Olga Ss Guide

OLGA Steady State is SPT Group’s steady state modelling tool for multiphase flow - a fully compatible companion to OLGA.

CONCEPT/ FEED

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,

PRODUCTION

OPERATIONS

… 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

Using OLGA Steady State Guide contents Concept and capability........................................................................................................ 1 1

Background ................................................................................................................. 3 1.1 Key features ........................................................................................................ 4

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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

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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

4

Current limitations of OLGA SS .............................................................................. 29

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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 package 1 , 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 1

OLGA SS is able to read fluid property tables written by the following PVT packages: Calsep’s PVTsim, Infochem’s Multiflash & Well Flow Dynamics Mud Property Table generator.

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Using OLGA Steady State 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 Black Oil based: Mud Property Table

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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 CASE FILES OPTIONS INTEGRATION GEOMETRY NODE BRANCH HEAT TRANSFER Boundary

SOURCE *

PROFILE OUTPUT TREND

Notes relating to OLGA SS

Steady State is necessarily set to ON Compositional option must be consistent with Files keyword ENDTIME and STARTTIME are overridden and are set to 0s. Time-step information is ignored but must be set. Pipe sectioning should continue to satisfy length ratio of 1: 0.5 - 2 between neighbouring sections.

Input must be consistent with the selected Temperature option 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 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. An arbitrary value must be set for DTPLOT An arbitrary value must be set for DTOUT. 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. If the temperature option is set to ‘WALL’, then the following Keywords are required to fully define the OLGA SS case:

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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: Crosssection Shape

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Used to define the location of pipelines within the shape. An arbitrary value must be specified for DTPLOT and DELTAT 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.

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2.1 Example cases in OLGA SS 2.1.1 Network solver OLGA 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.

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Topside manifold to common separator

Twin 12″ 20 km tie-back pipelines and 125m

Sub-sea manifold routing wells to either

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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

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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 Material Wall Pipe Boundary

Heat transfer Source

Valve Crosssection

Likely uses in OLGA SS parametric studies Insulation material thermal conductivity Insulation layer thickness Diameter, roughness Inlet and outlet pressure specification. Specification of temperature, gas fraction and water cut at pressure boundaries. U-value Sensitivity to ambient conditions Specified flow rate or Leak rate Fluid gas fraction or GOR Water cut Inlet temperature Inlet pressure (with flash to pipeline pressure) Opening 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.

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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).

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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 Supply pressure at platform Ambient temperature of water

Base case value 50 bara 10 °C

Values 40, 60, 70 bara 4, 15, 20 °C

The procedure is as follows:

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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.

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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 ‘Matrix..’ button, and then entering each value in the field to the right of the respective parameter. The supply pressures to be investigated are

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Using OLGA Steady State 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.

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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 after the case simulation is complete.

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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. Variables for a given case and branch are then selected by ticking the boxes in the upper pane of the window. The plot is generated by clicking the ‘OK’ button in the lower right corner of the window.

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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.

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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.

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Using OLGA Steady State 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.

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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.

8.

In order to create 3-dimensional plots in other software (such as Microsoft Excel), the profile of some selected variables can be

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Using OLGA Steady State exported. First, click the ‘Export’ button. Next, paste the data into the other software (Microsoft Excel, Notepad, etc.).

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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.

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3.2 Parametric study examples 3.2.1 Line sizing This 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

Outlet Pressure, bara (inlet pressure = 150 bara)

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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.

Leak location

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Leak location

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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.

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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 Check-valves

Leaks Counter-current flow

In-line booster pumps

Separators

Controllers

Heated walls

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Current status in OLGA SS Ignored

Ignored Gas, oil and water flow must have same direction Centrifugal and simple pump work included Displacement pump and pump battery are ignored. Separators are ignored (flow downstream remains 2-phase)

• 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. Subkeys POWER etc in WALL are ignored.

Suggested work around To prevent reverse flow into a dead-leg, specify a small positive mass source (e.g. 0.001 kg/s). Model using a negative source No work around.

For displacement pump and pump battery, undertake separate modelling of the pipeline upstream and downstream of this equipment. Use separate branches to model the inlet and outlet flowlines of a separator. Each branch should use a different fluid definition to reflect singlephase gas/liquid flow in the outlet branch of the separator. 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.

No work around.

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Using OLGA Steady State 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.

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