companion to OLGA.
Concept and capability
OLGA SS represents a new concept in design of multiphase pipelines … ….. build one model and use it throughout the design life cycle, … without tying up valuable dynamic resources when steady state is sufficient, … to maximise results while
minimising software cost and
unproductive time spent re-working models. FIRST OIL DETAILED DESIGN PRODUCTION CONCEPT/ FEED OPERATIONS FIRST OIL DETAILED DESIGN PRODUCTION CONCEPT/ FEED OPERATIONS
Guide contents
Concept and capability... 1
1 Background ... 3
1.1 Key features ... 4
2 How to use OLGA SS... 5
2.1 Example cases in OLGA SS ... 7
2.1.1 Network solver... 7
2.2 FEMtherm example ... 9
3 Parametric studies ... 10
3.1 A step by step guide... 10
3.1.1 Base case OLGA model... 11
3.1.2 Building the parametric study... 12
3.1.3 Parametric study results ... 17
3.1.4 Multi-variable parametric studies ... 23
3.2 Parametric study examples ... 25
3.2.1 Line sizing... 25
3.2.2 Leaks in pipe ... 26
3.2.3 Fluid arrival temperatures ... 28
1 Background
OLGA Steady State (OLGA SS) builds on more than 30 years of heritage in multiphase flow modelling that has been encapsulated in the OLGA dynamic software of SPT Group. The release of OLGA SS as a stand-alone product recognises that low cost, fast turn around steady state results can be highly valuable in certain phases of project development.
Typical applications of steady state multiphase flow simulations include: 1. Sizing of pipeline options during early field evaluation
2. Screening pipeline thermal design to meet steady state criteria (e.g. minimum arrival temperature to avoid wax formation)
3. Optimisation of well deployment through multiple flow paths, and pairing of co-mingled wells through a given pipeline
4. Optimisation of gas lift deployment
5. Assessing the expected performance of leak detection from pressure-flow instrumentation
OLGA SS reads the same input files as OLGA so the same model, built in the same way, can evolve from early screening through to dynamic simulation, operator training, and on-line pipeline surveillance in real time.
OLGA SS has the same industry-leading 3-phase (gas, oil, water) flow pattern mapping and multiphase flow simulation basis as OLGA. Compositional aspects can be modelled in OLGA SS where appropriate.
Simulations using fixed fluid properties, including black oil simulations, can be completed by reading fluid data from an externally supplied property table file written by a compatible PVT simulation package1, such as Calsep’s PVTSim which is supplied with OLGA SS. For black-oil simulations, OLGA SS is supplied with the Mud Property Table generator from Well Flow Dynamics, which includes the Standing and Vazques & Beggs correlations to allow rapid modelling of production fluids from minimal information (Gas and oil density, and Gas Oil Ratio).
OLGA SS is equipped with a powerful general purpose parametric study facility that allows for evaluation of the impact of changes to one or more model
parameters by adjusting the current case description to explore multiple cases defined by the user. A quick-edit 2-parameter study feature is also included which builds a case study matrix for either single or 2-parameter studies covering the specified range of interest in a series of intervals set by the user. Any variable
specified in the OLGA SS input file is available for cross-comparison amongst the parametric cases, and the individual cases are themselves stored in temporary files for detailed examination should that be necessary.
The advent of OLGA SS removes the need to re-build a pipeline model from another simulator when moving from steady state to dynamic pipeline operational simulations. A substantial step forward for the oil industry is represented in
working smarter to deliver cost savings throughout project life cycle.
1.1 Key features
OLGA SS enables the following multiphase flow aspects and process equipment to be modelled:
• 3-phase flows (gas, oil, water) • Well and pipeline networks
• Compositional tracking (e.g. mixing of streams with different compositions) • Well inflow performance
• FEMtherm (Finite Element thermal modelling) • Bundles and annuli
• Corrosion
• Non-Newtonian fluid behaviour (Complex fluid) • Heat exchanger
The OLGA SS package includes both black oil based and composition based fluid property generation.
