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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

skoge@ntnu.no

AIChE Annual Meeting, Philadelphia, USANovember, 2008

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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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Gas processingareaControl

room

Kårstø plant

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Nyhamna

Europipe II

Europipe I

Norpipe

Emden

ÅTS

Norne

Åsgard

Haltenpipe

Heidrun

Franpipe

Zeebrugge

Zeepipe I

St Fergus

Vesterled

Frigg

Statfjord

Kårstø

Kollsnes

Melkøya

Snøhvit

Ormen Lange

Easington

Langeled

Ekofisk

Sleipner

Troll

Dunkerque

Kristin

Tjeldbergodden

North Sea gas network

• Kårstø plant: Receives gas from more than 30 offshore fields

• Limited capacity at Kårstø may limit offshore production (both oil and gas)

Norwegiancontinental shelf

Oslo

UK

GERMANY

TRONDHEIM

5 Kårstø plant – 20 years of development

1985

2005

20001993

2003

How manipulate feeds and crossovers?

Europipe IIsales gas

Statpipesales gas

Propane

N-butane

I-butane

Naphtha

Ethane

Condensate

Sleipnercondensate

Tampen rich gas

Halten/Nordland rich gas

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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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• Feasible remaining feed capacity for unit k:

• 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!

Remaining capacity (using local MPCs)

current feed to unit k

max feed to unit k within feasible operation

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Local MPC applications

• Kårstø: Most local MPC applications are on two-product 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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MV

CV

Half of the plant included:

6 MVs22 CVs7 DVs

Rich gas

Rich gas

Condensate

Export gas

Export gas

CV CV CV CV

CV CV

CV

CV

CV

CV CV

CV

CV

CV

CV

CV

CV

MV

MV

MV

MV

MV

KÅRSTØ MPC COORDINATOR IMPLEMENTATION (2008)

13Step response models in coodinator MPC

+ more…

Remaining capacity (R) goes down when feed increases…

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Coordinator MPC in closed loop

• Test runs January to April 2008

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MV

CV3

Rich gas

Rich gas

Condensate

Export gas

Export gas

CV CV CV CV2

CV CV

CV

CV1

CV

CV CV

CV

CV

CV

CV

CV

CV

MV1

MV

MV2

MV

MV

DV

TEST 07 FEB 2008

16 TEST 07 FEB 2008

MV1

DVMV2

CV1

CV2

CV3

t = 0 min: Turn ont = 250 - 320 min: Change model gains (tuning)t = 500 min: Adjust back-off for R in demethanizert = 580 – 600 min: Feed composition change (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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COORDINATORIN CLOSED LOOPDATE=?

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MV

CV

Rich gas

Rich gas

Condensate

Export gas

Export gas

CV CV CV CV

CV CV

CV

CV

CV

CV CV

CV

CV

CV

CV

CV

CV

MV

MV

MV

MV

MV

DATE=?

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9 hrs 6 hrs

CV: Pipeline pressure MV: Feed

CV: Remaining capacityMV: Crossover

Increasebackoff

New constraintfrom pipelinenetwork operators

DATE=?

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COORDINATORIN CLOSED LOOP07 FEB 2008

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CV: Pipeline pressure MV: Feed

CV: Remainingcapacity

MV: Crossover

9 hrs 6 hrs

DV: Feed composition

Model adjustment

Compositiondisturbance

07 FEB 2008