FLOW ASSURANCE WITH OLGA EXERCISES
FLOW ASSURANCE WITH OLGA - EXERCISES TABLE OF CONTENTS 1.
SLUGGING......................................................................................... 3 1.1 Preliminary Pipeline Sizing ........................................................ 3 1.2 Terrain Slugging – Normal Operation........................................ 7 1.3 Terrain Slugging – Mitigation Alternatives ................................. 9 1.4 Production Ramp-up................................................................ 10 1.5 Decretisation Sensitivity (Optional).......................................... 10 1.6 Slugtracking Module ................................................................ 11
2.
PVTSIM – FLUID PROPERTIES...................................................... 13 2.1 Gas Condensate Fluid Property File ....................................... 13 2.2 Harthun Fluid Property File...................................................... 16 2.3 Mixing condensate and water.................................................. 17
3.
PIPELINE SHUT-IN AND START-UP .............................................. 18 3.1 Shutdown Simulations ............................................................. 20 3.2 Start-up Simulations ................................................................ 21 3.3 Depressuring Simulations........................................................ 22 3.4 Depressuring Simulations – Comp. Tracking (Optional) ......... 23
4.
GAS CONDENSATE PIPELINE....................................................... 24 4.1 Geometry Modification............................................................. 24 4.2 Steady State Simulations ........................................................ 27 4.3 Pigging Simulations ................................................................. 28 4.4 Turndown Ramp-up................................................................. 29
5.
THREE PHASE FLOW – WATER MODULE ................................... 30
6.
YOU GET A FAX – RESULTS WITHIN TWO HOURS? .................. 31
7.
(OPTIONAL) SEPARATORS AND CONTROLLERS ...................... 34
FLOW ASSURANCE WITH OLGA – SLUGGING EXERCISE 1.
SLUGGING The Harthun field has recently been discovered, located approximately 4.3 km due south of the existing Wigoth Alfa platform in 255 m water depth. It is proposed to develop this field via a single subsea wellhead and pipeline to the Wigoth Alfa platform to allow the field potential to be fully assessed during an extended test phase prior to full field development. The field will flow at production rates of between 5 and 15 kg/s. Wigoth Alfa is located in 270 m of water with the production deck located 30 m above sea level. There is an existing 300 m long 4 inch riser on Wigoth Alfa which was pre-installed during the construction phase of the platform to accommodate future subsea field developments. The riser has an internal diameter of 0.1 m with a wall thickness of 7.5 mm and no insulation. Recent topsides modifications involving the installation of multiphase well test meters has allowed the existing Test Separator to be freed up as a dedicated Harthun production separator. The Test Separator operating pressure is to be maintained at 50 bara to allow the gas to be feed forward to the export compression system. (Preliminary topsides studies indicate that there is not sufficient spare capacity in the flash gas compressor to handle the additional gas from Harthun). 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), can be assumed to be 6.5 W/m²/K in the absence of any other data. You are required to perform a study into the technical viability of producing Harthun over Wigoth Alfa, taking account the following:• pipeline size required, • production stability during both full production and well testing phases • insulation requirements during both normal production, well testing and shutdown • establish any limitations due to the existing topsides facilities during both normal operating and transient operations. A number of hints have been prepared to help you to complete this task. It is recommended that you read these prior to commencing each activity.
1.1 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 using an assumed pipeline profile. However, there is very little information currently available, specifically, there is no seabed profile with the only information being the water depth at the Harthun well and the existing platform information. Consequently, you will need to assume a rough pipeline profile. You will need to establish the insulation thickness required, (initially assume 20 mm), to prevent both hydrate and wax formation in the pipeline during steady state and turndown operations. It can be assumed for this case that the minimum required arrival temperature at Wigoth Alfa is 27°C to avoid wax formation. Assume that the flowing wellhead temperature for all cases is constant at 62°C in the absence of any other data from the Production Department. You have been
FLOW ASSURANCE WITH OLGA – SLUGGING EXERCISE advised that the maximum allowable pipeline inlet pressure is 80 bara at a flowrate of 15 kg/s. The properties of the steel and insulation can be assumed to be as follows:Material
Density [kg/m³]
Specific Heat [J/kg/K]
Thermal Conductivity [W/m/K]
Steel
7,850
500
50
Insulation
1,000
1,500
0.135
The pipeline roughness is assumed to be 0.028 mm. You can assume that the topsides pipe work has an internal diameter of 0.1 m in line with the riser. The fluid is thought to be very similar to the Wigoth wellstream fluid. In the absence of any fluid compositions, the fluid files generated for Wigoth are to be used. They may be found in the Harthun directory, (Wigoth.tab). The steady state simulations should be performed for two flowrates, specifically, 5 and 15 kg/s. The former case will dictate the insulation levels required whilst the latter will tend to dictate the pipeline internal diameter. The gas fraction for the SOURCE should be set to -1. HINTS: Create a new project in the directory FA Exercises OLGA 5.0 Harthun called {Steady State.opp} Please do not work in the Solutions directory. Create a new case called {Steady State.opi} based on the OLGA Basic Case Template.
This will create a complete case which 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 current project. Remember to use the Verify button to establish where there are errors in the simulations. Note that the case will run straight away but this will not be the case as the case is modified. It is good practise to modify the default labels to labels which make sense. This will help not only you to understand the model but also the quality assurance checkers and most importantly the course instructors to understand your model. For example, the first material is steel, so change the label from the default ‘MATER-1’ to ‘Steel’, the second material, ‘MATER-2’, is insulation so rename it accordingly. Two different wall structures will need to be defined, one for the pipeline, (with 20 mm of insulation), and a second for the riser (with no insulation).
