The pipeline connection simulation

The pipeline connection

GIBBSim provides the capability to model the processes in pipelines, principally oil pipelines. While separate products for such modeling exist on the market, GIBBSim has the advantage of integrating pipeline simulation into a comprehensive package for industrial modeling. This allows the user to avoid separate spending for pipeline modeling software, and also enables work with pipelines as parts of more complicated industrial systems.

The principal challenge in pipeline simulation is taking into account non-Newtonian liquid properties. The non-Newtonian character of oil may lead to build-up of paraffins in the pipeline, causing it to malfunction eventually. Simulation allows the engineer to ascertain whether a pipeline will be able to function reliable under various sets of conditions. The characteristics of the pipeline’s walls and insulation, which may be different for different parts of the pipeline, need to be known; the model can also be used to check the impact of possible changes in the pipeline’s walls and insulation. The modeling system can take into account real data from various measurements on the pipeline, thus improving the precision of the results.

Pipeline modeling example: sea-based platform

In an example setting, a sea-based oil platform is connected to the shore via a long oil pipeline, which has a transitional point on a ship.

Example pipeline layout

The profile of the pipeline is listed in the following table:

No	Pipe section	lengh, m	Outside diameter, mm	Wall thickness, mm	Insulation thickness, mm
1	Riser downward	70	426	16	35
2	Horizontal	2955	426	16	35
3	Riser upward	70	426	16	35
4	Riser downward	70	426	16	35
5	Horizontal	16152	426	16	35
6	Horizontal	3504	325	16	45
7	Horizontal	546	325	16	0
8	Riser upward	70	8’’	flexible hose	0

The oil

The formation fluid has the following composition, in molar parts:

CO²	0.00010
H2S
Methane	0.42888
Ethane	0.08478
Propane	0.05623
i-Butane	0.01772
n-Butane	0.02687
i-Pentane	0.00981
n-Pentane	0.01220
n-Hexane	0.01650
С7	0.34691

The C₇ fraction has the density of 835 kg/m³, and the molecular weight of 253.2 g/mole. The fluid is, immediately on the platform, subjected to separation at the pressure of 2.5 bar, at the temperature of 86 ^oC. Then the resulting oil is pumped into the pipeline, at the pressure of 15 bar. The oil shows non-Newtonian properties at temperatures below 34 ^oC; its congelation temperature is 21 ^oC. The oil is to be supplied to the shore at the rate of 10300 tons per day.

The modeling

First we need to process the experimental data and get a fitting viscosity model. GIBBSim supports this in its Viscosity Modeling subsystem; the system is able to automatically select the optimal model as well as its parameters. For our case, the Hershel-Buckley model will be selected.

Next, we can start modeling the pipeline. To represent the oil, we create a set of components:

To model the C7+ fraction, we create a hydrocarbon (HC) pseudocomponent, setting its density and molecular weight according to our incoming data. We can easily find the boiling temperature of 580 K for it.

Next, we use the graphical model design features of GIBBSim to draw the following diagram:

Note that PPSeg-1 successfully represents all of the first five sections (all of them have the same pipe parameters). We will also need to set other parameters for the pipeline; some of them should be “typical” values as actual data is not available. Finally, we select the VM_test viscosity model, and indicate that a non-Newtonian fluid is to be modeled:

Now we can start the calculation. The following graphs show the resulting profiles of pressure and temperature in the pipeline, based on the calculation results. Points indicate actual data. Pressure profile:

Temperature profile:

Note that according to the temperature profile, the oil temperature never reaches 34 degrees or less; therefore the non-Newtonian properties of oil are not exhibited. We can also repeat the modeling with altered circumstances, For example, suppose that we want to find out whether we can use the final section of the pipeline (3504 meters long) without insulation. To do this, we create a second data set, initially as a copy of the first. Support for varying Data sets makes GIBBSim most flexible for calculation of different variants within the same broad process.

In the new data set, we can enter the insulation thickness for Tube-2 to 0, and repeat the modeling. But this time, the system will report that the calculation is impossible for the last section, Tube-1. The temperature of oil in this section will fall to 25 oC, and the oil’s increased viscosity will lead to additional resistance, and passing the same amount of oil through the tube will become impossible. The temperature profile without insulation of the last section:

The pressure profile for the same case; note how pressure increases because of viscosity: