OLGA7 Training Exercises

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1. GUIDED TOUR

This exercise will guide you through the actions required to create and run a simple two-phase flow transient pipe simulation using OLGA in a step-by-step manner. We shall start from a basic case since hardly anyone needs to build a case from scratch.

Double click on on the desktop to initiate the Graphical User Interface (GUI).

1.1 Create a Case

With OLGA you may work with a single simulation case in the GUI or you may gather several cases under a common Project.

In this course, you will work in the predefined folder on the desktop where you just stored the files from the USB stick.

Click Browse to locate and select:

DesktopFA Exercises OLGA 7.2  Guided Tour

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This will create a complete case labelled Basic.opi, located in the folder C:\Users\User1\Desktop\FA Exercises OLGA 7.2\Guided Tour

You will now also see the case label “Basic” on the top bar of the main GUI window.

By default OLGA makes a case container (a Project) with the same label as the case.

A Project file has the extension .opp i.e. the project file name is Basic.opp in this case. By opening a project file, all other associated case files will also open. However, if a project file should be deleted, the case files are maintained.

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By clicking on the Case label (Basic), you should see the case main window and you may expand/collapse the case by clicking the (+)/(-) squares.

The main OLGA window (also known as the Canvas) contains the graphic view of OLGA 7 model. The window pane to the left is called “Model View” described by a number of KEYWORDs.

Each element (keyword) in the Model View has a Properties window (normally to the right).

Essentially, the main structure of the model is made in the Model View, while the detailled modelling takes place in the Properties windows.

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You can learn more about each keyword by selecting it (click on it) and pressing F1. A user manual Help Window will appear.

For example, if you click on the keyword INTEGRATION and F1, the User Manual should open on the relevant page:

You may want to take a closer look at the case and the concept of Project. Select the main window by clicking:

File  Help  OLGA Help

Click on “Contents”  “Graphical User Interface (GUI)”  ”Introduction to projects and cases”.

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CHANGE THE CASE NAME

In order to re-label the Basic case to “Oil Case.opi”, right-click on the case label and select “Save Case As” to give the new file name:

The following steps will guide you through the process to modify the various model elements and make a simple OLGA model.

1.1.1 CASE

Expand Case Definition and Select CASE in the Model View window. Now, complete the case information in the Properties window on the right hand side of the screen.

1.1.2 FILES

We shall now change the fluid table file: Select FILES in the Model View window.

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Select the PVTFILE box in the Properties window and click on . Browse for the file {tour-oil.tab} in the Tour directory (FA Exercises OLGA 7.2\Guided Tour) and click “Open”. Select ./3phase.tab and click Remove to delete it from the list. Note that a fluid file may reside in any folder.

The error indicator at the bottom right of the main page should now turn RED, indicating the simulation is not ready to run.

Each fluid file has one or more distinct fluid label(s), which define what fluids are present in the table file (more on this later). When defining fluids in OLGA system, these fluid labels are referred to, in order to indicate which fluid is present in the respective system component(s). You have just changed the fluid file; therefore, the fluid label that OLGA refers to needs changing. The fluid labels are specified for all nodes and branches (FLOWPATH) of the Geometry.

In this case, we shall work with one fluid label only, namely Label “1”. Note that the initial case we started from used “Fluid 1”. 1

Select BRANCH in the Model View to be able to switch FLUID to “1” in the Properties window:

Fluid tables in this course are made using a 3rd party software called PVTsim. In this program, you may specify fluid labels – or leave it to the program to label the fluids. In that case, the first fluid in the table is given the label “1”, the next label “2” and so on.

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We need to do the same for the 2 nodes (i.e. change fluid). The procedure is as follows: Click on the INLET node in the Model View

In Properties Window, scroll down to find Pressure Conditions and expand to locate the FLUID keyword and select the proper label (“1”).

Do the same with OUTLET node.

1.1.3 Time INTEGRATION

Select INTEGRATION in the Model View window and enter the data below in the Properties Window for the INTEGRATION keyword. STARTTIME = 0 s

ENDTIME = 5 h DTSTART = 0.01 s MINDT = 0.01 s MAXDT = 20 s

The correct time-unit should ideally be selected before the data is entered. The value will be automatically converted when the unit is changed.

Note that you can override the time unit conversion by holding down the <Shift> key while changing the unit.

Also note that the initial time step (DTSTART) cannot be outside the maximum and minimum time steps specified (MAXDT and MINDT).

Refer to the descriptions at the bottom of the Properties Window for more information on each item (key).

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1.1.4 Global Simulation OPTIONS

Select OPTIONS: we use the default values for the current case.

OPTIONS define the global calculation options used in the simulation; for example, the methods used to calculate the fluid temperature (temperature profiles).

You may also select to use the standard OLGA flow model – or the OLGA HD model (developed by SPT Group, sponsored by a number of oil companies in the Horizon project.)

In this course you will modify the TEMPERATURE option, but for the Guided Tour we leave them at default.

1.1.5 Pipe Walls and Materials

Wall Materials

The materials surrounding the fluid in the pipe such as pipe wall, insulation, concrete, soil, etc must be specified for heat transfer calculations. That is to calculate the heat transfer coefficient and the accumulated heat during a cool-down or shut-in scenario.

Walls and Wall Materials are pre-specified in the Basic model. Therefore, as a first step, right click on WALL_2 to delete it as shown below.

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Take a look at the properties of the two materials in the Library MATER-1 and MATER-2.

Which material would dominate the heat exchange to the surroundings and which would dominate the rate of cool-down during a shut-in?

To make the model easier to read, change the labels of the predefined materials:

Change MATER-1 to “Steel” and MATER-2 to “Concrete”. The instructions for this are given below: In Model View, navigate to Library  MATERIAL and in Properties window, change the LABEL as shown.

Note that you can also import materials from a predefined list in the Library. Simply right-click on Library in the Model View window as below and select “Import from user’s library”.

Moreover you may add new “customised” materials with relevant properties to the user’s library: Right-click on a MATERIAL and select “Add to user’s library”. These materials will then be available when you use OLGA again (as the same user).

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12 Walls

As you have now changed the MATERIAL labels in the model, we need to update these in the Wall definitions.