FLOW ASSURANCE WITH OLGA – SLUGGING EXERCISE Change the description of the geometry to PRELIMINARY and adjust the ‘Y’ coordinate of the start of the pipeline to -255 m, (corresponding to the water depth at Harthun). You should also adjust the ‘Y’ coordinate of the INLET node to the same elevation and change the LABEL to Harthun Wellhead. You will also need to change the ‘X’ and ‘Y’ coordinates of the OUTLET node and change the LABEL to Wigoth Test Sep. Changing the coordinates of the nodes will allow the pipeline profile to be visualised in 3 dimensions. Remember to specify the outlet pressure as 50 bara and the gas fraction as 1, (i.e. only gas will flow back into the pipeline). The separator temperature can be assumed to be 27°C, (the minimum allowable arrival temperature).
A preliminary pipeline profile is provided below.
The initial line sizing will be done using OLGA as a conventional steady state simulator, therefore, the ENDTIME specified under INTEGRATION should be set to the same value as the STARTTIME, i.e. 0 seconds. The inlet flowrate is to be specified using a SOURCE. Change the LABEL to Harthun. Set the GASFRACTION to equal -1. This means that fluid table will be used to establish the ratio of gas to hydrocarbon liquid present in the SOURCE flowrate. The SOURCE temperature and local pressure are taken into account in establishing the equilibrium gas mass fraction. The SOURCE WATERFRACTION should be set to 0, i.e. no water.
FLOW ASSURANCE WITH OLGA – SLUGGING EXERCISE The following output specifications will also need to be given as a minimum to allow the results of the simulations to be visualised. Remember that the variables specified in the OUTPUTDATA keyword will export the data to the *.out file which allows the data to be viewed in a text editor. (This file contains a print of the input file and certain other useful information from the case). Variables specified under TRENDDATA and PROFILEDATA will be able to be viewed graphically using the GUI. Specify the following variables to be exported to the output file every hour. • Hold-up (HOL) • Pressure (PT) • Temperature (TM) • Flow Regime Indicator (ID) Remember that some variables (HOL, PT and TM) are volume variables which are averaged properties for each pipe section whilst ID is a boundary variable, i.e. it is plotted at the edge of each pipeline section. The flow regime indicator will output a code which corresponds to different flow regimes. The flow regime codes are as follows:1 Stratified Wavy Flow 2 Annular Flow 3 Hydrodynamic Slug Flow 4 Dispersed Bubble Flow Specify the following variables to be written to the trend file (*.tpl file) every 10 seconds, (DTPLOT on TREND in the Property window). The variables may be at a particular point in the pipeline network or variables which relate to individual flowpaths or to the entire simulation. These variables are of different type and cannot be mixed in one single trend specification. Plot flowline inlet pressure and temperature. Also plot pressure and temperature in the last pipe on top of the platform. Note that this should be done on a separate TRENDDATA entry. Plot the total mass flowrate (GT), the gas mass flowrate (GG), the total liquid mass flowrate (GLT), the volumetric gas flowrate (QG) and the volumetric flowrate of liquid (QLT) out of the topsides piping. These should be plotted on a separate TRENDDATA entry. Plot the total amount of liquid (LIQC) in the pipeline as function of time. Again, this should be done on a separate TRENDATA entry as LIQC is a different type of variable from the other variables that you have specified. Specify the following variables to be written to the profile file (*.ppl file) every 5 minutes. • Hold-up (HOL) • Pressure (PT) • Temperature (TM) • Flow Regime Indicator (ID) Create and run the simulation at a rate of 5 kg/s to ensure that the file runs. Then use a parametric study to create and run steady state simulations for flowrates of 5 and 15 kg/s over a range of pipeline internal diameters, i.e. 8, 10, 12 and 14 cm.
FLOW ASSURANCE WITH OLGA – SLUGGING EXERCISE Establish the correct pipeline size first and then update the model prior to performing establishing the insulation level required to achieve the desired arrival temperature. The parametric study option is accessed from the Tools menu as shown. The Parametric Study window is then opened.
Select the number of #Parameters to be studied. There are two parameters to be considered in these cases, the pipeline diameter, (PIPE-1) and the flowrate. Right click in Case and select Insert Case and repeat until the desired number of cases have been created. Right click on the next column header and select the parameter to be adjusted. The units may be changed in the next column. Once completed, the parametric study may be run by clicking on [Run Study]. View the results as either a trend or a profile plot to establish the required pipeline diameter and then update the base model with the correct pipeline diameter. Note that the results from the parametric study are referred to by the case number but can be edited in the graphical interface to allow the graphs to be more descriptive. This is done by right clicking in the graph and selecting Configuration... The name may be edited by clicking on the [Title] button. It is possible to perform a parametric study on items relating to each individual flowpath only. Consequently, the effect of changing the insulation levels may not currently be studied using a parametric study. You will therefore need to gradually change the insulation levels until the correct arrival temperature has been established at a flowrate of 5 kg/s. (Hint: increase the total insulation thickness in increments of 5 mm). You may also create duplicate cases to allow the different insulation levels to be compared. As a result of the above steady state simulations, you should now have established the pipeline size required to achieve the desired production rates and established the minimum insulation level required on the pipeline. The pipeline model should now be updated to reflect this configuration.
1.2 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.
