available from the home page of S. Skogestad:
http://www.nt.ntnu.no/users/skoge/publications/thesis/2009_aske/
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21
COORDINATOR
IN CLOSED LOOP
DATE=?
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Export gas
DATE=?
Rich gas
CV
MV
CV
Export gas
MV
Rich gas
CV
CV
CV
CV
CV
CV
CV
MV
MV
Condensate
MV
CV
CV
CV
MV
CV
CV
CV
CV
CV
CV
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DATE=?
CV: Pipeline pressure
MV: Feed
New constraint
from pipeline
network operators
CV: Remaining capacity
MV: Crossover
Increase
backoff
9 hrs
6 hrs
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COORDINATOR
IN CLOSED LOOP
07 FEB 2008
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07 FEB 2008
CV: Pipeline pressure
MV: Feed
9 hrs
CV:
Remaining
capacity
Model adjustment
6 hrs
MV: Crossover
Composition
disturbance
DV: Feed composition