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Transient Multiphase

Flow Simulator

Enabling reliable and efficient flow of production fluids

OLGA

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MANUAL

Transient Multiphase Flow Simulator

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CHAPTER 1 INTRODUCTION

1. INTRODUCTION...2 1.1 General Information ...2 1.2 Potential Applications ...4 1.2.1 Pipeline Design ...5 1.2.2 Pipeline Operation...5 1.2.3 Safety Analysis...6

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1. INTRODUCTION

1.1 General Information

OLGA was originally developed as a dynamic one dimensional modified two fluid model for two-phase hydrocarbon flow in pipelines and pipeline networks, with processing equipment included. Later, a water option was included which treats water as a separate liquid phase.

OLGA was originally based on the computer program OLGA 83, developed by IFE in 1983 for the Norwegian State Oil Company, Statoil.

Since 1984, OLGA has been improved continuously due to the experimental data-base from the large scale two-phase flow laboratory at SINTEF and extensive use and numerical testing at IFE and in the oil companies involved. In the later years, more two-phase and three-phase field data have become available for the testing of OLGA.

The dynamic capability of OLGA is its most important feature. Multiphase flow is a dynamic phenomenon and should be modelled as such. This dramatically increases the range of applicability compared with steady state models. OLGA is capable of dynamic simulation of pipeline networks with process equipment such as compressors, pumps, heat exchangers, separators, checkvalves, controllers and mass sources/sinks.

OLGA has full network capability, that is, it handles both diverging and converging networks. Complete topside process systems can therefore be simulated, e.g., a system where several flowlines are connected to one manifold, which is connected to parallell separators that have compressor trains, separator trains and water drains further downstream.

Computing a transient multiphase flow situation with a dynamic model requires increased CPU-time expenditure compared with ordinary steady state models. The additional time dimension also increases the amount of output produced by the code. The dynamic feature of the program imposes additional requirements on the user, compared with steady state models, but the results of the transient program are significantly more useful in design of the pipeline and its attendant facilities than steady state methods.

A steady state pre-processor is also included in OLGA, where the steady state conservation equations are solved. Although it may be used independently, it is mainly intended as a generator of initial values for dynamical simulations.

OLGA is a modified two-fluid model, i. e. separate continuity equations for the gas, liquid bulk and liquid droplets are applied; these may be coupled through interfacial mass transfer. Only two momentum equations are used; one for the continuous liquid phase and one for the combination of gas and possible liquid droplets. The velocity of any entrained liquid droplets in the gas phase is given by a slip relation. One mixture energy equation is applied; both phases are at the same temperature. This yields six conservation equations to be solved: three for mass, two for momentum, and one for energy.

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With the water option, continuity equations for bulk water and water droplets are added. The bulk water velocity is obtained from a correlation for water velocity relative to the average liquid bulk velocity.

Two basic flow regime classes are applied; distributed and separated flow. The former contains bubble and slug flow, the latter stratified and annular mist flow. Transition between the regime classes is determined by the program on the basis of a minimum slip concept combined with additional criteria.

To close the system of equations, boundary and initial conditions are required. The specification of initial conditions is a fundamental difference between transient and steady state model where these are not required. The user decides, and later specifies in the input, whether the simulation is to start with an empty, filled, or shut-down pipeline, or from full production. The steady state initial pre-processor in OLGA can be used to provide reasonable initial values. In addition, the restart capability may be used to start with data saved from a previous simulation.

The boundary conditions define the interface between the pipeline system and its surroundings. There are several options available, but basically either flow rate or pressure must be specified at each pipeline inlet and outlet boundary.

Due to the numerical solution scheme, OLGA is particularly well suited for simulating rather slow mass flow transients. The semi-implicit time integration implemented allows for relatively long time steps, orders of magnitudes longer than those of an explicit method (which would be limited by the Courant Friedrich Levy criterion based on the speed of sound). This is important for the simulation of very long transport lines, where typical simulation times in the range of hours to several days will require long time steps, to have efficient run times.

