Performing engineering studies with dynamic simulation is becoming a critical requirement for new liquefied natural gas (LNG) plants. Typically, the major target for such a study is the investigation and review of critical plant design elements, such as compressors, flares and boiler systems. Dynamic engineering studies can identify design changes that will significantly improve plant performance and the safety and reliability of plant operations. Furthermore, if such design changes are identified early, they can be implemented at a low cost and provide significant savings during a plants lifetime.13
LNG plant design is based on steady-state process simulation. This approach typically does not take into account rotating equipment characteristics, holdups or actual pressure drops. However, the use of a dynamic process simulator is required to understand actual plant transients and dynamics, to examine and verify control schemes, and to review plant procedures. The dynamic model allows for the calculation of process variables as a function of time (i.e., as a movie instead of a series of snapshots). Moreover, it is possible to examine process upsets, including process startups and shutdownsa critical functionality not offered by the steady-state simulator.
This article presents experiences obtained from a detailed engineering study for Sonatrachs GNL3Z project, a grassroots baseload LNG production plant in Algeria. A brief description of the LNG production process is provided, along with a number of test cases demonstrating the design challenges faced and their resolutions. Lessons learned during this study with regard to project execution and project management are offered, and the benefits obtained through engineering studies are described.
Systems examined and simulation test cases
All compressor systems in the GNL3Z plant have been examined, and a series of simulations has been performed to verify the process design. In this section, the approach used in engineering studies and in the overall liquefaction process is described. Next, details are provided on the compressor systems and on the test cases reviewed.
Engineering study approach. A dynamic engineering study involves a number of steps before delivery of the final results and recommendations, as outlined below:
- Exact scope definition, which includes the modeling boundaries and the test cases to be examined
- Careful data collection and reduction to a form usable by the simulator; these data should be easily accessible, reusable and verifiable, which, in practice, means maintaining specific project folders and spreadsheets and ensuring version control
- Calculation of pipework volume and resistance to flow through analysis of isometric drawings
- Model building as per piping and instrumentation diagrams (P&IDs) and agreed scope, incorporating plant logic (e.g., cause-and-effect charts) and isometrics; elements typically included are:
a. Compressor system (gas/steam turbines, electric motors, heat exchangers, vessels, etc.)
b. Compressor control, including the anti-surge, hot-gas/cold-gas bypass and bleed valves
c. Regulatory control, emergency shutdown logic and related valves
d. Startup and normal shutdown procedures.
- Integration of sub-models and preparation of the final model, aligned at the agreed heat and material balance (H&MB)
- A first report to the final user clarifying data and general approaches used, comparing the model with the agreed H&MB, and supporting the model review
- Model update, which can include client comments, incorporation and review of data, preparation of repeatable scenarios for test runs, defining test run duration and specific integration time steps, etc.
- Execution of the test run, which involves collecting data, graphically reviewing the results (i.e., process variable as a function of time) for easy analysis, and discussing first results with the client
- Delivery of the individual test run and reporting of observations, conclusions and possible recommendations
- Delivery of the final report, including all individual run reports and final recommendations.
LNG production process description. Natural gas is first compressed in a feed gas compressor and then sent to the mercury-removal unit. The gas is further treated in an acid gas absorber to remove CO2 and H2S, if present. The sweet gas is dried in molecular sieve beds and further processed in the natural gas liquids (NGL) recovery unit to remove the C2+ compounds. The chilling requirement for the treatment is supplied by auxiliary propane (C3) refrigeration.46
The treated natural gas is then compressed in a residue gas compressor and cooled by C3 chillers. Afterward, it is fed to the bottom of the main cryogenic heat exchanger (MCHE), where it is liquefied by mixed refrigerant (MR). This is the typical liquefaction technology used in projects based on a propane/MR process. The LNG produced is sent to the helium recovery and nitrogen-stripping sections. The nitrogen- and helium-free LNG is then sent to the storage tanks. The boil-off gas (BOG) from storage is recompressed and sent to fuel gas. The gas coming from the helium-recovery and nitrogen-stripping units is compressed in the end-flash gas compressors and sent to the fuel gas header.
The following compressor systems are considered in the current dynamic simulation scope:
- Feed gas compressor
- Residue gas compressor and turboexpander
- Auxiliary propane compressor
- Refrigerant (C3/MR) compressors
- End-flash gas compressors
- Boil-off gas compressors.
Several test cases are reviewed in the following sections.
Feed gas compressor startup. The natural gas is fed to the feed gas compressor suction knockout drum to remove any entrained or condensed liquids. The overhead vapor is directed to the feed gas compressor. The compressed gas is cooled and then directed to other LNG plant units for further treatment. A simplified process diagram is shown in Fig. 1.