The OLGA SS package includes limited functionality versions of two third party software systems for fluid property generation:
• Composition based: PVTsim
2 How to use OLGA SS
OLGA SS is driven from the OLGA Graphical User Interface (GUI). Use of this interface is described in detail in the document ‘OLGA 6 GUI User Manual.pdf’, additionally full details of the keywords are given in the OLGA User Guide. The following keywords are the minimum required to build a case in OLGA SS, and the notes relate solely to their use within Steady State modelling:
Keyword Notes relating to OLGA SS
CASE FILES
OPTIONS Steady State is necessarily set to ON
Compositional option must be consistent with Files keyword INTEGRATION ENDTIME and STARTTIME are overridden and are set to 0s.
Time-step information is ignored but must be set.
GEOMETRY Pipe sectioning should continue to satisfy length ratio of 1: 0.5 - 2 between neighbouring sections.
NODE BRANCH HEAT
TRANSFER
Input must be consistent with the selected Temperature option Boundary If the boundary type is Closed, then a non-zero Source must be
placed in the pipe section next to the Closed boundary (i.e. first or last pipe section). Boundary may be of type Pressure for both pipeline inlets and outlets. The boundary may also be of type Massflow.
SOURCE * Source must be non-zero and may be placed at any location in the pipeline (e.g. to simulate a tee). A positive source implies fluid is flowing into the pipeline. A negative Source implies fluid is flowing out of the pipeline. A negative source may be used to simulate a leak.
PROFILE An arbitrary value must be set for DTPLOT OUTPUT An arbitrary value must be set for DTOUT.
TREND This keyword is required for parametric study, although the entries are arbitrary. An arbitrary value must be set for either NPLOT or DTPLOT.
ENDCASE
* Source is not required to fully define an OLGA SS case if the boundaries are specified as being Pressure Boundaries. Similarly, a WELL (inflow relationship) may be used in place of a Source.
MATERIAL Value must be specified for heat capacity, thermal conductivity, and density of each material.
WALL Wall layers should be discretised such that layer thicknesses of neighbouring layers are generally in the range 1: 0.5 – 2.
As with OLGA, heat transfer is considered to be 1-dimensional axi-symmetric. Buried pipelines have asymmetric heat transfer to the sea. Traditionally, this aspect has been ignored and a radial soil layer has been incorporated into the model. The thickness and properties of that soil layer may be adjusted to give a better match to the heat transfer for a buried pipeline.
However, the correct heat transfer may be calculated by using FEMTherm - a module which links to OLGA SS to solve the 2-dimensional heat conduction using a finite element grid of the soil in the vicinity of the pipeline at each pipe section position. FEMtherm can also be used to examine heat transfer within a pipeline bundle.
A case that makes use of FEMtherm requires, as a minimum, the following additional keywords:
Cross-section
Used to define the location of pipelines within the shape. An arbitrary value must be specified for DTPLOT and DELTAT Shape The size and shape of the overall FEMtherm region. A circle is
used to model bundles and wells; a rectangle is used for modelling buried pipes.
2.1 Example cases in OLGA SS
2.1.1 Network solverOLGA SS solves gathering networks of wells and pipelines where the boundary conditions may be a combination of pressure boundaries, fixed flow rate
boundaries and well inflow performance boundaries. However, it should be appreciated that inclusion of multiple pressure boundaries or wells reduces the ability of the model to solve successfully, and multiple solutions may exist (dynamic simulation would be required to establish whether or not the solutions are stable).
The model shown below describes a network of 20 wells completed to a sub-sea manifold which has twin 12″ 20 km pipelines tied back to 125m risers which are co-mingled at surface to supply production fluid to a common first stage
separator. In this example, 1 well is described by a pressure boundary, and the remaining 19 by fixed flow rate sources.
When the steady state model is solved, one of the key items of interest is load balancing between the twin pipelines (for example to maximise oil delivery). This can be achieved either manually or through parametric studies which route each well in turn to either one or other of the tie-back pipelines.
The other key concern in network solving is to identify any wells that are being stifled by the co-mingling selection. Backflow is also possible into any well that is represented in the model by a pressure boundary or by a well with an injectivity specified.
Gas lift can be included and optimised through parametric studies. If the gas lift distribution network is to be modelled, then valve performance curves can be included to correctly model the pressure in the network (which determines the allocation of gas between wells) and to correctly capture the position(s) at which gas enters the well through the gas lift valves.