FLOW ASSURANCE WITH OLGA – SLUGGING EXERCISE Location
Distance
Water Depth
(X Coordinate) [m]
(Y Coordinate) [m]
0
-255
1,000
-255
5
1,400
-250
2
1,800
-255
2
3,400
-255
8
Riser Base
4,300
-270
6
Riser Top
4,300
30
6
Topsides
4,400
30
2
Wellhead
# of Sections
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. The purpose of this exercise is to establish the possibility of severe slugging in the Harthun riser. Determine the extent of terrain slugging by varying the pipeline inlet flow rate (i.e. at well head) by creating 3 different cases at flowrates of 5 kg/s, 10 kg/s and 15 kg/s to avoid overwriting the results. Each simulation should be run for 2 hours, i.e. remember to change the ENDTIME in INTEGRATION to 2 hours. PLOT can be used to produce a *.PLT file that can be animated by the OLGA-Viewer,. (The OLGA Viewer is be started from the Tools menu.) It is particularly useful in helping visualise the change in some variables along the length of the pipeline with time. Add PLOT to the simulation file to animate the holdup profile (HOL) every 1 minute
HINTS: Create a new project called {Slugging.opp} and open the steady state case with the correct insulation level. 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. Make the necessary changes to {Slug 5.opi} and run the simulation. Duplicate {Slug 5.opi} two times (once the original file is correct), call the new files {Slug 10.opi} and {Slug 15.opi} and change the flowrate accordingly. Note that this study could also be performed using a parametric study but in this case use separate cases as the results will be required for subsequent simulations and some simulations will be modified later. The pipeline profile can be edited directly in the Geometry window. Additional points can be added in the pipeline by right clicking on the pipe name after which the new position is to be inserted. The pipes can then be re-ordered by selecting
FLOW ASSURANCE WITH OLGA – SLUGGING EXERCISE
. You will need to redefine the TRENDDATA positions if the pipes are re-ordered. The recommended profile for this case is presented below. Note the transition in PIPE-5 from the pipeline section lengths to the riser section length, (3 sections of 200 m followed by a section of 150 m, a section of 100 m and a final section of 50.125 m).
Compare the cases by checking the total liquid volume flow out of the flowline and the pressure at the inlet using trend plots. It is also recommended that the flow regime is checked along the length of the pipeline for each case.
1.3 Terrain Slugging – Mitigation Alternatives A number of mitigation options have been identified to prevent severe slugging in the riser. The two options are:• Choking the flow at the top of the platform • Injecting gas at the bottom of the riser It is recommended that these exercises be performed as a new project. Add the 5 kg/s case (Slug 5.opi) to the new project and create two duplicate cases, ( ), and call the new files {Topsides Choke.opi} and {Gas Lift.opi}. Then delete the original case from the Project to avoid inadvertent editing. Topsides Choking. A valve is added by right clicking on the flowpath as shown.
FLOW ASSURANCE WITH OLGA – SLUGGING EXERCISE Define a choke with the VALVE key at PIPE_7, section boundary 2. Specify a diameter of 0.1 m. The discharge coefficient (CD) has a default value of 0.84. Start with a value for OPENING of 0.12 (i.e. 12% of full cross sectional area). Decrease the valve opening. Try the following openings: 0.10, 0.04 and 0.01. This sensitivity may be done using a parametric study to allow the results of the different openings to be compared. What is the valve opening required to stabilize the flow at 5 kg/s and what is the down side of this option? Gas Lift Start with the Gas Lift cases create earlier. Add a gas SOURCE at the riser base. You will need to think about how you add only gas at the riser base. Try lift gas rates of 0.2, 0.6 and 1.2 kg/s. Use a gas source temperature of 32°C. How much gas injection is required to avoid the slugging problems at 5 kg/s (without choking)? What other issues may present problems with this option?
1.4 Production Ramp-up This case introduces the concept of changing variables over time. Investigate the effect of increasing the rate from 5 to 15 kg/s over 1 minute. Simulate for 1 hour at 5 kg/s, ramp up the rate over 1 minute to 15 kg/s and simulate the second hour at this rate. A time series can be entered by clicking on the Timeseries icon, ( ) in the Properties window. The following window will appear. Enter the data to generate the graph as shown.
1.5 Decretisation Sensitivity (Optional)
FLOW ASSURANCE WITH OLGA – SLUGGING EXERCISE You may also wish, (time permitting) to check the impact of the pipeline decretisation on the results of the 5 kg/s case, i.e. the effect of increasing the number of sections in each pipe. Try increasing the number of pipe sections by 2 times, 5 times and 10 times and run each simulation. These should be prepared as separate cases to allow the results to be compared. What is the down side of increasing the number of pipe sections? Have we gained any more information on the pipeline operation?
1.6 Slugtracking Module You have seen that the outlet liquid volume flowrate at 15 kg/s is stable. However, from the profile of the flow pattern indicator, we see that parts of the pipeline are in the hydrodynamic slug flow regime (ID = 3). We have no idea of the size of any slugs that may come out of the pipeline and cannot quantify any problems that may be encountered in the separator. Therefore, we need to use the Slug Tracking module to quantify the slug sizes expected. Slug Tracking simulations would normally be performed as a series of restart simulations from another case which allow some data to be excluded from the final analysis. However, slug tracking simulations can also be run without having to perform a restart. This is the method that will be used here, (restart is covered later). Duplicate the {Slug 15.opi} case and save it with a different name for example {Slugtracking 15.opi}.
The Slug Tracking module is accessed as follows:-
Turn on SLUGTRACKING. Activate the HYDRODYNAMIC slugging option only. Use default values for all other input in the Property window.
FLOW ASSURANCE WITH OLGA – SLUGGING EXERCISE
It is important to specify the relevant output variables before you start the simulation. Add the boundary variable ACCLIQ at PIPE_7, Section 3. (ACCLIQ is the accumulated total liquid volume flow on a pipe boundary and is a boundary variable so it can be added to one of the existing TRENDDATA entries). You may also add slug tracking trend plot variables at PIPE_6, Section 1 and at PIPE_7, Section 1. LSLEXP which is the length of a slug when the given boundary is within that slug. Run the case for 2 hours and inspect the result. You should also turn on the DEBUG option under OPTIONS in the Model View window. This will generate slug statistics in the output file. Add the trend variables NSLUG (total no. of slugs in the system) and HT (integration time step). These variables are global variables and should be entered under Output in the Properties window.