Select WALL-1 in Model View and click on the selection icon shown in the red box on the right hand side. A new window pops up:

In the Select MATERIAL data for [WALL] window, select all the material labels under Items

selected on the right side of the window. Then, click the left arrow button (<) to move them to Items available for selection: on the left side of the window (note: sometimes the materials get

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Then, highlight Steel under the “Items available for selection” pane and click on the right arrow (>) to substitute MATER-1. Do the same procedure for Concrete by clicking on (>) to substitute MATER-2. Note that you have to create two instances of Concrete to fully replace MATER-2.

The two layers of Concrete coating are specified in order to avoid too big jumps in the wall thickness discretisation, to improve the accuracy of the numerical solution.

Click on the Property Page icon in the WALL-1 Properties Window to open up the window below. Check that your model data are OK.

1.1.6 Pipeline Network

The next step is to model the pipeline profile in more details. We shall build a simple pipeline, which is 1200m long (along the pipe) with a total elevation of 1.5 m.

In the Model View window under Flow Component, navigate to FLOWPATH  Piping and choose PIPE-1.

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In the PIPE-1 Properties window, change: ELEVATION = 0 m

ROUGHNESS = 2.8e-005 m

Change elevations of PIPE-2 to 1.5 m and PIPE-3 to 0m following the same procedure as above (remember to change ROUGHNESS to 2.8e-005m for these pipes)

In the main window (also known as Canvas), double-click on the PIPELINE.

The GEOMETRY editor will appear, showing the actual pipe profile. The green squares indicate the numerical pipe sections (boundaries).

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To view the main properties of the pipeline and its geometry, click the second tab in the Geometry Editor window (Oil Case: GEOM-1 tab).

From the Geometry editor main menu, select File  Exit. Make sure to click NO in both confirmation windows.

In the Model View window under Flow Component, choose NODE : INLET, which is of TYPE CLOSED. Note that there is no mass or energy transfer across a CLOSED node.

In the NODE:OUTLET Properties window, complete these steps: a. Verify that the node TYPE is PRESSURE.

b. Expand Pressure Conditions and verify the information entered for those options.

Observe that you always must specify a temperature for this type of node. This is needed in the event that fluid flows from the (pressure) node into the pipeline.

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These three buttons at the top of the Properties window, from left to right respectively, let you sort the input alphabetically, in its original format, or according to the information entered (State).

Change the unit of PRESSURE to bara by clicking the unit field and selecting from the unit list:

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1.1.7 Heat Transfer

In this exercise, you will give input parameters for calculating the heat transfer from pipe wall to the surrounding area (ambient).

In the Model View window under Flow Component, navigate to:

FLOWPATH:PIPELINE  Boundary and Initial Conditions and choose HEATTRANSFER : HEATTRANS-1

In the HEATTRANSFER Properties window, enter these values for the ambient temperature options.

TAMBIENT = 50 F HAMBIENT = 8 W/m²/C 2

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HAMBIENT is the heat transfer coefficient for the external film, i.e. from the outside wall layer to the

ambient temperature, TAMBIENT. The external film coefficient can also be calculated by specifying the external fluid, (e.g. air or water), and fluid velocity under the Calculated heat transfer coefficient.

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1.1.8 Inlet Flowrate

The flowrate in this case is a constant mass flow, specified as a SOURCE.

To view the Properties Window for SOURCE-1, double click on SOURCE-1 in the main canvas.

In the SOURCE:SOURCE-1 Properties window, make the following changes:

MASSFLOW = 4 kg/s

TEMPERATURE = 60° C.

Observe that GASFRACTION = -1. This means the fluid is injected at the equilibrium gas mass fraction at the pressure and temperature conditions of the section 3

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Note that the pressure does not have to be specified. In the event that the pressure is specified, OLGA will perform an adiabatic flash of the fluids from the specified pressure and temperature to the pressure in the first pipe section to establish the actual temperature.

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1.2 EXTRACTING SIMULATION RESULTS

The model is ready to run, but we would like to modify the set of output variables of interest.

Since OLGA is a transient program, it calculates all output variables at each integration time-step for each pipe section. Normally, you do not need all these data for your evaluations and reporting. Therefore, you must specify which variables you want to see and how often you need to record them. Moreover, you must think about the presentation format that you want to apply.

The temporal parameters for result production are specified under the global "Output" specification (at the bottom of the model view).

1.2.1 Time Intervals for Simulation Results 4

In this part of the exercise, you will learn how to define time intervals for data recording in various forms of OLGA results.

OUTPUT file

OLGA produces an output text file (*.out) that contains various information and simulation results.

In the Model View window, expand Output level and choose the OUTPUT key. In the Properties window, enter 5 hours for DTOUT. This variable defines how often data are written to the output text file. In this exercise, all variables specified under OUTPUTDATA (defined later) will be saved every 5 hours of simulation time to the output file.

PROFILE

In the Model View, under the Output level, select PROFILE and specify the time interval (DTPLOT) to be 10 minutes in the Properties window.

This key defines how often profile variables (specified under PROFILEDATA later on) are written to the profile file (*.ppl). Results are printed at each time interval to the profile file, which is used by the

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OLGA plotting package to produce profile plots. This means that each profile variable is plotted against Pipeline Length along the FLOWPATH at all defined time intervals.

TREND

In the Model View, under the Output level, select TREND and specify the time interval (DTPLOT) to be 10 seconds in the Properties window.

This key defines how often trend variables (specified under TRENDDATA later on) are written to the trend file (*.tpl). This file can be read by the OLGA plotting package to produce trend plots.

In other words, each variable is defined at specific positions along the FLOWPATH and you plot the variation of the variable against time at those positions.

1.2.2 Output variables

You must define the output variables in order to extract useful information from your simulation runs. Since you created a case from a predefined and complete case template, you need to make some modifications.

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Note that OLGA calculates several hundred output variables. If you want to add variables to a model you have already run, you will need to re-run the simulation to see the results with the new variables. Try to consider what you really need before adding too many variables.

Profile Plots

In the Model View window under Flow Component, navigate to:

FLOWPATH:PIPELINE  Output  PROFILEDATA.

In the Properties window, click the ellipses button for the VARIABLE field. This will open up

Select output variables window.

This window displays and lists all of the variables that are specified currently as well as those that are available for selection. The variables are listed alphabetically by their label. Select the checkbox at the top of the window to display the variables already specified to the top of the list.

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Uncheck the Q2 variable (Overall heat transfer coefficient ) from the selection list.

You also can list the variables by group or subgroup by clicking on the Group or Sub Group column heading in the Select output variables window.