Fig. 1. Feed gas compressor system
simplified process flow diagram.
During startup, the feed gas compressor pressurizes other downstream units to the residue gas compressor. The volume of the downstream system is very large (about 3,000 m3). Thus, during a typical startup, when the downstream pressure is low, our simulations show that the compressor would operate in the stonewall region until the downstream system is pressurized, as shown in Fig. 2 (red line).
Fig. 2. Compressor maps for the feed gas
compressor system, with the movements of
the operating point during compressor
To avoid operating at high compressor flows, it was decided to isolate the compressor from the downstream system during startup. A valve bypassing the discharge on/off valve and controlling the downstream pressure was used to smoothly pressurize the downstream system. Results are shown in Fig. 2 (blue line). In this case, the anti-surge controller was allowed to control pressure while increasing the compressor speed from minimum to operating speed.
A further refinement to the procedure was implemented to allow the anti-surge system to control when the compressor reached the minimum speed. Results are shown in Fig. 2 (green line). The compressor startup procedure was optimized, achieving smooth system pressurization while the compressor was protected from surge.
End-flash gas system: Motor trip case. Nearly 85% of the fuel gas is made by the end-flash gas coming from the helium-recovery drum overheads and the nitrogen-stripping column overheads. Before combining together, both streams pass through the end-flash gas exchanger for heat recovery. The combined stream is then compressed to about 32 bara. There are two compressor trains with three stages each. A simplified process diagram is shown in Fig. 3.
Fig. 3. End-flash gas compressor simplified
process flow diagram.
The performance of the anti-surge system during motor trip was studied extensively. It has been observed that the first-stage compressor was surging upon trip (Fig. 4, blue line). To solve this problem, two options were examined to improve the system design:
1. Use a check valve at suction (Fig. 4, green line)
2. Use a hot-gas bypass (HGB) valve (Fig. 4, cyan line).
Fig. 4. First-stage fuel gas compressor map
during motor trip.
Both options appear valid. However, the results demonstrate that, if an HGB valve is used, high temperatures occur at the compressor discharge. This can be detrimental to the compressor seals. Based on these findings, it was recommended to modify the design and to use a check valve at the compressor suction.
Liquefaction section: Refrigeration compressor cases. Refrigeration for this process is provided by two major systems: the C3 system and the MR system, containing nitrogen, methane, ethane and propane. The low-pressure MR is compressed in three stages of MR compressors. The high-pressure vapor from the discharge of the compression train is cooled and partially condensed by four C3 chillers in series. The partially condensed MR is fed to a high-pressure separator, where heavy MR and light MR are separated before entering the MCHE.
The C3 refrigeration system utilizes C3 evaporating at four pressure levels to supply refrigeration to the natural gas feed and to the MR circuit. The chilled natural gas feed enters the MCHE, where it is liquefied.
Case 1: Verification of compressor valve size. One of the objectives of the simulations performed was to verify the sizing of the compressor anti-surge valve. For the majority of the runs performed, it was noted that, for the low-pressure C3 compressor anti-surge valve, a valve size (calculated as CV) of 1,600 was adequate to protect from the risk of damage due to surge. However, while performing startup of the C3 system, the valve size proved to be insufficient. Results of the test cases examined are presented in the following figures.
Fig. 5 shows a map of the C3 low-pressure (first-stage) compressor with anti-surge valve CVs of 1,600 (red line) and 2,600 (blue line), while starting up the system with propane. Using a CV of 1,600, the operating point appears to be in the surge region for a significant part of its path. The higher CV (2,600) is needed to start the compressor safely, with normal composition and pressure of 1.5 bara.
Fig. 5. First-stage C3 compressor map
during startup with propane (Liquefaction
Case 1). The pink line shows the compressor
curves at the highest and lowest speed and
the surge line.
However, when starting up the C3 system with defrost gas or N2, the low-pressure C3 anti-surge control valve requires an even higher CV to protect from surge. A CV of 3,000 is needed to start the compressor safely with N2 or defrost gas, together with a starting pressure increase to 1.52.5 bara. The case of defrost gas is shown in Fig. 6.
Fig. 6. First-stage C3 compressor map
during startup with defrost gas (Liquefaction
Case 1). The pink line shows the
compressor curve at the nominal speed and
the surge line. The blue, green and pink lines
demonstrate the operating point using
different CVs. A high CV is required for safe
operation when defrost gas is used.
Compare with Fig. 5.
To start the compressor with different compositions (C3, N2 and defrost gas), a semi-automated approach was developed, based on the aforementioned results. The plant operator knows the current operation case (e.g., startup with defrost or propane gas) and presses the appropriate button on the distributed control system. The operator action ensures passing the necessary information to the compressor control system. The compressor control then adjusts the value of the first-stage C3 compressor anti-surge valve maximum opening/CV through a software clamp, and it also defines the startup pressures and permissives required.