To make a partly automatic calculation of liquid slug and surge volume, plot the accumulated liquid volume at the pipeline outlet as a trend plot and right click in the graph. Select and paste the data into an Excel spreadsheet. In the Excel spreadsheet, perform the following calculation on the ACCLIQ time series: Vsurgej+1 = MAX(0, Vsurgej + ACCLIQj+1 - ACCLIQj - Qdrain*( tj+1 - tj )) For Qdrain, one can use the average liquid flow rate or, if known, the maximum drain capacity of the inlet separator. Open the output file from the icon in the output toolbar to view the slug statistics.
FLOW ASSURANCE WITH OLGA – PVTSIM EXERCISE 2.
PVTSIM – FLUID PROPERTIES For any OLGA simulation, it is necessary to specify a fluid property file, which contains information about the amounts and properties of gas and oil (and water if applicable) for a given range of pressure and temperature. PVTsim is used to make such fluid files. Open PVTsim. Select and store the database in the PVTsim directory. FA Exercises OLGA 5 PVTsim Alternatively, you may create a new database by clicking the [New database] button in the Fluid Management window. (This window can be opened from .)
Then select , (or click on the button in the Fluid Management window), to enter a new fluid into the database. The following window appears.
2.1 Gas Condensate Fluid Property File You shall now make an OLGA fluid property file for a gas condensate. This file will be used later in the course. Fill in the fluid text labels with the following information:as in the window above and • Well = Condensate • Fluid = Condensate • Sample = Condensate00 • Text = Gas Condensate
Enter the composition for the fluid “Condensate00” given in the table below:
FLOW ASSURANCE WITH OLGA – PVTSIM EXERCISE Molecular Weight [kg/kmol]
Density [g/cm³]
0.65
100.21
0.690
C8
1.1e-4
210.00
0.750
C9+
1.5e-4
300.00
0.800
Component Name
Mole %
Nitrogen
0.1
Carbon Dioxide
1.64
Methane
87.39
Ethane
5.75
Propane
2.30
I-Butane
0.48
N-Butane
0.85
I-Pentane
0.26
N-Pentane
0.30
Hexane
0.28
C7
The “C9” component is a plus component (if the fluid has a plus component, the last component is always considered to be the plus component). No experimental data is given.
Check Save CHAR Fluid and then click [OK] in the fluid window. The fluid is now characterized (i.e. the heaviest component is broken down into a number of new pseudo components). The characterised fluid becomes the last fluid in the database and is to be used for the remainder of the exercise. Make a phase envelope using Phase Envelope option in the Simulations window or by clicking on the icon, ( ) on the toolbar.
Perform a PT flash of the condensate at 15°C and 1 bara using the Flash option in the Simulations window or by clicking on the icon, (
) on
FLOW ASSURANCE WITH OLGA – PVTSIM EXERCISE the toolbar. The following results window will appear. Note that there is a [Save Phase] button. This allows the individual gas and liquid phase compositions to be saved to the database as separate fluids. This is useful when recombining wellstream fluids to achieve a specific GOR.
Generate an OLGA fluid property file with the file name {condensate00.tab} using the [OLGA 2000] button on the Interfaces window or by clicking the OLGA 2000 icon, ( ), on the toolbar. (The Interfaces window is opened from either or from the Simulations window.) The following window will appear.
The OLGA table range should be: • Pressure range • Temperature range = • Number of Pressure points • Number of Temperature points
= 1 to 120 bara -20 to 100°C = 50 = 50
Note that it is possible to generate a single {*.tab} file containing a number of different fluids by entering the details of the other fluids under the Fluid 2, Fluid 3 etc Tabs. Use GasCond for Fluid Label. Use the defaults for the other input, click [OK] and wait until the window below appears. Close the window and click [Cancel].
FLOW ASSURANCE WITH OLGA – PVTSIM EXERCISE
2.2 Harthun Fluid Property File In this exercise, you shall make an OLGA fluid property file for the Harthun oil using PVTsim. The composition for the fluid “Harthun” is given below: Molecular Weight [kg/kmol]
Density [g/cm³]
19.50
100.21
0.690
3.62
210.00
0.750
Component Name
Mole %
Nitrogen
0.03
Carbon Dioxide
1.23
Methane
39.29
Ethane
7.65
Propane
6.43
I-Butane
2.14
N-Butane
4.44
I-Pentane
1.99
N-Pentane
2.57
Hexane
4.35
C7 C8
C9+ 6.76 300.00 0.800 No experimental data is given. The last component C9+ is a plus component. Repeat the same as for the gas condensate fluid
FLOW ASSURANCE WITH OLGA – PVTSIM EXERCISE Complete the following activities as for the gas condensate fluid above:• Input the Harthun composition as a new fluid. • Make a phase envelope using Phase Envelope option. • Make a PT flash at 15°C and 1 bara using the PT flash option. • Generate an OLGA fluid property file termed {Harthun.tab}. Use ‘Harthun’ for the fluid label. The OLGA table range should be: • Pressure range • Temperature range = • Number of pressure points • Number of temperature points
= 1 to 200 bara -10 to 100°C = 50 = 50
The OLGA fluid property file should contain data for two phases only. Everything else is as per the default settings in PVTsim.
2.3 Mixing condensate and water Select from the PVTsim main menu. Duplicate the composition Condensate. The duplicated composition appears as the last composition and is identical to the original, see below.
Select the duplicated composition, change the name to “Three phase” and add 0.05 mole % of water to the total composition and press Normalize. Generate an OLGA fluid property file “threephase.tab” with the following specifications: • • • • •
Pressure range Temperature range = Number of Pressure points Number of Temperature points Check three phase.
= 1 to 120 bara -20 to 100°C = 50 = 50
Use “GasCondWet” for the fluid label, the defaults for everything else and generate the new fluid property file.
FLOW ASSURANCE WITH OLGA – TRANSIENT EXERCISE 3.