Alternatively, you can search for variables of interest by typing in the <Search> field in the Select

output variables window. The Search feature covers all variables’ Name, Group, Sub Group and

description.

Following one of the above methods, locate and select (check) the additional Boundary variables:

 UG (Gas Velocity)

 UL (Liquid Velocity)

 USG (Superficial Gas Velocity)

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Click OK and the Properties window should now show all the profile output variables specified:

Note that it is always possible to enter several output statements (i.e. PROFILEDATA, OUTPUTDATA, TRENDDATA or SERVERDATA) for each Flowpath. To add a new output statement. To add a new output statement, follow the steps below:

In the Model View, right-click on Output under the FLOWPATH and add either PROFILEDATA, OUTPUTDATA, TRENDDATA or SERVERDATA. A new variable of the type that you selected will be displayed in the Model View with a number appended to the variable name, for example TRENDDATA[2].

Note that one can also define PROFILEDATA in the Output level directly under the OLGA case name, known as Case Level output statement, for it to apply to the entire case. Therefore, all the variables added to the PROFILE statements are recorded for all the existing branches.

Trend Plots

By following these steps, you will instruct OLGA to record the trend data for various points along the pipeline, such as inlet and outlet.

Under the Flowpath Output, delete the SERVERDATA specifications5

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Select the TRENDDATA statement under the FLOWPATH.

In the TRENDDATA Properties window, complete these steps:

a. Remove the default values in the VARIABLE field, except PT. You can do this by simply highlighting and deleting them from the list. Moreover, the short codes for each variable can be typed in directly in the VARIABLE field.

b. In the PIPE field, ensure PIPE-1 is selected as below. Also make sure in the SECTION field, SECTION 1 is selected.

Add two new TRENDDATA specifications for the pipeline outlet by following these steps:

In the Model View, navigate to FLOWPATH  Output, right-click and add a new TRENDDATA. Repeat this step to add a second TRENDDATA parameter.

In the first newly added TRENDDATA, specify temperature (TM) at the outlet of the pipeline. To enable this, you need to have the following entries in the Properties window:

VARIABLE = TM

PIPE = PIPE-3

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In order to add/change the PIPE entry, click the ellipses button in PIPE field in the Properties window. In the Select Pipe window, under Items available for selection, choose PIPE-3 and click the right arrow button to move it under Items selected. To close the window, click ok to confirm the pipe selection.

The Properties window should now look like this:

Following the same procedure, the trends for Total Mass flowrate (GT), Total Gas Volume flowrate (QG) and Total Liquid Volume flowrate (QLT) at the pipeline outlet can be specified.

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Specify the following in the last TRENDDATA field: VARIABLE = GT, QG, QLT

PIPE = PIPE-3

SECTION = 11

You may have noticed the difference in SECTION numbers in the two TRENDDATA for pipeline outlet. As you know, PIPE-3 has 10 sections and 11 section-boundaries. For temperature (TM) which is a volume variable, you have to choose the last section. For boundary variables such as mass or volume flow rates (in this case GT, QG and QLT), you need to specify the last

section-boundary in the pipe.

Note that it is advisable to define volume variables and boundary variables in separate TRENDDATA.

As the final step, it is recommended to monitor the liquid volume fraction in the branch/FLOWPATH variable LIQCFR.

In the Model View, navigate to Output  TRENDDATA[1] and add LIQCFR in the Properties window to the existing list of variables.

Note that this TRENDDATA is located in the Output level directly under the OLGA case name, known as Case Level output statement as it applies to the entire case. Therefore, all the variables added to any of the output statements (e.g. PROFILEDATA, TRENDDATA) are recorded for the entire case. In this case, LIQCFR is plotted for all the existing branches.

The Case Level output statements are different from the previous output statements under the FLOWPATH, where trends and profiles were recorded for that specific FLOWPATH only. Hence, any output statements added under FLOWPATH are known as Branch Level.

When specifying a Case Level TRENDDATA, only variables that do not belong to a specific position (i.e. do not need a defined PIPE and SECTION) in a FLOWPATH can be specified. For example, as LIQCFR applies to the entire branch, it can be added to the Case Level TRENDDATA.

1.3 Run the Simulations and View Results

1.3.1 Verify the case and run simulation

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 Click on Verify icon in the toolbar of the OLGA main window.

 Click F7

If the model is verified successfully, the message “Verification succeeded” displays in the Output window and the green bar at the bottom of the screen turns green displaying “Ready to Simulate” as below. Note that Output window is located at the bottom of the screen as shown.

If there are any errors, you can go directly to the error by clicking on the arrow icon next to the error message in the Output window.

Once you have checked the model, click on the Run simulation icon in the toolbar or press F5 to run the simulation in interactive mode (more on this feature later).

Successful completion of the simulation will be indicated by the following message in the Output window:

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Alternatively, it is also possible to run the simulation in batch mode, whereby a DOS window pops up with information on the simulation status and progress.

1.3.2 Select units

If you have not selected your preferred unit system, you can do so before looking at the results of the simulation.

From the OLGA main menu, select File  Options

In the Application options window, navigate to Default Units tab.

You may select from pre-defined unit systems or make your own unit set(s) which you can save for re-use. For these exercises, either of the Metric or the OilField system should be relevant.

CONGRATULATIONS, you have now built your first OLGA simulation case.

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1.3.3 Viewing Profile plots in OLGA

A profile plot gives the values of a variable along the entire length of a particular flowpath at specific points in time. In this exercise, we have recorded the data for default profile variables. These variables have been put in the model in the branch level PROFILEDATA as seen below:

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The simulation profile plots may be viewed by selecting the Profile Plot icon in the toolbar:

The Select variables window displays, allowing you to choose the variables that you wish to plot:

In the Select variables window, select the following variables:

 Geometry (pipeline profile)

 HOL (liquid hold-up, defined as liquid volume fraction in the pipeline)

 PT (pressure)

 TM (temperature)

The units for plots may also be changed in this window by selecting the unit and choosing from the drop down menu as below:

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Note that there are a number of filters available to screen out the cases, variables or branches to ease the variable selection process. For example, you can choose which variables to view in the main window by selecting/de-selecting the variables in the Green box below. In the Blue box, you can choose to view variables belonging to a specific branch in your case.

You have the option of adding other profile files (*.ppl files) by choosing Add files in the red box above.

