Pipeline Engineering Studies Using Dynamic Simulation

Designing pipeline systems requires consideration of a number of factors, including the distance to be travelled, terrain profiles, the expected volumes to be received and delivered over the pipeline, and the physical properties of the products to flow through the pipe.

Dynamic simulation of pipelines provides essential information on the development of liquid holdup profiles as well as pressure and temperature profiles. This information is used to determine expected pressure drops, required pigging frequency, and slug sizes. There are commercial software tools that provide the functionality to perform the required engineering calculations; Aspen Upstream Hydraulics is an application, which provides such capabilities inside the Aspen HYSYS environment. Aspen Hydraulics allows steady-state modelling of pipe networks, as well as transient analysis of multiphase flows, features not available in the HYSYS pipe segment. This article provides tips to create a pipeline model in Aspen Upstream Hydraulics and for running the model in Dynamics mode; we assume familiarity with the HYSYS environment. 

Setting up the model

When using HYSYS Hydraulics (steady-state or dynamic mode), the pipeline is configured inside of the “Aspen Hydraulics Sub-Flowsheet”. The Hydraulics Subflowsheet is different from standard HYSYS subflowsheets in that all the relevant information is contained within tabs in the subflowsheet view. The following unit operations can be added within the Hydraulics Subflowsheet: 

• Valve 
• Pipe 
• Swage 
• T-Junction Mixer 
• T-Junction Splitter  

The Swage is required when connecting pipes of different diameters. The Aspen Hydraulics solver will not solve if pipes of different diameters are connected without using a Swage. 

In addition, all streams connected to a Mixer or Splitter must be linked to pipes of the same diameter. The elevation profile, pipe diameter, length and heat transfer characteristics can be defined using the pipe interface in the Hydraulics subflowsheet.

When solving the Hydraulics subflowsheet in steady-state mode, it is critical that inlet/outlet streams are specified as follows: 

• Inlet streams must have temperature, composition, and flow rate specified. 
• Pressure must be specified at the outlet stream. 
• Transfer basis for the inlet/outlet streams must be set to T-P Flash.

In modelling of hydraulics, the pressure is typically known downstream and will be back-calculated in the system. However, the Hydraulics solver requires an initial guess of the inlet conditions to solve the case. The initial estimate could be provided in the Reference tab of the inlet stream inside the subflowsheet. Alternatively, you could use a Recycle unit operation on the inlet streams to provide the initial guess. Using this approach, the pressure transfer direction in the Recycle unit operation should be set to “backwards”. To start the calculations, flow, temperature and composition need to be initially specified in the product stream of the recycle while pressure needs to be specified for the inlet stream connected to the recycling.

If other equipment needs to be added to the model, this should be done in the main flowsheet, outside the Hydraulics subflowsheet.  

Going to Dynamics

The case should be fully converged in steady-state mode before switching to dynamics mode. In dynamics mode, all the boundary streams should have valves set up as is typical with HYSYS dynamic simulation in general. These valves can be automatically set up using the dynamics wizard but it is important to properly set up the boundary specifications. If the flowrate of all input streams is to remain constant during the dynamic run then the flow specification, rather than the pressure specification, should be made active. HYSYS will give a warning when moving to dynamics but you can still successfully run the case with this type of dynamic specification.

On the Dynamics tab of the Hydraulics subflowsheet, many solver control options are available. Some of the options are self-explanatory, while some are not as clear and are summarized as follows: 

Pigging

A pigging scenario can be easily added by selecting the “View Pig Options” button on the Dynamics tab. The following parameters for the pig need to be specified: 

• Model: Determines the pig velocity. Aspentech recommends using “high-performance pig” which does not restrict the pig velocity.
• Entry pipe/Exit pipe: These are the pipe segments where the pig is launched and received.
• Exit Location: Exit location specifies where along the pipe length the pig is received. HYSYS will not allow values larger than the pipe length.
• Leakage: By default, this parameter is set to 0.01. This value dictates how much liquid leaks around the pig as it goes down the line. Setting this value to 0 will usually result in an unstable solution.

To launch the pig, simply click on the “Launch Pig” button while the integrator is running. Alternatively, the Event Scheduler can be used to launch pigs. You can find the pig position, slug front location, and transit time of the pig in the “View Pig Options” interface. 

Aspen Hydraulics provides powerful capabilities to perform pipeline dynamic simulation, however, it can be complex and a solid understanding of the various options is critical to ensuring good results.

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