PIPELINE SHUT-IN AND START-UP The main goal in this exercise is to conduct well shut-in and start-up simulations in order to determine the thickness of the insulation layer needed to keep the fluid temperature a minimum of 5°C above the hydrate formation temperature after a 8 hour shutdown period, (i.e. a ‘no-touch’ time of 8 hours during which time the operators do not need to do anything to the pipeline). The hydrate formation curve has been prepared based on the composition in the PVTsim exercise and is as follows;Temperature [°C]
Hydrate Pressure [bara]
-20
2.1
-15
2.7
-10
3.4
-5
4.2
0
5.3
5
11.4
10
23.3
15
48.6
18
78.2
20
138.9
21.1
200.0
At the end of the shutdown, the operator will then have two options, namely to restart production or to depressure the pipeline to ensure that the pipeline contents remain outside hydrate formation conditions in the event that production cannot be restarted. The liquid surge volume out of the pipeline for both the start-up and depressurisation options will be determined along with the gas rate to flare. The Production Engineers have also provided the proposed well profile and the expected reservoir conditions. You have been requested to include the wellbore in the simulation model to allow the interactions between the well and the flowline to be assessed. The wellbore is a 1,000 m long deviated pipe with an inclination of 45° followed by an 800 m long vertical pipe to the wellhead. The tubing has an inner diameter of 0.101 m and the thickness of the tubing wall is 6.88 mm. The inner roughness of the tubing is assumed to be 0.025 mm. The formation outside the tubing can be approximated by a 0.6 m thick concentric formation layer. The formation layer should be modelled as a number of layers. The physical properties of the formation rock are given below Material
Density [kg/m³]
Specific Heat [J/kg/K]
Thermal Conductivity [W/m/K]
Formation
2,243
1,256
1.59
Assume a linear geothermal temperature gradient between the perforations and the seabed (70°C to 6°C).
FLOW ASSURANCE WITH OLGA – TRANSIENT EXERCISE The simulations will be performed as a series of restart simulations to reduce the computational time required to address both the shutdown and the restart/depressurisation issues. HINTS: Create a new project and use the {Slug 5.opi} case as the base case. Duplicate the original case and call the new case {Shutdown.opi}. The original case should then be removed from the project. Create a new node to represent the bottom of the wellbore and name it Reservoir. Define the X and Y coordinates based on the information below. You will also need to redefine one of the existing nodes as it changes from being a terminal node. (See the following sketch.) Wigoth Separator
Elev. = +30
Sea Level (LAT) Elev. = 0
Harthun Wellhead
Harthun Pipeline
Elev. = -255 Harthun Wellbore
Elev. = -1,762 Harthun Reservoir
Add a new WALL structure with the formation layer included (divide this layer into six layers with thicknesses of 10, 20, 40, 80, 150 and 300 mm). Add the two new wellbore pipes as a separate FLOWPATH named Harthun Wellbore. Note that the X coordinates are relative to the wellhead and the Y coordinates are relative to the sea level or LAT;Location
Reservoir Wellhead
Distance
Depth
# of Sections
(X Coordinate) [m]
(Y Coordinate) [m]
-707
-1762
0
-1055
5
0
-255
4
FLOW ASSURANCE WITH OLGA – TRANSIENT EXERCISE Should be specified using the HEATTRANSFER keyword. Use a vertical INTERPOLATION. Use an ambient heat transfer coefficient (HAMBIENT) of 6.5 W/m²/K. You are to use the new Harthun fluid composition provided for the PVTsim exercise. The fluid property file to be used is {Harthun.tab}. The inflow to the model will be simulated using a WELL keyboard as opposed to the SOURCE previously used. (You will need to delete the source from the previous case). The WELL option is accessed from the same location as the SOURCE option. Add a new WELL and enter the following data in the Property window:• TIME = 0 s • RESPRESSURE = 180 bara • RESTEMPERATURE = 70°C • PIPE = PIPE-1 • SECTION = 1 • GASFRACTION = -1 • WATERFRACTION = 0 Specify a linear inflow performance relationship (IPR) for the reservoir by setting PRODOPTION to LINEAR and repeat for INJOPTION. The formula for a linear IPR is Q = A + B*ΔP. You need to specify A and B for both Production and Injection (corresponds to specifying backflow conditions). The well productivity index, (PI or BPROD) and injectivity index, (II or BINJ) is assumed to be 0.000003 kg/s/Pa. Set the AINJ and the APROD to 0. The WELL is located at the middle of the wellbore. Add two new valves located at the Harthun Wellhead, (PIPE 2, section 5) and at the Harthun Riser, (PIPE 7, section 2). The wellhead valve has a diameter of 0.089 m and the platform valve has a diameter of 0.1 m.
3.1 Shutdown Simulations The initial steady state and shutdown operations will be performed as a single simulation. This is because the insulation thickness cannot be changed as part of a restart simulation. You are to run the revised model for a 2 hour period followed by an 8 hour shutdown. Is the insulation sufficient to maintain the minimum pipeline temperature at least 5°C above the hydrate formation temperature at the local pressure at the end of the 8 hour shutdown? If not, determine the required insulation level to achieve this specification. HINTS: Both valves shall be fully open for the first two hours of the simulation. Close the wellhead valve and the platform valve simultaneously over a period of 60 seconds to
FLOW ASSURANCE WITH OLGA – TRANSIENT EXERCISE shutdown the production. Simulate a shut-in period of 8 hours. The total simulation time adds up to 10 hours, (ENDTIME = 10 h). The simple way to do the valve manipulations is to use time series for valve openings. This is done by clicking on the Timeseries icon, ( ) in the Properties window. The time is given in minutes with the corresponding valve openings specified, (1 is fully open and 0 is fully closed). The valve moves linearly with time between the time points specified with the exact position determined by interpolation. (You may specify any opening between 0 and 1.) The difference between the local temperature and the hydrate formation temperature at the local pressure may be plotted directly in OLGA since the hydrate formation curve is provided. The hydrate formation curve is entered under the HYDRATECHECK option which is accessed from the Properties window as shown. Enter the hydrate formation curve presented above. Add the variable DTHYD to the PROFILE plot for each FLOWPATH and use this variable to evaluate the insulation requirements.