Once you have chosen the variables above, click OK in Select variables window to view the results as below:

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View changes to the profile variables over time by selecting the black arrow (Play button) in the replay toolbar at the bottom of the plot. Replay speed can also be varied by specifying it in terms of frames per second (fps) in the reply toolbar. Note that real speed will depend somewhat on

computer capacity and size of the actual graph. You may also use the slide to view changes with time.

To view the exact values of the plot variables, take one of these actions:

 Right-click on the graph and select View  Track Values.

 Click the Track Values button from the toolbar at the top of the Profile window.

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A window opens and displays the values for each variable in the graph, depending on the position of the mouse pointer. Move the mouse pointer along the pipeline length to observe the change in the value tracker. By left-clicking on a particular point, the variable values for that specific pipeline length will appear on a separate window on the left hand side of the profile (See below).

To disable the value tracker and remove it from the graph, simply right-click on the graph.

The values can be copied/pasted into an open Excel-worksheet in the following two methods:

 To copy all profile data from the graph, Right-click on any part of the Profile window and select: Edit  Copy  Copy Data

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 To copy specific tracked values in the chosen value box, right-click on the value box and select

Copy all data as below

Individual graphs can be turned on and off by selecting (or deselecting) the tick in the legend. There are also several features which may be accessed by right clicking in the OLGA Graph.

1.3.4 Viewing Trend plots in OLGA

A trend plot gives the values of a variable as a function of time. This value could be recorded at a specific location, could apply to the entire branch or be related to the entire simulation case (such as VOLGBL, HT which will be covered later on.)

The simulation trend plots may be viewed by selecting the Trend Plot icon in the toolbar:

The Select variables window will appear, similar to when plotting Profile graphs. Select the following variables both at the outlet of the pipe.

 Temperature (TM) at the outlet of the pipe

 Total liquid volumetric flowrate (QLT) at the outlet of the pipe.

Change the temperature unit to °C by following the procedures in the previous section.

The resulting graph is presented below. Note that although the Total Liquid flowrate coming out of the pipeline has reached a stable condition, the temperature seems to be changing slightly, which is due to the highly detailed scale of the Temperature axis.

As with the Profile Plot, there are a number of additional graphing features that you can access through the pop-up menus when you right-click on the graph.

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1.4 Effect of flowrate change

In this exercise, you investigate the effect of changing the flowrate. You will reduce the mass flowrate entering the flowpath from 4 kg/s to 0.25 kg/s and compare the results. To do this, you will create a new case based on the original case.

To make a new case, based on an existing case select the Oil Case that you created previously in the Model View window. Right-click and select Duplicate Case.

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In the Duplicate case window, you can enter Turndown Case.opi as the case name and click Save. The new case will display in the Model View window.

To change the mass rate for Turndown Case, double click on SOURCE in the main canvas, change the MASSFLOW to 0.25 kg/s in the Properties window and run the case.

Plot the total liquid volume flowrate at the outlet of the flowline for both cases to compare the results. Add the *.tpl file for the first case (Oil Case.tpl) in the selection window.

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Now try repeating this last exercise, changing the fluid file to the file tour-gas.tab with the mass flowrate at 4 kg/s to see the effect.

You are encouraged to look into some of the additional options available that allow the layout to be customised by changing the default units, the screen appearance, auto-save and so on. These options are accessed via:

FILE  Options

Additional functionality in the GUI includes the ability to write an input report which summarises the model. This is accessed from Toolbar icon highlighted below:

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2. EXERCISE 1: OIL PRODUCTION AND SLUGGING

2.1 The story

The Harthun field has recently been discovered, located south of the existing Wigoth Alfa platform. It is proposed to develop Harthun via a single subsea wellhead and pipeline to the Wigoth Alfa platform to assess the full field potential during an extended test phase, prior to full field development. There is an existing riser on Wigoth Alfa which was pre-installed during the construction phase of the platform to accommodate future sub-sea field developments.

Recent topsides modifications on Wigoth Alfa involving the installation of multiphase well test meters has allowed the existing Test Separator to be used as a dedicated Harthun production separator. The Test Separator operating pressure is to be maintained constant to allow the gas from Harthun to be fed to the export compression system.

2.2 The data

The Harthun wellhead is located on the seabed in a water depth of 255 m and is 4.3 km away from the Wigoth Alfa riser base. The Wigoth Alfa platform stands in 270 m of water with the production deck located 30 m above sea level. The riser at Wigoth Alfa is fully vertical and is 300 m long with a

4 inch diameter. The riser has an internal diameter of 0.1 m with a steel wall thickness of 7.5 mm

and no insulation. There is a 100 m horizontal top-side pipe with the same properties as the riser. A common pipe roughness is assumed to be 0.028 mm.

The Test Separator pressure is constant at 50 bara.

It can be assumed that the minimum required arrival temperature at Wigoth Alfa is 27°C (to avoid wax formation). The maximum allowable pipeline inlet pressure is 80 bara at a flowrate of 15 kg/s. The flowing wellhead temperature is assumed to be constant at 62°C.

Harthun

Wigoth Alpha

4.3 km

300 m

255 m

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The properties of pipe steel and insulation are the following: Material Density [kg/m³] Specific Heat [J/kg/K] Thermal Conductivity [W/m/K] Steel 7850 500 50 Insulation 1000 1500 0.135

The minimum ambient temperature can be assumed to be 6°C. The ambient heat transfer coefficient, (from the outside of the pipe structure to the surroundings), is 6.5 W/m²/C for the entire pipeline-riser system - in the absence of any other data.

In the absence of any fluid compositions, the fluid file (Wigoth.tab) generated for Wigoth shall be used (the fluid from Harthun is thought to be very similar to the Wigoth well stream fluid).

2.3 The task

You are required to perform a study into the technical viability of producing Harthun over Wigoth Alfa, taking account of the following:

 Pipeline size (inner diameter) and insulation required

 Production instabilities during well testing as well as full production

 In case of instabilities, you will evaluate various mitigation alternatives and estimate maximum liquid surge volumes into the separator

Note that in Exercise 3, you will extend the model you make here and study:

 Insulation requirements during a shut-down/cool-down

 Establish any limitations due to the existing topsides facilities during both normal and transient operations

2.4 Preliminary Pipeline Sizing

The first task is to establish the pipeline size and insulation level required to achieve the desired production and turndown rates. This can be done by performing a series of steady state simulations. However, there is very little information currently available for the system; for example, there is no given seabed profile. Consequently, you will need to assume a rough pipeline profile. As part of this exercise, you will perform steady state simulations for two flowrates, specifically 5 kg/s and 15 kg/s. The lower flowrate dictates the insulation level required due to the larger temperature drop along the pipeline. The highest flowrate determines the minimum pipeline diameter required in order to keep the wellhead pressure above certain limits. In other words, to optimise the cost associated with the new pipeline, you shall determine the minimum insulation required at minimum flowrate and the minimum allowable inner pipe diameter at maximum flowrate. As per exercise description above, the arrival temperature (at the outlet of the topside pipe) must be above and as close as possible to 27oC and the pressure at the pipeline inlet must be below and as close as possible to 80 bara.