The necessary fluid properties (gas/liquid mass fraction, densities, viscosities, enthalpies etc.) are normally assumed to be functions of temperature and pressure only, and have to be supplied by the user as tables in a special input file. Thus, the total composition of the multiphase mixture is assumed to be constant both in time and space for a given branch. The user may specify different fluid property tables for each branch, but has to ensure realistic fluid composition when several pipeline branches merge into one. It is also possible to perform a simulation using compositional tracking, where the compositional data is provided in a separate feed file and the code calculates the fluid properties internally. This means that the total composition may vary both in time and space, and that no special consideration is needed for a downstream branch.

The purpose of this manual is to assist the user in the preparation of the input data for an OLGA simulation. Chapter 2 gives an overview of the required and the optional input to OLGA. It also describes in some detail, the different modules for simulating process equipment, slugtracking and water as a separate phase. A detailed description of all input data and the required fluid property tables can be found in chapters 3 and 4. The output is described in chapter 5.

The user is also advised to examine the input variable definitions in the comment records in the sample cases given in the appendices.

The sample cases presented in appendices are intended to illustrate important program options, but they can also serve as benchmark during program installation.

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OLGA comes in a basic version with a number of optional modules. These modules are the Water module, the Slugtracking module, the Bundle module, the Soil module, the FEMTherm module, the Multiphase pumps module, the Corrosion Module, the Advanced Well module, the Wax Deposition module, the MEG Tracking module, the Compositional Tracking module, the Complex Fluid module, the Tuning module, and the Server module. In addition there is a number of additional programs like the OLGA GUI and the FEMThermViewer for preparation of input data and visualisation of results. These are available to the user according to the user's licensing agreement with Scandpower Petroleum Technology. The modules are described in the following chapters.

1.2 Potential Applications

The dynamic multiphase flow model in OLGA has a wide range of applications. The model is basically verified against data from the SINTEF Multi-Phase Flow Laboratory, IFE’s small/medium size high density flow loops, and field data, in addition to small scale laboratory data. The applied numerical method makes it particularly well suited for simulation of flow transients. This was also the original motivation for the development of the model: available steady state models are of very limited use for the design engineer who is considering two-phase fluid transportation in pipelines.

An important example of an unstable flow situation of great practical importance is the occurrence of slug flow. Two types of slug flow are recognised in OLGA; hydrodynamic or "normal" slug flow, and terrain induced slug flow. The terrain induced slugs are created by the accumulation of liquid at low points in the pipeline, in dips or bends, and may be many orders of magnitude longer than the slugs occurring in hydrodynamic slug flow. Slugs represent a serious challenge to the design and operations of the receiving process equipment. The OLGA slugtracking module can follow the growth or decay of each individual slug.

Some highly relevant applications of OLGA are briefly presented below. The discussion that follows is based on experience from the use of OLGA in the oil and gas industry during the last years.

Typical systems that OLGA may be applied to, are:

- Oil and natural gas flowlines or transportation lines - Wet gas or condensate pipelines

- Well stream from a reservoir - LNG/ LPG/ NGL pipelines - Dense phase pipelines

- Network of merging and diverging pipelines - Artificial lift and other mass source injections - Pipelines with process equipment

- Single phase gas or liquid

- Small diameter pipelines with various fluids - Laboratory experiments

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Three major fields of application are pipeline design studies, operational studies and safety analysis.

Chapter 6 contains a list of papers describing the OLGA model and its applications.

1.2.1 Pipeline Design

OLGA is a powerful instrument for the design engineer who is considering different concepts for fluid transportation in pipelines, e.g. oil and gas from a subsea production well. The dimensions and layout of a pipeline must be optimised under given restrictions. The limiting conditions may be the available total pressure drop, a time varying field production rate, a minimum temperature that has to be avoided (e.g. due to the formation of hydrates), a flow regime that gives high pipe corrosion or erosion, an outlet receiving capacity limitation or the occurrence of terrain induced slugs.

For example, in a flow capacity study, an undersized pipe will give a prohibitively high pressure drop, and a critical erosional velocity may be reached at some point in the pipeline. Conversely, if the diameter is too large, terrain slugging might occur. A possible suppression of terrain slugging can be studied with available process equipment in OLGA, such as chokes, compressors, check valves, etc. If terrain slugging is allowed, OLGA can indicate the transients the system must withstand (e.g. the value of the largest pressure peak) and the required capacity of the outlet slug catcher. Thermal calculations may help when making decisions regarding pipe material and wall thickness, and whether the pipe will have to be insulated or buried at the sea bottom.