3.2 Start-up Simulations You will now need to address the pipeline start-up operation once the correct insulation has been established. This simulation can be performed as a RESTART simulation using the pipeline conditions in the Harthun pipeline at the end of the shutdown simulation as the initial conditions for the restart simulation. HINTS: Create a duplicate case based on the shutdown simulation {Shutdown.opi} and call the new case {Start-up.opi}. Modify the new case to include the RESTART keyword from the Model View window as shown. Specify the FILE to be used as the initial conditions for this simulation as {Shutdown.rsw} in the appropriate location in the Property window.
Select the INTEGRATION window and change the ENDTIME to 12 hours and delete the STARTTIME. This will ensure that the end time from the Shutdown simulation is used as the start time for the Start-up simulation.
FLOW ASSURANCE WITH OLGA – TRANSIENT EXERCISE Run the simulation and estimate the maximum surge volume in the slug catcher during start-up. Assume a separator liquid drain rate equal to the average steady state liquid production from the first two hours of the case {Shutdown.opi}. Use the variable ACCLIQ and/or QLT to establish the required surge volume. Do not forget to open the valves!!!! Can the surge volume at start-up be reduced using a different start-up procedure?
3.3 Depressuring Simulations The pipeline must be depressurised to avoid hydrate formation in the event that the pipeline cannot be restarted immediately following the shutdown. A 2”NB (5 cm ID) manual valve to flare is provided upstream of the Wigoth Separator isolation valve. You are required to confirm that the pipeline can be depressurised through this valve without the fluid temperature falling into the hydrate formation region. You are also to confirm the total liquid volume generated during the depressurisation operation and check to make sure that the flare capacity is not exceeded. The wellhead and riser valves will be left closed during the depressuring simulations which will be performed by modelling the route to flare as a LEAK. LEAKS are controlled using a CONTROLLER which may be added under Library in the Model View window.
Set-up the CONTROLLER as a manual controller with the following parameters:• TYPE = MANUAL • STROKETIME = 10 s • MAXCHANGE = 0.2 The CONTROLLER set point should be 0 for the first 10 hours of the shutdown (i.e. the leak is closed) and then opened over a 1 minute period. This time series may be entered as a time series by clicking on the Timeseries icon, ( ) in the Properties window.
Add a LEAK in the pipe section immediately upstream of the topsides isolation valve with a diameter of 5 cm. (A LEAK is an item of Process Equipment). The leak should have a back pressure of 5 bara corresponding to the flare system backpressure. The CONTROLLER is CONTROLLER-1 which has just been specified. Specify the following leak variables to be exported to the trend file.
FLOW ASSURANCE WITH OLGA – TRANSIENT EXERCISE • • • • •
Accumulated mass of gas released downstream of leak (ACGLKEX) Accumulated volume of gas released downstream of leak (ACQGLKEX) Accumulated volume of oil released downstream of leak (ACQOLKEX) Gas mass flow downstream of leakage (GGLKEX) Liquid mass flow downstream of leakage (GLLKEX)
Run the simulation and confirm that the fluid temperature in the pipeline does not fall into the hydrate formation region. Also check the peak gas and liquid rates and the total volume of liquid generated during the depressurisation.
3.4 Depressuring Simulations – Comp. Tracking (Optional) This exercise is a repeat of the previous depressuring case but uses a full compositional approach as opposed to the table approach from the previous simulations. The shutdown simulation has already been performed {Shutdown CompTrack.opi} and the resulting restart file is located in the Harthun directory. The Compositional Fluid file you require is located in the PVTsim directory and is called {Harthun.ctm}. You are required to duplicate the depressuring case {Depressure.opi} and rename it {Depressure CompTrack.opi}. There are a number of other modifications required to change the new file to run in compositional tracking mode. Make the necessary changes and run the simulation. Compare the results with those from {Depressure.opi} and explain the differences.
FA WITH OLGA – GAS CONDENSATE EXERCISE 4.
GAS CONDENSATE PIPELINE In this exercise, a gas-condensate pipeline through hilly terrain shall be simulated. The fluid property file to be used is {condensate00.tab} which was created in the PVTsim exercise. A preliminary case has been prepared called {Initial.opi} which may be found in the Gas Condensate directory. This file has not been completed and a very rough pipeline profile has been assumed. Your task is to get the file to run and then to modify the pipeline profile to reflect the geometry supplied by your Pipeline Engineer as a tabulation of x-y coordinates in the file {geo.xy}. The pipeline outlet pressure is 1015 psia. The fluid inlet temperature is 120°F. The wall thickness of the pipeline is 0.5 inches, the pipeline is buried and the ambient temperature is 3°C. The burial is modelled by adding two 1 ft thick layers of soil to the wall.
4.1 Geometry Modification This exercise will guide you through the steps required to import the new pipeline profile into the simulation model. Duplicate the original case {Initial} and save the new case as {Simplified.opi}. Expand out the file structure in the Model View window. Right click on GEOMETRY : INITIAL to access the Geometry Editor. Left click and drag one of the tabs down to allow the data entry window and the flowpath profile graph to be viewed together by creating a new horizontal tab group as shown below.
Enter the Diameter (19 inches) and Roughness (0.0018 inches) in the first row as shown above. These values will become the default values when the new geometry is imported. Select and select the file {Geo.xy}. (Note that it is also possible to cut and paste from excel.)