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Note that pipe and insulation normally are produced in standard sizes/thicknesses. This means you only need to do the sensitivity analysis for certain increments in size/thickness.

2.4.1 Preparing the simulation cases

Create a new case called SteadyState.opi from the OLGA Basic Case template and save it in the following folder:

\FA Exercises OLGA 7.2\Exercise 1-Oil Production and Slugging

Please refrain from working in the Solutions folder.

The case and the project are updated with the name that you specified and a new tab is created in the main window. Note that the GUI will automatically create a Project with the same name.

Click the SteadyState tab to display the case details.

The new case is complete and can be run. Expand the case in the Model View window so that the entire model can be visualised. The template now needs to be edited to reflect the Harthun project.

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Updating the fluid in OLGA case:

You need to change the fluid file in the OLGA case to set up the Harthun project conditions. In the

Model View, navigate to Case Definition  FILES and update the PVTFILE in the Properties

window to Wigoth.tab.

To change the fluid file, click on the ellipses button, in the PVTFILE field in the Properties window. A new window opens up which allows you to select a new fluid file. Note that Wigoth.tab is located in

Exercise 1-Oil Production and Slugging folder.

Remember to remove the original 3phase.tab file from the fluid file list.

You will now have to change the fluid references in BRANCH and NODES as before. Below is a reminder on how to do this:

In the Model View window, navigate to:

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Change the FLUID to 1 as shown below.

In the Model View, navigate to:

Flow Component  NODE : OUTLET

Expand the Pressure conditions field to change the FLUID to 1 as shown below.

Changing the simulation time:

The initial pipeline sizing will be done using OLGA as a conventional steady state simulator. Therefore, the simulation times needs to be modified to change the simulation mode from a dynamic run to a Steady State run. To do this, you would have to equalise the simulation start and end time as described below.

In Model View, navigate to Case Definition  INTEGRATION. In the Properties window, change ENDTIME to 0, i.e. the same value as the STARTTIME. This means that only the steady state solver will be used. Remember to use the Verify button to establish where there are errors in the simulation model.

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Options

In this exercise, you will model a two phase system. In other words, the mass transfer is taking place only between oil and gas. To do this, follow the instructions below:

In Model View, navigate to Case Definition  OPTONS. Change FLASHMODEL to HYDROCARBON as shown.

Pipe materials

It is good practise to modify the default labels to reflect the actual materials. This helps you and others, such as the quality assurance checkers and most importantly, your instructor, to understand your model.

For the Harthun field, the first wall material is steel, so change the label from the default MATER-1 to Steel. The second material is insulation, so change the label from the default MATER-2 to insulation. You can use the instructions given in the Guided Tour to change labels (Section 1.1.5).

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Once the MATERIAL labels have been changed, ensure to change CAPACITY, CONDUCTIVITY and DENSITY to match the properties given in Exercise 2 description in Section 2.2. The final outcome should look like the following:

Pipe walls

As with the Guided Tour, there are two pre-defined pipe wall structures, WALL-1 and WALL-2. Re-label WALL-1 to W-Pipeline for the pipeline and WALL-2 to W-Riser for the riser/topside piping. This is to help you as the user and Quality Assurance person to better understand your model. The re-labelling process is similar to that of MATERIALs in the previous section.

To re-label the WALL, in the Model View window, navigate to Library  WALL. In the Properties window, delete the default LABEL and enter the desired label (W-Pipeline or W-Riser).

You now need to update the MATERIAL field with Steel and Insulation, in the same way as Section 1.1.5. To do this, in the Properties window of W-Pipeline WALL, click on the Property button as highlighted below. In WALL window, update MATER-1 to Steel and MATER-2 to Insulation. Also in the same window, right-click on the empty cell next to the last MATER-2 and select Delete. The final WALL window should look like the following:

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For W-Riser WALL, you need to remove the insulation layers as explained below: In the Properties window of W-Riser, delete the following:

 In THICKNESS field, highlight and delete 2:0.02

 In MATERIAL field, highlight and delete both MATER-2 labels.

Update MATER-1 to Steel according to the procedure explained for W-Pipeline. The final Properties window should look like the following:

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 W-Pipeline consisting of 9mm Steel and 20mm Insulation layer

 W-Riser consisting of 7.5mm Steel and no Insulation layer

Pipeline Geometry:

In this step, you will set up the overall pipeline profile/geometry, which will include a subsea pipeline and a riser to the receiving facilities.

In the Model View, navigate to Flow Component  FLOWPATH : PIPELINE  Piping and select the Geometry (GEOM-1) field shown below. In the Properties window, change LABEL to PRELIMINARY and adjust the YSTART (y coordinate of the start of the pipeline to

-255 m, (corresponding to the water depth at Harthun assuming sea-level to be at Y = 0).

To make your model more consistent, change the LABEL of the INLET node to Harthun Wellhead. You can do this by navigating in the Model View window and expanding Flow Component  NODE : INLET as shown below:

In the same field in the Model View, navigate to NODE : OUTLET and make the following changes in the Properties window:

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 Enter Wigoth Test Sep in the LABEL field.

 Expand Pressure conditions and set these values:

PRESSURE = 50 [bara]

GASFRACTION = 1 (only gas will flow back into the pipeline) TOTALWATERFRACTION = 0 (because this is a two phase oil-gas system) TEMPERATURE = 27° [C] ((the minimum allowable arrival temperature)

FLUID = 1

The base OLGA case has a predefined pipeline profile. In order to change it, double-click on the PIPELINE flowline in the main canvas to open the Geometry Editor window.

In the Geometry – SteadyState : PRELIMINARY window, drag and drop the highlighted tab to the centre of the window. Select New Horizontal Tab Group, in order to view both geometry tabs simultaneously.