Use of the compositional description within the model enables fluid properties of the co-mingled lines to automatically adjust depending on the flow rate from the various wells. Key components such as CO2, H2O and H2S can also be tracked
for corrosion and materials specification purposes. CO2 corrosion rates can be
calculated by OLGA using the three corrosion models that are included within OLGA.
Topside manifold to common separator
Twin 12″ 20 km tie-back pipelines and 125m
Sub-sea manifold routing wells to either
2.2 FEMtherm example
As with OLGA, the FEMtherm module can be used to solve for asymmetric heat transfer by using a finite element mesh external to the pipeline. Multiple pipelines can be included within the FEMtherm grid. FEMtherm is applicable when
designing bundles or considering accurate representation of buried pipelines. The results can be viewed using the FEMtherm viewer, which can be used to illustrate temperature through sections either radially through the pipe, or axially along the pipe. FEMtherm viewer can also be used to animate the results in a given cross-sectional slice as it is moved along the length of the pipeline.
Temperature, °C Axial section through the pipeline
Radial cross-section through the pipeline, at the pipeline inlet end
Line plots at defined length positions
3 Parametric
studies
The parametric study capability within OLGA SS automatically clones the current case and varies the selected parameters according to the parametric study definition.
Parametric studies in OLGA SS are likely to involve the following keywords:
Keyword Likely uses in OLGA SS parametric studies
Material Insulation material thermal conductivity Wall Insulation layer thickness
Pipe Diameter, roughness
Boundary Inlet and outlet pressure specification.
Specification of temperature, gas fraction and water cut at pressure boundaries.
Heat transfer
U-value
Sensitivity to ambient conditions Source Specified flow rate or Leak rate
Fluid gas fraction or GOR Water cut
Inlet temperature
Inlet pressure (with flash to pipeline pressure) Valve Opening
Cross-section
Burial depth when using FEMtherm
Other keywords are available for use within the parametric study tool but are not listed here since there use would be uncommon.
3.1 A step by step guide
This example concerns gas flow from an offshore platform to onshore facilities. Due to seasonal and diurnal demand swings, the arrival pressure can vary in the range 30 – 70 bara. The ability of turning up or turning down the flow due to changing onshore requirements by altering the supply pressure at the platform should provide an understanding of the steady state hydraulics of the proposed line.
There are also concerns regarding the thermal aspects of the pipeline. Due to seasonal changes, the temperature of the seawater ranges from 4 °C to 20 °C, hence posing a potential hydrate problem as the fluid is predominantly gas with a small percentage of water. The temperature profile of the fluid along the line during normal operations would be of importance at various ambient conditions.
3.1.1 Base case OLGA model
In order to perform a parametric study, a base case OLGA SS model has to be built.
A brief description of the base case OLGA model, ‘flow.opi’, is presented here. Detailed model build using the OLGA Graphical User Interface (GUI) can be found in the “Getting Started with OLGA” Guide.
The OLGA model consists of a 20-inch single branch flowline bounded with a pressure boundary at the platform end, and a second pressure boundary at the onshore end. The steel pipeline is insulated with a layer of FBE and concrete, and is exposed to seawater at an ambient temperature of 10 °C. The onshore facilities are designed for a minimum arrival pressure of 30 bara, while the fluid enters the line at 50 °C. If the inlet pressure is specified to be 50 bara, then the resulting gas flow rate is 95 kg/s (340 MMscf/d).
The fluid table ’fluid.tab’ describes a two-phase mixture (predominantly gas), with an approximate 2 molar % of water.
3.1.2 Building the parametric study
This section describes the steps for defining a two-parameter study in the OLGA GUI, running it and obtaining the results. The parameters to be studied are the boundary pressure at the platform and the ambient temperature of the seawater.
Parameter Base case value Values
Supply pressure at platform 50 bara 40, 60, 70 bara Ambient temperature of water 10°C 4, 15, 20 °C The procedure is as follows:
1. From the ‘Tools’ menu select ‘Parametric Studies’, and then ‘Add Study…’ to open the Parametric Study input form.
2. The number of parameters is entered in the field ‘#Parameters:’. The parameters are then selected by right-clicking on the column title header, and then selecting ‘Select Parameters..’.
