Introduction
Aspen HYSYS is the dominant process simulation tool in upstream oil and gas, and the vast majority of simulation work is carried out in steady-state mode. Steady-state simulation is the right tool for equipment sizing, heat and material balance development, and FEED-level process design — it is fast, intuitive, and directly produces the outputs that equipment datasheets require.
But steady-state simulation has a fundamental limitation: it describes the process at a single, fixed operating point. It tells you what the process looks like when everything is behaving normally. It tells you nothing about what happens during startups, shutdowns, process upsets, emergency trips, or any other transient condition — and it is precisely those conditions that most frequently cause incidents, equipment damage, and production loss.
Dynamic simulation — HYSYS Dynamics mode — extends the model into the time domain. Conditions evolve from one moment to the next, driven by control loops, valve movements, equipment responses, and process disturbances. This article describes when dynamic simulation is the right tool, what it can and cannot do, and how to get useful results from it.
When Steady-State Is Not Enough
Emergency Shutdown and Depressuring
When an emergency shutdown (ESD) is initiated, every control valve and isolation valve moves to its fail-safe position within seconds. The resulting pressure transients — trapped volumes isolated between closed valves, high-pressure gas expanding through blowdown orifices, liquid surging through knockout drums — cannot be analysed with a steady-state model.
Dynamic simulation allows you to:
- Confirm that the blowdown system can reduce pressure from design pressure to 50% MAWP (or the target defined by API 521) within the required time
- Verify that the flare knockout drum does not liquid-flood during peak depressuring flow
- Check that relief valves do not open during normal ESD (indicating an undersized blowdown system)
- Confirm that the ESD valve sequencing (which valves close first, in what order, with what time delays) does not create hydraulic hammer or uncontrolled pressure spikes
The API 521 methodology for emergency depressuring is inherently dynamic — it defines pressure-time targets that require a time-domain model to verify.
Control Valve Sizing and Stability
A control valve sized correctly for steady-state design conditions may behave poorly at other points on its operating range — hunting at low flow, failing to open quickly enough to prevent a low-level trip, or causing pressure oscillations in the downstream system.
Dynamic simulation is used to:
- Tune PID controller parameters (Kp, Ki, Kd) against the actual process dynamics
- Verify that level controllers on separators have sufficient turndown before level alarms trip
- Check that pressure control valves respond fast enough to prevent overpressure during ramp-up scenarios
- Identify integrating process instabilities (common in large vessel liquid levels with poorly tuned controllers)
This is particularly important for gas compression systems, where the interaction between antisurge controllers, recycle valves, and inlet pressure control can create complex dynamic interactions that steady-state simulation cannot reveal.
Slug Flow and Riser Systems
Offshore pipelines and risers are subject to terrain-induced slugging — periodic accumulation and expulsion of liquid slugs caused by the pipeline profile. A slug arriving at the production separator can be significantly larger than the steady-state liquid holdup, and if the separator and its level control system cannot absorb and process it, the result is liquid carryover into the gas stream or a low-level trip shutting down the production train.
Dynamic simulation can:
- Model a representative slug arrival scenario (based on multiphase flow calculations from a tool such as OLGA or LedaFlow)
- Confirm that the separator volume is sufficient to buffer the slug without tripping on high level
- Verify the level controller tuning and check whether a feedforward controller on the liquid outlet valve provides better performance
- Size the slug catcher if one is required
Slug flow simulation requires integration between the pipeline multiphase model and the process simulation model. HYSYS Dynamics handles the process side; the pipeline model (typically external) provides the boundary conditions.
Startup Procedure Development and Validation
Bringing a new facility online is a high-risk operation. The sequence of valve opening, equipment startup, and process stream introduction must be correct — wrong sequencing can result in liquid hammer, compressor surge, or rapid overpressure.
Dynamic simulation allows the commissioning team to:
- Test startup procedures in simulation before the first hydrocarbon introduction
- Identify steps in the procedure where automatic shutdowns would trip (and refine the procedure to avoid them)
- Determine how long the facility takes to reach stable production conditions
- Train operators on the expected behaviour of the process during startup
This is particularly valuable for complex facilities — multi-train gas processing plants, floating production systems with marine motion effects, or facilities with long pipeline tie-in distances — where the interaction between systems during startup is non-obvious.
Compressor Train Dynamics
Gas compressor trains are among the most dynamically sensitive systems in an oil and gas facility. The interaction between the process gas conditions, the antisurge system, and the driver (gas turbine or electric motor) creates a coupled dynamic system that can behave very differently from its steady-state design point.
Dynamic simulation is used to:
- Verify antisurge controller performance across the full operating range
- Confirm that the recycle valve size and Cv are adequate for the worst-case surge avoidance requirement
- Check the time-to-surge under ESD conditions (particularly important for motor-driven compressors where the motor trips faster than the antisurge valve can open)
- Model the effect of molecular weight variations on compressor performance and surge margin
What Dynamic Simulation Cannot Do
It is important to be clear about the limitations:
It is not a substitute for multiphase flow modelling. HYSYS Dynamics models single-phase or simplified two-phase systems adequately. For detailed pipeline multiphase hydraulics — severe riser slugging, wax and hydrate deposition, terrain locking — a dedicated multiphase flow tool is required.
It requires validated steady-state models as a starting point. A dynamic model built on incorrect steady-state thermodynamics will give incorrect dynamic results. The steady-state model must be validated against design data or operating data before the dynamic model is developed.
Controller tuning from simulation needs field validation. PID parameters determined from dynamic simulation are starting points, not final values. Real process equipment — control valves with mechanical friction, sensors with measurement noise and lag — behaves differently from ideal simulation models. Field tuning during commissioning is always required.
It is slow. Dynamic simulation runs in real-time or slower, depending on model complexity. A one-hour startup simulation may take four to eight hours to complete, even on modern hardware. This makes dynamic simulation unsuitable for fast parametric studies; that work belongs in steady-state mode.
Practical Approach: Building a Useful Dynamic Model
Start with a converged steady-state model
A dynamic HYSYS model is built by converting a steady-state model. Ensure the steady-state model is fully converged, all equipment specifications are complete, and the thermodynamic package is validated before making the conversion. Trying to build the dynamic model while the steady-state model is still in development is a reliable way to create a model that produces misleading results.
Add valve sizing and controller parameters carefully
In steady-state mode, valves are specified by pressure drop. In dynamic mode, they are specified by Cv (or Kv) and position. Every control valve and block valve in the model needs a Cv that matches the actual valve datasheet. Controller parameters need to be appropriate — start with conservative tuning (low gain, long integral time) and tighten from there.
Use meaningful events for scenario testing
Define each scenario as a sequence of events — a valve closing, an ESD signal, a step change in feed conditions — with realistic timing. Document the scenario so results are reproducible and can be reviewed by others.
Deliver the model, not just the report
A dynamic simulation model that is handed over to the client with the final report allows the operator's engineers to run additional scenarios during detailed design, commissioning planning, and operations. A report without the model is a one-time answer to a fixed question. Deliver the model.
Conclusion
Dynamic simulation is not a replacement for steady-state modelling — it is a complementary tool that answers a different set of questions. If your project involves complex ESD and depressuring scenarios, large or multi-train compressor systems, slug-prone pipeline tie-ins, or a commissioning team that needs to rehearse startup procedures before first hydrocarbon, dynamic simulation pays for itself many times over in avoided commissioning incidents and production delays. Used at the right stage and for the right questions, HYSYS Dynamics is one of the most powerful tools available to the process engineer.