Simulation of different pipe network configurations is also possible. OLGA may be used in the design of merging flow lines from different production wells with different fluids into a manifold that is connected to several multiphase transport lines.

1.2.2 Pipeline Operation

OLGA can be of great assistance in defining the operational strategies of a multi-phase flow system. Consequences of changes in operating conditions are difficult to foresee but can be predicted with OLGA. Some typical events during operation of oil and gas pipelines that can be simulated with Olga, are discussed below.

Pipeline shut-down

If the flow in a pipeline for some reason has to be shut down, different procedures may be investigated. The dynamics during the shut-down can be studied as well as the final conditions in the pipe. The liquid content is of interest as well as the temperature evolution in the fluid at rest since the walls may cool the fluid below a critical temperature where hydrates may start to form.

Pipeline start-up

The initial conditions of a pipeline to be started is either specified by the user or defined by a restart from a shut-down case. The start-up simulation can determine the evolution of any accumulated liquid slugs in the system. A start-up procedure is often sought whereby any terrain slugging is minimised or altogether avoided. The slugtracking module is very useful in this regard.

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In a network case a strategy for the start-up procedure of several merging flow lines could be particularly important.

Change in production

Sometimes the production level or type of fluid will change during the lifetime of a reservoir. The modification of the liquid properties due to the presence of water, is one of the important effects accounted for in OLGA.

A controlled change in the production rate or an injection of another fluid are important cases to be simulated. Of particular interest is the dynamics of network interactions e.g. how the transport line operation is affected by flow rate changes in one of several merging flowlines.

Process equipment

Process equipment can be used to regulate or control the varying flow conditions in a multi-phase flow line. This is of special interest in cases where slugging is to be avoided.

The process equipment simulated in OLGA includes critical- and subcritical chokes with fixed or controlled openings, checkvalves, compressors with speed and anti- surge controllers, separators, heat exchangers, pumps and mass sources and sinks.

Pipeline pigging

OLGA can simulate the pigging of a pipeline. A user specified pig may be inserted in the pipeline in OLGA at any time and place. Any liquid slugs that are created by the pig along the pipeline can be followed in time. Of special interest is the determination of the size and velocity of a liquid slug leaving the system ahead of a pig that has been inserted into a shut-down flow line.

Hydrate plugs

A simple model for hydrate plug growth and release is included in OLGA. Based on user specified hydrate formation rate as a function of fluid temperature and other characteristic hydrate data, the growth, the release, and the transport of hydrate plugs can be simulated.

Drilling

Both conventional and underbalanced drilling (UBD) can be simulated in OLGA. A drilling path must be predefined, and then the drilling operation with reservoir interactions and varying penetration rate is performed. The Underbalanced Interactive Transient Training Simulator (UBitTS) is a stand-alone tool delivered by Scandpower Petroleum Technology that is specially adapted for simulating and visualising this operation.

1.2.3 Safety Analysis

Safety analysis is an important field of application of OLGA. OLGA is capable of describing propagation of pressure fronts. For such cases the time step can be limited by the velocity of sound across the shortest pipe section.

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OLGA may be useful for safety analysis in the design phase of a pipeline project, such as the positioning of valves, regulation equipment, measuring devices, etc. Critical ranges in pipe monitoring equipment may be estimated and emergency procedures investigated.

Consequence analysis of possible accidents is another interesting application. The state of the pipeline after a specified pipe rupture or after a failure in any process equipment can be determined using OLGA.

Simulations with OLGA can also be of help when defining strategies for accident management, e.g. well killing by fluid injection.

Finally it should be mentioned that the OLGA model is well suited for use with simulators designed for particular pipelines and process systems. Apart from safety analysis and monitoring, such simulators are powerful instruments in the training of operators.