3. A description can be given for the study by entering text in the ‘Description:’ field.
4. The parameters are specified by first selecting the relevant ‘Flowpath’, ‘Node’ or, Library element from the upper drop-down menu; and then selecting the keyword from the list in the lower left pane. Keywords are added to the selection by highlighting on the row and then using the ‘>’ button to move the parameter to the right side pane.
5. Parameters are selected after the keyword has been specified by right clicking on the second column header and choosing the desired
parameter. Note that only parameters that are applicable are
selectable, while those that are not available are shown with grey text.
Steps 2 – 4 are repeated for the second parameter. In the example given the inlet pressure is selected by choosing the ‘NODE : Platform’, then selecting ‘PARAMETERS’, and finally choosing ‘PRESSURE’. 6. The values of the parameters to be evaluated are input by clicking the
entered: 40, 60, 70 (comma separated) [bara]. The ambient
temperatures are then entered: 4, 15, 20 [C]. The respective units are selected by the text ‘[unit]’ and selecting from the drop-down menu.
7. The name of each case in the parametric study can be defined by selecting a format from the ‘Decoration:’ drop-down menu. If the pressure and ambient temperature were to be included in the case name then the format ‘(V1)%1,(V2)%2’ could be selected, and then changed to ‘Tamb %1 C, Pin %2 Bara’.
8. It is not necessary to define the units on each row for each parameter, as the units are specified for each column. If units differ for a given row then the correct units could be selected from the corresponding drop-down menu for that row.
9. The definition of the two-parameter study matrix has now been completed!
10. The ‘Run Study’ button is pressed to perform the matrix of steady state simulations. Each parametric study model input (*.sinp) and output files (*.tpl, *.ppl, *.rsw, and *.out) are stored in a new folder of the same directory that the case file (*.opi) is located in. For a case ‘flow.opi’ a folder named ‘flow_Study[1]’ would be generated for the first
parametric study added to the case.
11. The cases are highlighted as the simulations run. The ‘Case’ row colour is orange while the case is in queue to run, and then light blue
3.1.3 Parametric study results
The results of the parametric study can now be viewed. Results can be viewed from the parametric study window by clicking the ‘Trend Plot’, ‘Profile Plot’, or ‘XY Plot’ buttons.
1. Profile plot is then selected from the Parametric Study menu where the values of the pre-defined variables at steady state (time = 0) along the pipeline are plotted. These variables are specified in the PROFILE keyword in the base model. It should be noted that an arbitrary data collection interval must be defined using DTPLOT. Similarly, arbitrary values for DTOUT in the OUTPUT keyword and DTPLOT in the TREND keyword must be defined.
More definitions and descriptions of available variables are detailed in the OLGA user guide.
2. Profile curves can be defined by selecting variable to be plotted for each case in the parametric study. A single case can be selected, or multiple cases can be plotted on the same graph. The outputs to be plotted can be filtered by selecting the desired case, branch, and variables to be displayed in the selection pane by ticking the boxes in the ‘File’, ‘Variable’, and ‘Branch’ panes of the ‘Filter’ section.
3. For example, by setting pressure to be the primary variable, the fluid temperature profiles as a function of position along the line at a supply pressure boundary of 60 bara at ambient temperatures of 4, 15 and 20°C could be plotted. The following plot is returned when the above selections are made and the ‘OK’ button is clicked, indicating that when the ambient temperature increases, the temperature gradient between the fluid and the surroundings decreases, and hence, less heat is lost from the fluid to the environment.
4. Similarly, with TAMBIENT as the primary parameter, the total mass flow rates in the line at a seawater temperature of 15 °C for various supply pressures of 40, 60 and 70 bara can be obtained.
5. An additional axis, the horizontal (or alternatively the pipeline) length, can be considered when the XY Plot (and subsequently from Profile variable) is selected from the Parametric Study Menu. Here, the selected variable can be plotted as a function of either the supply pressure or ambient temperature (defined as the primary parameter) at a fixed position along the flowline.
6. For example, the total mass flow rate is presented as a function of supply pressure at various ambient temperature defined. As the supply pressure increases, the pressure drop across the line increases
correspondingly (fixed arrival pressure), and hence, the mass flow increases, with the dimension of the pipeline remaining constant. For such study, it would be advisable to define a larger number of pressure points when building the parametric study so as to obtain a total mass flow rate vs. pressure relationship with a higher resolution. As only three pressures were defined in this simple study, the pressures between the discrete points are linearly interpolated.