The different approaches used in valve design by vendors are worth mentioning. The typical valve design is based on a number of distinct/discreet operation cases at steady-state conditions. Each case may be based on different gas conditions at the suction and discharge of the valve (e.g., gas temperature, pressure, density, etc.).
With dynamic simulation, it is possible to explore a continuous set of plant conditions. For example, during a simulated compressor startup, all plant conditions from the settle out/trip to the final steady-state conditions are visited. These include intermediate conditions with increasing compressor speed, varying suction/discharge compressor temperatures, ΔP across the anti-surge valve, etc.
In brief, using dynamic simulation, the engineering study team was able to evaluate design modifications and to capture cases where the original valve design was not satisfactory for startup.
Case 2: MR compressor HGB valve design. Another objective of this dynamic simulation study for the low-pressure MR compressor was to confirm the requirement for an HGB valve and its stroking time. Note that the low-pressure MR compressor is provided with a bleed valve and a discharge pressure control valve to flare.
A number of sensitivity test runs were conducted. It was determined that the bleed valve opening time could be reduced to 1 sec. Furthermore, it was shown that, by opening the bleed valve quickly and the discharge valve to flare relatively slowly, the HGB valve size could be reduced, or the valve itself could be removed. Results are demonstrated in Fig. 7. To compensate for any uncertainties, it was decided to maintain the original design (i.e., maintain the HGB valve with a small size).
Fig. 7. Low-pressure MR compressor map
upon MR compressor trip (Liquefaction Case
2). The pink line shows the compressor
curves at the highest and lowest inlet guide
vane positions and the surge line.
Case 3: Valve opening on compressor trip. As soon as the C3/HP MR compressor driver was tripped, the compressor speed decreased, as expected. However, the simulation run showed a pronounced effect on the MR compressors. The first stage of MR compressor was entering the surge region upon C3 compressor trip.
A series of simulation test cases revealed that, to protect the MR compressor from surge upon C3 compressor trip, both the low- and medium-pressure MR compressor anti-surge valves must open shortly after the C3 trip, following a feed-forward signal. The performance map of the low-pressure MR compressor with the aforementioned feed-forward signal to the low-pressure MR anti-surge valve can be seen in Fig. 8.
Fig. 8. Low-pressure MR compressor map
on C3 compressor trip (Liquefaction Case 3).
Lessons learned and execution methodology
It is challenging to perform a meaningful dynamic simulation study. To ensure useful results, some principles must be understood and basic guidelines must be followed:
- An engineering study is a dynamic process. It requires continuous interaction between the EPC contractor, the dynamic simulation provider, the equipment vendors and the client at all levels (managerial, technical, etc.). Excellent cooperation must be achieved to turn the study into important findings and appropriate process design changes.
- Small teams of engineers with good process understanding and the ability to cooperate will drive the project to success. The team should remain the same throughout the project execution.
- The plant data used for the study must be accurate and consistent. After some point, data changes should be minimized to have consistent and comparable results, while avoiding a series of costly sensitivity-simulation runs.
- For all test cases, detailed procedures are required and should be communicated in writing. This step ensures common understanding, targets and methodology.
- The simulation models must be of high fidelity. Special attention should be devoted to all critical parameters that could possibly invalidate the study results (isometrics, valve sizes and timings, compressor and driver inertia, compressor curves, etc.).
- Models should be approved before proceeding to results generation. The following should be reviewed and accepted:
a. Model topology and match with P&IDs
b. Main data used
c. Match with heat and mass balance
e. Compressor curves and inertia values used.
- After the simulation models are prepared, it is critical to debug the models by running a series of test cases. These can reveal model weaknesses. At the same time, early findings can be communicated so that the direction of investigation can be set.
From a project management point of view, a dynamic simulation study follows a relatively standard schedule that can be changed according to project-specific needs and requirements. The main elements are:
- The kickoff meeting is where the studys targets are confirmed and data are provided.
- When the model is prepared, a report describing the match of the models to the data and the heat and mass balance is delivered. The model validation test follows. The delivered models and associated reports are examined to ensure the match with the actual plant design. This model review can be an offline activity. However, face-to-face meetings are advisable to improve the communication of findings. Such meetings present ideal settings to finalize the procedure for every simulation test case.
- After the model review meeting(s) are conducted, the dynamic models are updated and retested, the individual cases are examined, and the individual run reports are delivered. Early findings are communicated, and the direction of investigation is discussed.
- A number of review meetings should be arranged to closely monitor the project execution and to discuss important findings or challenges that must be addressed to avoid delays and project risks.