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CHAPTER 2 INPUT DESCRIPTION

AND PROGRAM EXECUTION

2. INPUT DESCRIPTION AND PROGRAM EXECUTION...10

2.1 General Input Structure...10

2.1.1 Input file 1/General Rules ...10

2.1.2 Input File 1/Data Structure...10

2.1.2.1 Case information and execution modes ...11

2.1.2.2 Auxiliary information ...13

2.1.2.3 Geometrical system definition...13

2.1.2.4 Boundary and initial conditions ...14

2.1.2.5 Compositional model ...15

2.1.2.6 Process equipment ...15

2.1.2.7 Output options ...16

2.1.3 Input Data Dependency...17

2.1.4 Description of Input file 2/ Fluid Properties...17

2.1.5 Input file 3/Restart ...19

2.1.6 Input file 4/Compressor Data...19

2.1.7 Input file 5/Pump Data ...19

2.1.8 Input file 6/Wax Data ...19

2.1.9 Input file 7/Hydrate Curve Data ...20

2.2 Process Equipment...20 2.2.1 Separator...20 2.2.2 Compressor ...26 2.2.3 Controllers ...33 2.2.4 Leak...44 2.2.5 Mass Sources...44 2.2.6 Plug or Pig...48 2.2.7 Heat Exchanger...53 2.2.8 Check Valve ...53 2.2.9 Valves...53 2.2.10 Pumps ...57 2.3 Special Options...65

2.3.1 Slug/Pig Tracking Module...65

2.3.1.1 Slug Statistics ...71

2.3.2 The Water Module ...71

2.3.2.1 Gas-water simulations ...75

2.3.3 Well Description ...76

2.3.4 The Advanced Well Module...79

2.3.4.1 Reservoir inflow ...79

2.3.4.2 Drilling option ...82

2.3.4.3 High pressure displacement pumps (pump battery) ...86

2.3.4.4 Bit nozzles ...86

2.3.4.5 Pipe Upsets ...87

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2.3.4.7 Mud Property Table ...91

2.3.4.8 Gas Lift Valve (GLV)...91

2.3.5 The Complex Fluid Module ...95

2.3.5.1 The slurry plug extension ...95

2.3.6 Thermal Computations ...98

2.3.6.1 Thermally driven natural/free convection in the axial direction.100 2.3.7 Bundled pipelines (Bundle Module)...102

2.3.8 Soil Module...103

2.3.9 The FEMTherm module ...104

2.3.9.1 Limitations and recommendations...107

2.3.10 Corrosion Module ...109

2.3.11 Wax Deposition Module ...113

2.3.12 The MEG tracking module...118

2.3.13 The Compositional Tracking module...120

2.3.13.1 The compositional model in OLGA...120

2.3.13.2 PVT routine package ...121

2.3.13.3 User input to the compositional model ...121

2.3.14 The Black Oil module ...124

2.3.14.1 Black Oil Correlations ...125

2.3.14.2 Thermodynamic properties ...130

2.3.14.3 How to use the Black Oil Module...131

2.4 Program Execution ...132

2.4.1 Core Requirements ...132

2.4.2 OLGA execution ...132

2.4.3 Steady State Processor...132

2.5 Restrictions and Limitations ...133

2.5.1 Fluid Properties ...133

2.5.2 Two-Phase Model Limitations ...133

2.5.3 Vital Numerical Recommendations ...134

2.5.4 Array Size Limitations...135

2.5.5 Input/Output Limitations ...135

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2. INPUT DESCRIPTION AND PROGRAM EXECUTION

2.1 General Input Structure

The input system of OLGA consists of seven files. Input file 1 and 2 are always required while files 3, 4, 5, 6 and 7 are optional. The program execution is described in section 2.3.2.

The first file (input file 1) contains the data particular to a given case such as geometry, operational conditions, output variables etc. The second (input file 2) contains the fluid property tables or the compositional data (for compositional tracking). The third file (input file 3) is a restart file that is used to continue a previous calculation. If a compressor is simulated, the compressor data are given in input file 4, and if a pump is simulated the pump characteristics are given in input file 5. If wax deposition is simulated, the wax data are given in input file 6, and if formation of hydrates is to be detected, a hydrate curve can be given in input file 7. The input files 2, 3, 4, 5, 6 and 7 are referenced in the main input file (1). Most of the files have default extensions that are applied when the files are generated in the OLGA GUI etc., but the code will accept other extensions.

The following is an overview of the input files. An exhaustive description of the input is given in chapter 3.

2.1.1 Input file 1/General Rules

The default extension of this file is inp (e.g., simulation1.inp). The input data are organised in groups of similar physical contents. A data group consists of a keyword name followed by a list of keys with the appropriate data. Some keywords work as switches and require no data: program options are then determined by the keyword only.