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Change the x and y coordinates of the predefined geometry as shown in the window. Ensure to also fill in Diameter, Roughness and Wall for the 3 pipes accordingly.

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Observe that PIPE-2 and PIPE-3 have a different diameter from PIPE-1. The short and horizontal top-side pipe has been added to the model prior to the separator node for numerical stability.

From the Geometry editor main menu, select File  Exit. Make sure to click YES in the confirmation window to update the pipeline geometry.

You can now ensure that the FLUID identifier in FLOWPATH  BRANCH is correct:

In the Model View window, expand Flow Component  FLOWPATH : PIPELINE  Piping and choose BRANCH. In the BRANCH Properties window, change the value of the FLUID field from Fluid1 to 1

Source

The inlet flowrate is already given in the base case template as SOURCE-1. Make the following changes to this source by double-clicking on SOURCE-1 on the main canvas. In the

SOURCE:SOURCE-1 Properties window, make the following changes:

LABEL = Harthun

MASSFLOW = 4 kg/s

TEMPERATURE = 62° C

TOTALWATERFRACTION = 0

GASFRACTION = -1

The SOURCE temperature and local pressure are taken into account when establishing the equilibrium gas mass fraction.

Note that GASFRACTION of -1 means that the fluid table will be used to establish the ratio of gas to hydrocarbon liquid present in the SOURCE. Because we intend to simulate two-phase flow (gas and hydrocarbon liquid), setting TOTALWATERFRACTION to 0 ensures that no water enters the pipe.

Heat transfer

The template case used has a pre-defined heat transfer specification, which does not need changing. By default, it already applies to all pipes in the branch with a T ambient of 6 C.

Simulation results to display

To visualise the results of the simulations, the following output specifications need to be given as a minimum.

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As explained in sections 1.2.1 and 1.2.2 of Guided Tour, the output variables can be specified in OUTPUTDATA, TRENDDATA and PROFILEDATA. For example, the OUTPUTDATA keyword will export the data to the *.out file which allows you to view the data in a text editor. This file contains a copy of the textual input file and certain other useful information from the case.

The variables specified for the TRENDDATA keyword will export the data to the trend (*.tpl) file. For example, the liquid flowrate out of a pipeline should be specified in this field. This keyword is also useful for specific output parameters such as a source mass flow rate, or for global variables (such as the integration time step) and branch properties such as total liquid content in the entire branch. PROFILEDATA will be used for specifying certain system variables along the entire pipeline with time, such as holdup or flow regimes.

As before, you will use the plotting functions in OLGA to view the variables specified under TRENDDATA and PROFILEDATA.

You can also animate the variables specified in the PLOT key (under Case-level OUTPUT in Model

View window) by using the embedded OLGA Viewer program. This program is activated from the

OLGA interface which will be covered later in more details.

Following the procedures in Sections 1.2.1 and 1.2.2 of Guided Tour, carry out the following tasks for OUTPUTDATA, TRENDDATA and PROFILEDATA:

OUTPUT file:

As there are currently no OUTPUTDATA statements under the flowpath, you need to add a new one as specified in Section 1.2.2:

In the Model View window, expand Flow Component and choose FLOWPATH : PIPELINE. Right-click and select Add  Output  Outputdata.

A new OUTPUTDATA is created under FLOWPATH : PIPELINE in the Model View window. Select OUTPUTDATA[1] in Model View and type the following symbols in the VARIABLE field, separating each variable by comma like below:

 HOL (Liquid hold-up)

 PT (Pressure)

 TM (Temperature)

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The flow regime indicator will output a code which corresponds to a certain flow regime. The codes are:

1 Stratified Wavy Flow 2 Annular Flow

3 Hydrodynamic Slug Flow 4 Dispersed Bubble Flow

As explained before, you can also search for these variables if you do not know the variable symbol. Simply click on the ellipses button highlighted above to open Select output variables window and type the description of the variable in the Search field to filter and then, check the variable of interest. For example to search for holdup, you can type “holdup” and check the box next to HOL below.

Note that you do not need to specify position for OUTPUTDATA. This means that you get the variables specified printed for all calculation sections in your system.

Remember that some variables (HOL, PT and TM) are volume variables which are averaged properties for each pipe section (i.e. numerical section) and they are given at section midpoints, whilst ID is a boundary variable, calculated and plotted at each pipeline section boundary.

Following the procedure in Section 1.2.1 of Guided tour, set the .out file frequency to every 1h.

Hint: In the Model View, under the case name, navigate to the case level Output  OUTPUT and

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You can now follow the same procedures as above for both Trend and Profile plots.

Trend data:

In the Model View window, navigate to the first pre-defined Branch level TRENDDATA (i.e. under FLOWPATH  Output  TRENDDATA[1]). In the Properties widow, make the necessary changes to specify flowline inlet pressure (PT) and inlet temperature (TM):

Similarly, navigate to the second Branch-level TRENDDATA (TRENDDATA[2]) and make the necessary changes in the Properties window to define the pressure and temperature for the outlet of the pipeline.

Hint: The pipeline outlet position must be PIPE-3, SECTION 2. As explained before, the maximum

allowable section number in the Properties window is 3 for PIPE-3, which is the maximum section boundary number. As both pressure and temperature are volume variables and we have 2 sections in PIPE-3, SECTION 2 is the maximum allowable for this TRENDDATA. Therefore, if you make a mistake and specify SECTION 3 for pressure and temperature in this case, your OLGA case will run, but no pressure/temperature trend data will be recorded for the pipeline outlet.

You will need to add a third TRENDDATA entry to record the total mass flowrate (GT), the volumetric gas flowrate (QG) and the volumetric flowrate of total liquid (QLT) out of the topside pipe. As these are all boundary variables, it is advisable to enter them in a separate TRENDDATA entry. To do this, in the Model View window right-click on FLOWPATH : PIPELINE as shown below

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and add a new TRENDDATA entry on the Branch level. In the Properties window, modify the TRENDDATA entry to achieve the following:

In the Model View window, directly under the OLGA case name, a case-level TRENDDATA has already been specified for LIQC (the total amount of liquid in the branch), HT (Global timestep used by OLGA for calculations) and VOLGBL (Global maximum volume error). Also add GASC (the total gas volume in a branch), which is a "BRANCH" variable and thus, you do not need to specify any position.