7. Similarly, the liquid holdup as a function of ambient sea temperature at approximately middle of the flowline (8821 m from the platform) at various supply pressures can be obtained.
exported. First, click the ‘Export’ button. Next, paste the data into the other software (Microsoft Excel, Notepad, etc.).
3.1.4 Multi-variable parametric studies
The method described above provides a straightforward approach to cloning a matrix of cases in 2 parameters (of up to 25 x 25 cases). In some instances it is more appropriate for several parameters to vary simultaneously. This can be achieved by increasing “#Parameters:’.
1. A multi-variable parametric study can be carried out from the OLGA SS GUI. A maximum of 25 parameters can be specified. Values of each parameter are entered for each case and the individual case names are defined by the user. Building and running the cases is identical to that described for the two-parameter study described above.
3.2 Parametric study examples
3.2.1 Line sizingThis example consists of a single pipeline branch. The parametric study is used to determine the maximum throughput (of a given 2-phase fluid) as a function of pipeline internal diameter and pressure drop.
In this example, the inlet pressure is fixed at 150 bara while the outlet pressure is varied in the range 100 – 140 bara, giving pressure drops of 10 - 50 bar. The results show that for a 5,000 bbl/day development, either the 10 or 12 inch
pipeline size would be appropriate depending on the pressure drop which can be accommodated.
Parametric plot to establish liquid throughput for various pipe size options
3.2.2 Leaks in pipe
This example case investigates the pressure profiles along a pipeline when a leak is present near the outlet. In OLGA-steady state the leak is represented using a negative mass source, and for this example the boundaries are defined as pressure boundaries.
A parametric study can be defined to examine the behaviour of the pipeline with varying leak rates (0, -10, -20, -30 and -40 kg/s). The pressure and mass flow rate profiles at the corresponding leak rates can be obtained.
3.2.3 Fluid arrival temperatures
Often in pre-FEED design, a simple U value is used for understanding the range of fluid temperature in the pipeline. This is useful for gaining a broad
understanding of the thermal insulation required to ensure that the fluid temperature remains above a desired value – such as the wax appearance temperature. This example case investigates the arrival temperature of the fluid when different overall heat transfer coefficients (U) are used in the steady state heat transfer calculations.
Here, the ambient temperature is assumed to be 0 °C, and the U-Values in this study range between 0 and 16 Wm-2K-1. As the U-Value increases, there is more heat transfer between the fluid and the ambient conditions, and hence, the arrival temperature of the fluid decreases.
4 Current limitations of OLGA SS
SPT Group is continuing to extend the functionality of OLGA SS to match the wide range of simulations that can be conducted using OLGA. Future releases of OLGA SS will include the following functionalities that are already available in OLGA for dynamic simulations:
OLGA functionality
Current status in OLGA SS
Suggested work around
Check-valves Ignored To prevent reverse flow into a dead-leg, specify a small positive mass source (e.g. 0.001 kg/s).
Leaks Ignored Model using a negative source Counter-current flow Gas, oil and water flow
must have same direction
No work around.
In-line booster pumps Centrifugal and simple pump work included Displacement pump and pump battery are ignored.
For displacement pump and pump battery, undertake separate modelling of the pipeline upstream and
downstream of this equipment. Separators Separators are ignored
(flow downstream remains 2-phase)
Use separate branches to model the inlet and outlet flowlines of a separator. Each branch should use a different fluid definition to reflect single-phase gas/liquid flow in the outlet branch of the separator. Controllers • Over-ride and
Manual controllers function as
expected.
• The Bias setting is used for PID controllers.
• PSV is treated as being closed. • ESD is treated as
being opened.
Control action is necessarily associated with dynamic simulation. The configuration here enables a steady state model to be configured in readiness for use in OLGA dynamic simulations.
The following aspects of multiphase flow are necessarily associated with transient flow phenomena and are therefore not appropriate to OLGA SS: Slugtracking, Pigging, Wax phase formation and build-up. The Initial conditions keyword is also necessarily ignored.