Up to 60 characters are used to specify keywords. The user may abbreviate the keywords as long as the keyword can be uniquely defined.

The data in input file 1 is read according to the syntax rules presented in section 3. It is possible to include a data file into the case data input file using a READ-statement preceded by a (%) sign.

Ex: %read geometryl.dat

2.1.2 Input File 1/Data Structure

The input is presented in six parts.

1. Case information and execution modes 2. Auxiliary information

3. Geometrical system definition 4. Boundary and initial conditions 5. Compositional model 6. Process equipment 7. Output options

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2.1.2.1 Case information and execution modes

Case information and execution mode for a particular simulation is specified using the following keywords:

CASE DTCONTROL FILES HYDRATECHECK INTEGRATION OPTIONS RESTART CORROSION SLUGTRACKING PIGTRACKING WAXDEPOSITION WATEROPTIONS SHUTIN TUNING FLUID DRILLINGFLUID ENDCASE

CASE is used for identification of a case by defining project name, title of simulation, author, date and other relevant identification.

DTCONTROL specifies the options for time step control. Two options are available. a. Keep time step less than a theoretical limit for mass flow stability (CFL).

b. Keep time step small enough to keep error of pressure integration below a certain limit.

These options can be ON or OFF for any section or position in the system.

FILES specifies auxiliary data files for fluid properties, and compressor and pump characteristics.

HYDRATECHECK is used to give information on possible formation of hydrates. INTEGRATION is used to specify start and end time of a simulation and limits for time step control, min, max, and initial.

OPTIONS specifies options for temperature calculation, steady-state, pre- and post processing of data and number of phases to be simulated. A number of different calculation options can be selected.

Five different temperature options are available.

1. TEMPERATURE = OFF No temperatures are calculated. Temperatures must be specified with the INITIAL keyword statement.

2. TEMPERATURE = ADIABATIC Adiabatic flow is assumed, no energy exchange with pipe walls.

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3. TEMPERATURE = WALL Calculation of heat transfer between fluid and inner pipe wall surface, heat conduction and heat storage in pipe wall and heat transfer on outer pipe wall surface.

4. TEMPERATURE = UGIVEN A user specified overall heat transfer coefficient is used for calculating heat exchange between fluid and ambient conditions.

5. TEMPERATURE = FASTWALL An overall heat transfer coefficient is calculated from wall data and is used for wall heat transfer calculations. The thermal transient in the wall materials is neglected.

A simulation can be a continuation of a previous case.

RESTART specifies the name of the restart file that contains the data from the previous case. Optionally, it also specifies timepoints for (writing data to) and reading data from the restart file, and for writing to the restart file produced by the current case.

CORROSION specifies the use of the corrosion module. This module is described in section 2.3.10.

SLUGTRACKING and PIGTRACKING specifies the use of the slugtracking module. This module is described in section 2.3.1.

WAXDEPOSITION specifies the use of the wax deposition module. This module is described in section 2.3.11.

In case the three phase option is selected under OPTION, the keyword WATEROPTION can be used to specify the three-phase flow options available. The water module is described in section 2.3.2.

In the case that the WATEROPTION is selected, some numerical oscillations can arise, particularly during shutdown simulations. In order to reduce such oscillations, the keyword SHUTIN can be used during the shutdown period. Within a user defined time period, the flow regime will always be stratified/annular and some non-physical flow regime flipping can be avoided.

The TUNING keyword may be used for tuning certain parameters in the OLGA model to optimise for specific sets of measured data or for sensitivity studies. The tuning parameters available are described in the TUNING section in chapter 3 (3.3.56). This keyword is available both in batch and server mode. Note: TUNING should be applied with great care, as the validation and verification of the OLGA model may not be valid for such cases.

FLUID specifies the use of the module. This module enables simulation of non-Newtonian fluids. Further description in chapter 3 (3.3.20).

DRILLINGFLUID defines a drilling fluid. ENDCASE defines the end of input file 1.

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2.1.2.2 Auxiliary information

For valves and wells, a table can be defined for specifying the flow performance. Such a table can be specified using the keyword TABLE. For a valve, the discharge coefficient, CV, can be specified as a function of the valve opening and for a well, the flow rate can be specified as a function of pressure difference between the reservoir and the well inflow section.