Observe that when any output variable is specified at Case level, it applies to all flowpaths in the network. In this case, this is equivalent to specifying the TRENDDATA on the branch-level since we have only one branch in the entire model.

Finally, specify all the trend variables to be recorded every 10 seconds.

Hint: In the Model View window, expand Output under the OLGA case name, select TREND and change DTPLOT in the Properties window from 15 to 10 s.

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Note that if you wish to edit all your trend specifications for a specific flowpath (without navigating back and forth between different TRENDDATA entries), you can follow the procedure below:

In the Model View window, open a library function by right-clicking on a branch-level TRENDDATA entry and selecting Local Instances:

This opens a new window containing a table with all your TRENDDATA entries for that flowpath. This provides a great overview of all your trend variables and allows you to make changes more easily.

Moreover, you can customise the table by choosing which columns to become visible, and which to be hidden. Simply right-click on a column header, select Columns and check/uncheck any of the desired columns to add/remove from the table.

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Note that the Local/Global Instances feature is a general library function available for most of the OLGA model components.

Profile data:

Similar to trend variables, in the Model View window, navigate to the existing branch level PROFILEDATA entry and modify the VARIABLE field to record the following profile variables:

 Liquid hold-up (HOL)

 Pressure (PT)

 Temperature (TM)

 Flow Regime Indicator (ID)

Hint: Simply delete all the rest of the variables in the VARIABLE field so you are left with the above four profile variables.

Finally, specify all the profile variables to be recorded every 5 minutes.

Hint: In the Model View window, expand Output under the OLGA case name, select PROFILE and change DTPLOT in the Properties window to 5 minutes.

2.4.2 Minimum Pipeline Diameter: A Parametric Study

Now your OLGA case should be ready for simulation runs. Following the procedures outlined in Section 1.3.1 of Guided Tour, verify that the model is runnable. If there are any errors, rectify them and save your case.

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Hint: To verify the model, click on the following button on the main toolbar:

Once the model is verified, run the simulation to test if your OLGA case runs to the end of the simulation without any errors. Refer to Section 1.3.1 if you need a reminder of the procedures. In this exercise, you will establish the correct pipeline diameter to keep the inlet pipeline pressure just below 80 bara (note that you will keep the riser and top-side diameter during this exercise as it is). To do this, you will have to set up and run a number of simulation cases where you vary the pipeline diameter to determine the minimum pipeline size resulting in an inlet pressure lower than 80 bara. You will then update the model using the correct pipeline diameter.

The pressure drop along the pipeline is expected to be the highest at high flowrates (maximum 15 kg/s in this case). Therefore, this exercise is done for 15 kg/s flowrate to ensure the pipeline design is suitable for all flowrates throughout the field life.

To change the flowrate to 15 kg/s, click on the Harthun source in the main OLGA window to select the SOURCE system component. In the Properties window, change MASSFLOW field to 15 kg/s.

For the purpose of this exercise, you only need steady state results. Therefore, remember to set ENDTIME = 0 in INTEGRATION.

Hint: As explained in Section 1.1.3 of the Guided Tour, you can set the simulation end time in the Model View window by navigating to Case Definition  INTEGRATION and changing ENDTIME in

the Properties window.

Once you have made these changes, right-click on the case label and select Save Case As … to save it with a new label: SteadyState 15kg-s.

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The quickest way to do such a sensitivity analysis is to use the Parametric Studies tool to establish the inner diameter. You will create and run steady state simulations for the flowrate of 15 kg/s over a range of pipeline internal diameters 8, 10, 12 and 14 cm by following the steps below: The parametric study option is accessed from the Tools menu in the main window Toolbar as shown.

Click on the Tools symbol and select Parametric Studies in the drop-down menu:

A new tab will open in the main OLGA window called Parametric Study. In the new tab in the Study field, click on the Add button to add a new parametric study to your case. This button is highlighted below.

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Ensure that #Parameters to be studied is 1, since the parameter to be studied is the pipeline diameter (of PIPE-1) at the high flowrate 15 kg/s.

Right click on Case and select Insert Case. Repeat the procedure until you have 4 cases listed (one case for each diameter).

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Click on the Decoration field to select the following from the drop-down menu.

Note that this option will modify the way the Parametric Study cases are labelled. This specific Decoration will ensure the value of the parameter you are changing is included in the label of the case. This is particularly helpful when you are plotting the same trend or profile variable for a number of different cases in the same graph, as you will find out later on.

Right click on the middle column header and choose ‘Select Parameter’, which opens up a new window.

In the Select Parameter window, click on the top drop down menu and select FLOWPATH : PIPELINE as the primary system parameter.

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In the bottom left window pane, select PIPE-1 and click on [ > ] button to move to the right pane. Once PIPE-1 appears in the right window pane, right click on the column heading to its right and select DIAMETER. Click OK at the bottom of Select Parameter window to conclude the parameter selection process.

Once PIPE-1 diameter has been selected, fill in the diameter values for each case as below. You can also change the unit for the diameter by right-clicking on the unit column and selecting the desired unit.

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The parametric study may now be run by clicking on either Run or Run Batch in the Simulate field of Parametric Study tab. Run Batch feature is usually the quicker simulation run; however, you do not specifically see the simulation progress or failure of any of your cases (unlike the standard OLGA runs in Batch mode). Therefore, the advantage with Run option is the OLGA interface that would show you the progress of the simulation in the Parametric Study tab. Specifically, the Case label will turn green/blue upon successful completion and it will glow red if failed to reach completion. To see the difference and clarify, try out both methods of simulation runs.

Note that only the cases with a checked box next to them would be included in the Parametric Study run.

To view the results for Parametric Studies, you should use a slightly different method. In the

Parametric Study tab, click on Trend and Profile buttons in the Plot field shown below. To view the

inlet pressure for this exercise, you can plot pressure as either a trend or profile plot. Although this is a steady state case (ENDTIME = 0), in OLGA you may plot a trend even if you have only one point at time = 0. In this case the time scale is artificially from 0 to 1.

Using this method of plotting, compare the inlet pressure for all the diameters above and determine the required design diameter for the pipeline. Once you have determined the correct pipeline diameter, update the base OLGA model (in this case, SteadyState 15kg-s) if required and save the case.

2.4.3 Minimum Insulation Thickness: A Parametric Study

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In the Model View window, select the case label SteadyState 15kg-s, right-click and select duplicate the case. Call this new case SteadyState 5kg-s as you will use this flowrate for the exercise (for reasons below).