- The dynamic study completion is accompanied by the appropriate deliverables, including the final project report. This report includes:
a. An executive summary
b. The study objectives
c. Model scope and description
d. Stream comparison
e. Individual studies and results.
The work performed for the GNL3Z project illustrates that dynamic simulation can be an excellent tool for the support and verification of process design during the engineering, procurement and plant construction (EPC) stages. The advantages of dynamic simulation are becoming more important and valuable, as evidenced by the fact that dynamic simulation is increasingly included as a fundamental engineering step. Experience has revealed several attractive attributes of dynamic simulation:
- A dynamic simulation study is a useful tool for process verification and optimization of operating procedures, along with control and protection systems. Through critical results analysis and comparison with project constraints, it is possible to improve the quality of process design. In turn, the early implementation of improved solutions reduces total project cost.
- Dynamic simulation can reproduce the behavior of a real plant and offer process insight that cannot be obtained with traditional steady-state simulators. Using dynamic models and studies through all of the engineering phases improves knowledge of process dynamics. This understanding is fundamental during plant startup and for the identification and resolution of process bottlenecks.
- The evaluation of sequences and procedures involving rotating equipment and important packages during the engineering stage increases process controllability and reliability. Furthermore, the machine/package vendor can verify and implement suggested improvements in control functionality.
- By maintaining the dynamic model (i.e., ensuring that it tracks the adjustments applied onsite during the commissioning and startup phases), it is possible to extend the life and value of the simulation well beyond the engineering stage. Any update/improvement of the control schemes, the creation of what-if scenarios, etc., can be easily implemented using the simulation study model.
In the current project, the use of dynamic simulation delivered significant estimated value: around 1% of capital expenditure ($30$60 million) at plant commissioning, taking into account the minimization of reworks and the avoidance of prolonged commissioning activities. Increased process reliability is estimated to yield around $10 million per year. To capture these benefits and boost confidence in the plant design, it is critical to have an appropriate project execution strategy that includes:
- A recognized and validated modeling tool
- An experienced project team with a significant track record
- An accurately defined scope of work and a flexible project execution plan
- Maturity of design and availability of actual equipment data during the early construction stage, which allows for the timely implementation of study findings into the actual plant.
Failing to take into account the aforementioned measures significantly increases the risk that potential benefits will not be realized. HP
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3 Panigrahy, P., J. Balmer, M. A. Alos, M. Brodkorb, B. Marshall, Dynamics break the bottleneck, Hydrocarbon Engineering, Vol. 16 (9), September 2011.
4 Price, B. C., Small-scale LNG facility development, Hydrocarbon Processing, January 2003.
5 Foglietta, J. H., Consider dual independent expander refrigeration for LNG production, Hydrocarbon Processing, January 2004.
6 Harrold, D., Design a turnkey floating LNG facility, Hydrocarbon Processing, July 2004.
|The authors |
Anton Marco Fantolini is the LNG technology manager at Saipem. He holds a degree in chemical engineering and has 14 years of experience in the oil and gas industry, with concentrations in LNG plant design and development during the conceptual, FEED and EPC phases.
||Luigi Pedone is a lead control engineer with Saipems automation and control department. He holds a degree in chemical engineering and has 12 years of experience in the oil and gas industry. His areas of focus include dynamic and operator training simulators, management information systems, and advanced process control.|
||Luca DOrazi is a process engineer with Saipem. He holds a degree in chemical engineering and has six years of experience in the oil and gas industry, with a concentration in plant simulation.|
Ramona Prodan is a technical engineer specializing in control systems and automation. She has more than five years of professional experience in the oil and gas industry. She holds a BS degree in chemical engineering from the Petroleum Gas University of Ploiesti, Romania.
Ankush Sood is a team leader with Hyperion Systems Engineering in Pune, India. He has seven years of experience in the oil and gas industry, including five years of delivering engineering studies for centrifugal compressors and developing high-fidelity operator training simulators. He holds a BSc degree in chemical engineering from Punjab Technical University.
Girish Bhattad is a team leader with Hyperion Systems Engineering in Pune, India. He has eight years of experience in the oil and gas industry, including four years of developing high-fidelity operator training simulators and delivering engineering studies. He holds an MTech degree in process engineering and design from The Indian Institute of Technology Delhi.
Dionysios Stavrakas is a senior engineer at Hyperion Systems Engineering in Athens, Greece. He has five years of experience in delivering high-fidelity engineering studies and developing operator training simulators. He holds an MSc degree in chemical engineering and energy production and management from The National Technical University of Athens.
Vassilis Harismiadis is the real-time optimization and training simulation manager at Hyperion Systems Engineering. He has 12 years of experience in the oil and gas industry, with a focus on using dynamic process modeling to improve plant efficiency. Dr. Harismiadis holds a PhD in thermodynamic modeling of complex systems from NTU Athens.