FA WITH OLGA – GAS CONDENSATE EXERCISE Save this new geometry as {SIMPLIFIED.geo}. You will now need to define the Wall structure for each pipe. This can be done by using standard windows copy and paste functions. The next step is to check the angle distribution by selecting . The following Angle Distribution window will appear.
The colour of the bars and the % values in the output window indicate the difference between the average angle of the pies within an angle group and the average value of the angle group. Green, (and a low % deviation) represents a good match whereas red (and a high % deviation) represents a poor match. The angle groups can be modified to provide a better match. Select and the following window will appear. Additional angle groups my be added by clicking [Add] and entering the angle of the new angle group. The angle groups will automatically be re-ordered.
Modify the Angle Groups to give the following groupings. Clicking on [OK] will give the following results which represents a better match between the angle groups and the actual profile.
FA WITH OLGA – GAS CONDENSATE EXERCISE
The profile will then be simplified by using the filter. This is done by selecting from the main menu. Select the Angle Distribution tab. Move the ‘slider’ for the X End position to the right to select the entire pipeline and set the Pipe Length for Generated Pipes to 2,000.
Save the new geometry as {FILTERED.geo}. Now compare the results by selecting the respective graphs in the Geometry Editor window.
The angle distributions should also be checked to ensure that there is a reasonable match.
FA WITH OLGA – GAS CONDENSATE EXERCISE
The filtered geometry will now be used as the new pipeline profile. Close the {Actual.geo}. Enter the pipeline Diameter (19 inches), Roughness (0.0018 in) and Wall definition (WALL-1) in the tabular format. (The units may be changed by right clicking on the column heading). Note that the cells may be Copied and Pasted using standard windows protocols. We will now specify the length of each pipe section. Select from the menu. The Discretize selected profile window will appear. Specify the Min number of sections per pipe as 2 and the Max length of sections as 1,000 m. Save the geometry as {SIMPLIFIED.geo} and select . The pipeline geometry should now be updated. Rename PIPE : PIPE-1 to INLET and the last pipe to OUTLET. The case should now be ready to simulate.
4.2 Steady State Simulations Perform simulations with inlet flowrates of 20, 40, 60, 80 and 100 kg/s. Tabulate the inlet pressure and total liquid inventory of the line at each flowrate. The parametric study option should be used for this task. The simulations ENDTIME should be set to 0, i.e. this simulation will be run as a traditional steady state simulation. Make a graph of the inlet pressure and the liquid inventory as function of flowrate (Excel). Try to explain the form of the inlet pressure curve. Plot the Total Liquid Content (LIQC) and the Pipeline Inlet Pressure (PT) as trends.
FA WITH OLGA – GAS CONDENSATE EXERCISE 4.3 Pigging Simulations We shall pig the line with a liquid inventory resulting from running at 80 kg/s. However, the gas velocity in the pipeline at a flowrate of 80 kg/s is above the maximum allowable pig velocity. Therefore, the production flowrate should be reduced to 20 kg/s during the pigging operation to keep the pig velocity below acceptable limits. The flowrate is to be increased to 80 kg/s thirty minutes after the pig has arrived. The pig supplier has provided the following data for the pig;• TYPE = SHORT • INSERTTIME = 90 minutes • STATICFORCE = 19000 N • WALLFRICTION = 9500 • LINEARFRIC = 0 • QUADRATICFRIC = 4750 • MASS = 275 kg • DIAMETER = 19 inches • LEAKAGEFRACTOR = 0 You are required to determine the surge volume required at the pipeline outlet to handle the pig generated liquid surge. HINTS: The pig launch and trap positions are defined trough the POSITION keyword. Define a launch position in the second section of the first pipe and a trap position in the second section of the last pipe. Use the PLUG keyword, (under FAmodel in the Model View window), to enter the pig data.
The simulation should be run for 60 minutes at a constant flowrate of 80 kg/s. Then start to reduce the flowrate from 80 to 20 kg/s over 30 minutes and insert the pig after 90 minutes. Run a short case to measure the pig velocity. Assume that the pipe length is 70 km and calculate arrival time for the pig. The flowrate should then be increased to 80 kg/s starting 30 minutes after the pig has arrived at the trap. Now complete the case with ramp-up from 20 to 80 Kg/s over 30 minutes and simulate for a total period of 24 h. Calculate the require surge capacity in the slug catcher using the spreadsheet generated for the Slug Tracking exercise. Assume a drain rate equivalent to 1.2 times the volumetric liquid flowrate at the pipeline outlet for a production flowrate of 100 kg/s, (from the steady state simulations). (The additional capacity represents the over design inherent in any control valve.) Add the variables UPIG and ZZPIG to the TREND plot file.
FA WITH OLGA – GAS CONDENSATE EXERCISE You should also use the PLOT keyword to allow the hold-up profile in the pipeline to be visualised during the pigging operation.
4.4 Turndown Ramp-up You are also required to determine the surge volume generated as a result of a turndown and subsequent ramp-up operation. Create a new case and run at a steady state flowrate of 100 kg/s for 1 hour. Reduce the flowrate to 40 kg/s over a 30 minute period and run for a further 48 hours. Increase the flowrate again to 100 kg/s over a further 4 hour period and run for more 8 hours. The total simulation time should be 61.5 h. Be sure to adjust plotting frequencies and DTOUT in the OUTPUT in order to avoid generating large output, trend and profile plot files. Determine the size of the liquid surge volume required at the pipeline outlet and compare with the pigging case? What can be done to reduce the required surge volume.
FLOW ASSURANCE WITH OLGA – WATER EXERCISE 5.