In order to ensure the arrival temperature at the facilities stays above 27oC, you need to ensure such temperatures are maintained throughout the entire field life. Fluid temperature is expected to be at its lowest at the minimum flowrate (in this case 5 kg/s). Hence, change the SOURCE flowrate to 5 kg/s (minimum rate) to establish the minimum insulation needed to meet this design requirement.

Note that so far, it is assumed that the pressure and temperature targets are independent, from the pipeline design point of view.

The effect of changing the insulation levels may also be determined with a parametric study to achieve outlet temperature just above 27 C. Following the same procedure as the previous exercise (Section 2.4.2), make a new Parametric Study to determine the insulation thickness needed to obtain the correct arrival temperature at a flowrate of 5 kg/s.

Hint: The parameter to vary is the THICKNESS of the wall W-Pipeline, which is specified under

the primary model feature Library. Note that the parameter THICKNESS holds all the wall layers (2 layers in this case, 1 x Steel and 1 x Insulation). The innermost layer is the 9mm steel-layer while the insulation layer is 20mm thick. Increase the insulation thickness in increments of 5 mm to include 40 mm.

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Check the results for outlet fluid temperatures to decide on the minimum insulation required to maintain the temperature just above 27 degrees. Once you have determined the thickness, update and save the base OLGA model with the correct insulation thickness, if applicable.

2.5 Terrain Slugging – Normal Operation

The Project Pipeline Engineer has now provided a more detailed pipeline profile from Harthun to Wigoth Alfa. This profile is presented below.

Positions Distance (X Coordinate) [m] Water Depth (Y Coordinate) [m] # of (numerical) Sections Wellhead 0 -255 ---- 1000 -255 5 1400 -250 2 1800 -255 2 3400 -255 8 Riser Base 4300 -270 6 Riser Top 4300 30 6 Topsides outlet 4400 30 2

It is suspected that terrain slugging may cause serious problems to the process facilities on the platform due to the presence of a low point at the riser base (check Y-coordinate). The purpose of this exercise is to establish the possibility of severe slugging in the Harthun riser.

In this exercise, you will determine the extent of terrain slugging by varying the pipeline inlet flow rate (at the wellhead) by creating 3 different cases at flowrates of 5 kg/s, 10 kg/s and 15 kg/s to avoid overwriting the results.

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2.5.1 Preparing the simulation cases for Terrain Slugging

OLGA Project and Case

For organisational purposes, you should create a new project to keep all your slugging cases together in the OLGA interface:

To create a new project, in the main OLGA window navigate to File  New  Empty project Name the new project file Slugging.opp.

Open the steady state case you used for the previous exercise with the correct insulation level and pipeline diameter. This case should now be SteadyState 5kg-s, which can be opened in File  Open Case…

Duplicate and name the new case Slug 5.opi. It is recommended that the original case is removed from the new project to avoid inadvertent editing. To remove the old Steady State case, simply select the case label in Model View window, right-click and select Remove. In the pop-up window, select Remove again to only remove the steady state case from the OLGA project. Note that the OLGA case will not be deleted from your hard drive.

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Pipeline Geometry

The new pipeline profile can be entered directly in the Geometry editor.

You could follow the methods described in the previous exercises to do this. However, there is another straight forward process to do this:

In the main OLGA window, navigate to: File  Tools  GeometryEditor

A new Geometry window opens up. Navigate to File  New to create a new Geometry file. Once again, navigate to File  Save As… and save this file as GeoNew.geo for future use.

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After saving the new *.geo file, follow the instructions below to enter the new geometry according to the new data given:

 You need to have 8 rows (X-Y coordinate entries) available in total. By default, there are currently two rows available; hence, you have to insert 6 new pipes. To do this, right click on the pipe label PIPE-1 and select Insert After until you have 8 rows from start to end.

 Enter the new X-Y values by starting from the beginning of Wellhead coordinates, all the way to the end.

 Enter the correct diameter you determined from the previous exercise for the Pipeline and the original 0.1 m for the riser–topside pipes.

 Use a common roughness of 0.028 mm.

 Enter the number of sections from the table given above.

Note that for PIPE-5 only, the sections need to transition from the pipeline section lengths (long sections) to the riser section length (short sections). In order to comply with the section length rule of thumb in OLGA, you will need to define, in exact order: 3 x 200m section, 1 x 150m section, 1 x 100m section and a final section of 50.125m. In order to customise the section lengths, double click on the relevant cell for Length of sections in the geometry table to open the following toolbox. Finally, enter the correct section lengths as described above and select OK to update the section lengths.

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The final Geometry window is shown below.

Note that the walls are not available before the case Geometry has been updated in the OLGA case.

 Keep the GeoNew.geo open and navigate to the OLGA Model View window. Right-click on FLOWPATH : PIPELINE and select Exchange Geometry  GeoNew.geo

The geometry of the pipeline is now updated and it is safe to close the geometry editor window. Now you have to update the pipes with Walls. The easiest way to do this is via the Geometry Editor:

In the main OLGA window of Slug 5 tab, double-click on the PIPELINE to open the Geometry –

Slug 5 : GeoNew window. Navigate to the tab shown below and change the walls for each pipe

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68 Trend data

As the geometry has now changed, two of the TRENDDATA entries for the pipeline outlet need to be modified to reflect the correct POSITION in the FLOWPATH.

In the Model View window, in the branch-level output field select TRENDDATA[2] (outlet temperature and pressure) and TRENDDATA[3] (outlet flowrates) and exchange 3 with PIPE-7 in the relevant specifications. An example is shown for outlet pressure and temperature trend entry below.

Simulation time

To be able to visualise the slugging behaviour, set the simulation end time to 2h (i.e. a non-zero value).

Hint: In Model View window under Case Definition, select INTEGRATION and change the ENDTIME to 2 hours.

OLGA7 Training Exercises

Table of Contents

1.

GUIDED TOUR

1.1

Create a Case

1.2

EXTRACTING SIMULATION RESULTS

.

1.3

Run the Simulations and View Results

1.4

Effect of flowrate change

2.

EXERCISE 1: OIL PRODUCTION AND SLUGGING

2.1

The story

2.2

The data

Harthun

Wigoth Alpha

4.3 km

300 m

255 m

2.3

The task

2.4

Preliminary Pipeline Sizing

2.5

Terrain Slugging – Normal Operation