1. 1 1 Subsea process systems engineering Integrated and new processes for separation (vapor pressure and dewpoint control) and water handling, with emphasis on producing cleaner water and dryer gas Design of control systems, including multiphase flow systems and compressors Performance monitoring for fault detection, reliable operation and shorter shutdown periods Fault detection and safety systems for operation of large subsea systems

2. 2 2 Want to closely monitor the process –Flows, pressures, temperatures –Pressure drop in equipment –Efficiency for compressor, separator –UA for heat exchanger Traditionally in process industry: “Data reconciliation” –Need to wait for steady-state Common in control: Kalman filter / Observer –Dynamic, so no need to wait –Need extension to apply for process monitoring Performance monitoring

3. 3 3 How to best implement close-to-optimal operation using a simple an robust control structure Optimal operation (active constraints) Self-optimizing CVs – Example: Get optimal split in compact separator Operation and control

4. 4 4 Subsea gas-liquid separation: Åsgard Hodne (2012)

5. 5 5 Marlim subsea separation (Brasil) A – Multiphase desander B – Harp C – Tubular separator D – Outlet vessel E – Water desander F – Hydrocyclone G – Pump H – OIW monitor I – Ejector I Strong interactions between different process components Stiff system dynamics due to small hold-ups and low GOR Pressure drops of inlet cyclonic equipment need to be balanced Constraints on valve opening/closing speed Instrumentation is limited compared to top-side

6. 6 6 Process Control: Marlim SSAO Level controller Pereira et al. (2012) Harp Pipe separator Outlet Vessel Quick response is required (3 sec) To avoid constant speed changes a dead band is used Filtration of input can be used

7. 7 7 Process Control: Marlim SSAO Two pump flow rate controllers (Min/Max flow) Pereira et al. (2012) To keep the operating point inside the pump envelope Min opens when flow is less than minimum Max closes when flow is more than maximum

8. 8 8 Process Control: Marlim SSAO Multiphase choke valve-DP controller Pereira et al. (2012) To maintain stable backpressure for the rejects from desnader and hydrocyclone Asymmetrical dead band is applied to reduce choke movements

9. 9 9 Process Control: Marlim SSAO Two hydrocyclone controllers Pereira et al. (2012) To keep the reject rate between 2-6%

10. 10 Sand removal Flow assurance e.g. Hydrates (MEG injection, depressurizing) Secure foundation & Leveling of subsea separators Durability & Low maintenance Modularization Power distribution Compactness for deep-water applications Laboratory test (Technology Qualification Program) Summary Main challenges in design

11. 11 Simpler (compared to topside control systems ) Strong interactions between control loops Redundancy Safety & Emergency Shutdown (ESD) Fast dynamics of compact separators Data transfer to topside –Comes-on-power (slow, suitable for old bulky systems) –Fiber optic communication (fast, high rate, long distance) Modeling and dynamic simulations Controller tuning Wear and tear of control valves (constraint on speed of valves) Slugging flow and well-test Summary Main challenges in process control Strong interaction + constraint handling + economic operation  MPC

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