THREE PHASE FLOW – WATER MODULE The water option is used in this case to determine the effect of water accumulation in the pipeline at a production flowrate of 60 kg/s. Create a new project under the gas Condensate directory called {Three Phase.opp}. Open the case {Simplified.opi}, create a duplicate case called {Water 60.opi} and remove the original case from the project to avoid inadvertent editing. Change the inlet flowrate to 60 kg/s. OPTIONS should be revised to reflect a three-phase flow problem. The WATEROPTIONS FA-model shall be activated with the WATERFLASH ON, the DISPERSIONVISC ON and the WATERSLIP ON.
Use the fluid properties file threephase00.tab which you made in the PVTsim exercise (remember to modify the label in the FILES keyword and change the FLUID label for the flowpath). The WATERFRACTION in the SOURCE should be set to 0.05, (i.e. the inlet fluid contains 5% by weight of free water ignoring any saturated water in the gas phase). The pressure at the outlet boundary should be reduced to 715 psia. The integration time should be set to 2.5 days. Make sure plotting frequency is reasonable. Add the plotting variable total water content in the flowpath (WATC) to the TREND plot keyword and water volume fraction (HOLWT) to the PROFILE plot. Also check "MASS SOURCE INFORMATION" in the output file (*.out). Save the case with a different name and change TOTALWATERFRACTION to -1 in the source, i.e. the total amount of water in the inlet fluid is determined from the free water in the PVT table plus any saturated water in the gas phase. Re-run the case, compare the results and explain the difference.
FLOW ASSURANCE WITH OLGA – TEST EXERCISE 6.
YOU GET A FAX – RESULTS WITHIN TWO HOURS? A fax is attached and you have to respond within 2 hours. HINTS: The fluid property table must cover standard conditions Create the geometry in the Geometry Editor. Ensure that the following variables are generated for analysis:• PROFILE variables: HOL, ID, QLT, QG • TREND variables: PT at inlet • TREND variables: QLT, QG at outlet The temperature option is UGIVEN and the U-value is set in HEATTRANSFER
FLOW ASSURANCE WITH OLGA – TEST EXERCISE
OIL Ltd
Fax To:
Academy of Petroleum
From:
Dynamics Fax:
+ 47 64 84 45 00
Phone: Re:
Stability of tie-back pipeline
; Urgent
For Review
Pages:
2
Date:
1/18/2009
CC:
Please Comment
Please Reply
Please Recycle
We are considering a 5.6 mile (9 km) tie-back pipeline in relatively shallow waters to one of our existing platforms offshore Abu Dhabi. In the initial production phase the flow is stable, practically single phase oil. Our concern is flow stability as gas breakthrough is expected to occur in year 4 of the production. The GOR is expected to go from 280 to 985 Sft³/Sbl in year 4. We need some preliminary flow stability analysis to establish whether our concern is justified with a GOR of 985 Sft³/Sbl ( ≈ 175 Sm³/Sm³). Some data: Pipeline I.D. = 11.7 in 1st stage separator operating pressure = 840 psia Flowing Well Head Temperature = 175°F The sea floor temperature = 57°F Overall U value = 0.44 Btu/ft²/h/F Design production = 18,000 Sbbl/day Water cut = 0
(≈ 0.297 m) (≈ 58 bara) (≈ 79°C ) (≈ 14°C) (≈ 2.5 W/m²/C) (≈ 2,862 Sm³/d)
Please find attached the reservoir composition (which has a GOR of 1125 Sft³/Sbbl) and a rough drawing of the pipe profile. A quick reply would be highly appreciated. Best Regards
Project coordinator OIL E.P.
FLOW ASSURANCE WITH OLGA – TEST EXERCISE
Fluid analysis Component N2 CO2 C1 C2 C3 IC4 NC4 IC5 NC5 C6 C7+
Mol % 0.69 0.54 54.85 4.85 2.23 2.15 2.44 2.56 5.31 5.57 18.81
C7+ properties: Molecular weight = 350 kg/kmol, Density = 870 kg/m3
80 ft (24 m)
295 ft (90 m)
265 ft (81 m)
131 ft (40 m)
5.6 miles (9 km)
FLOW ASSURANCE WITH OLGA – SEPARATOR EXERCISE 7.
(OPTIONAL) SEPARATORS AND CONTROLLERS Case Description In this exercise we will include a separator with controllers to demonstrate how to combine a pipeline with receiving facilities. We will start from the input file made in the Gas Condensate exercise with a flowrate of 20 kg/s and make the following changes: • •
• •
•
•
Adjust the elevation of the last pipe in the flowline so that it is horizontal. Add an additional pipeline section at the end of the pipeline for the separator. (Hint: This section can have the same diameter as the rest of the pipeline, but the volume should be equal to the separator volume.) The separator is 8 feet in diameter and 20 ft long. Add two 150 m long sections after the section with the separator. These sections will model the separator gas outlet. Add a controller for liquid level control in the separator. The controller should have an amplification factor of 10, and integral constant of 1E10, and a derivative constant of 0. It should hold the level at 25% of the vessel height (Hint: OLGA uses the term HOL to indicate fraction of vessel volume). Add a vertical two-phase separator. OLGA asks for a “train” variable, which in this case is gas, since the geometry we defined downstream of the separator is the gas outlet. Use 3-inch valves for the oil level control valve and emergency drain valve. The backpressure for the two valves should be 300 and 200 psia respectively. Use a CD of 1 for the valves. The “highhigh” level is set at 40% of the volume, while the low level switch is set at 15% of the volume. The emergency dump reset is at 50% of the volume. Add trend plot variables for gas flow at the separator gas outlet (GG), liquid level in the separator (LIQLV), oil mass flow at the separator normal oil drain and the emergency oil drain (GNODHL and GEODHL). Also add the controller output (CONTR) for the liquid level controller, which will give the opening of the oil level control valve.
Run the simulation for 15 hours and observe the liquid level in the separator and the liquid flow out of the separator to see if the level controller is working correctly. Check the controller output for the level controller to determine if the sizing of the liquid drain valve is reasonable.