Basically, OLGA works with SI units. The user can, however, select from a given number of units for different physical quantities for both input and output. Should the user wish to use another unit than those predefined, the possibility exists to define such a unit for any of the possible physical quantities, pressure, flow, energy etc. This is achieved by using the keyword UNIT.

For fluid properties and compressor characteristics the selection of units is limited to SI or BRITISH units.

2.1.2.3 Geometrical system definition

The OLGA model accepts a network of diverging and converging branches. Each branch consists of a sequence of pipes and each pipe is divided into sections. These sections correspond to the spatial mesh discretisation in the numerical model.

Each branch starts and ends at a node. There are three different types of nodes: • Terminal (free end) nodes, where boundary conditions must be specified • Split nodes, where branches split

• Merge nodes, where branches are coupled together

A staggered spatial mesh is applied. That is, flow variables (velocities, mass flows, fluxes, etc.) are defined at section boundaries, while pressure, mass, phase fraction, temperature, etc. are average values in section volumes, (refer to Fig. 2.1).

1 2 3 4 5

1,2,3,…,5 (inside) : section volumes 1,2,3,…,6 (outside): section boundaries

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Fig. 2.1 Pipeline discretisation

Each pipe in the system can have a pipe wall consisting of layers of different materials. For the specification of the geometrical network with pipes and pipe walls the following keywords are applied.

Keyword Description

NODE Defines terminal and merging nodes.

BRANCH Defines start and end node, geometry of the branch and fluid name for that branch.

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GEOMETRY Defines name and starting point for a particular sequence of pipes. All pipes defined by keyword PIPE directly thereafter belong to the same geometry.

PIPE Specifies name and end point or length and elevation of a pipe. Further are specified discretisation, diameter, inner surface roughness, and wall name.

POSITION A number of positions with names can be defined for later reference.

MATERIAL Different wall materials with name and properties can be specified.

WALL Different pipe walls with name, radial discretisation and material for each layer can be specified.

REROUTE The outlet of a branch can be re-routed from one node to another during a simulation.

ANNULUS Defines the configuration of pipes that are bundled together and have thermal interaction.

BITNODE Specifies the branches that connect at the drilling bit for drilling simulation

BUNDLE Defines which lines belong to a bundle and which OLGA-pipes it covers.

COVER Modifies properties of cells in a soil group by specifying that the cell is in the sea or contains another material.

CROSSOVER Defines coupling of bundle lines.

CROSSSECTION Defines what lines, bundles, branches and shapes that belong to a cross section.

GRID Specifies a grid that can be used to define a SOIL group. LINE Defines a bundle line (dimensions, fluid and wall properties)

to be used with the bundle module.

SHAPE Describes the external contour of a material.

SOIL Defines a soil group by selecting a grid and main material.

2.1.2.4 Boundary and initial conditions

For the solution of the flow equations, necessary boundary conditions must be specified. All points in the system where mass flow into or out of the system can occur must be specified. Initial conditions must also be either specified or calculated.

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The following keywords can be used to specify the flow of mass and energy into and out of the pipeline system:

BOUNDARY For each terminal node, open or closed condition is specified. For an open node, values for pressure, temperature and mass fractions are specified.

HEATTRANSFER Definition of the heat transfer parameters.

INITIAL Defines initial values for flow conditions in the system for the case when no steady state calculation is performed.

LEAK Defines the position for a leak in the system with leak area and back pressure.

SOURCE Defines mass source with name, position, and data necessary for calculating the mass flow into or out of the system. The source flow can be given by a time series or determined by a controller.

WELL Defines a well with name, position and flow characteristics.

2.1.2.5 Compositional model

For the use of compositional tracking, keywords that define calculation options and feeds (fluids) can be defined.

The following keywords are defined:

COMPOPTIONS Specifies the different options used in the PVT routines for calculating material properties and flashing terms in the compositional module.

FEED Defines a feed and its components with belonging mole fractions.

2.1.2.6 Process equipment

In order to obtain a realistic simulation of a pipeline system, it is normally required to include some process equipment in the simulation.

Various types of process equipment can be simulated. A description of the different process modules can be found in section 2.2.

It should be notified that the steady state pre-processor ignores some of the process equipment. These are marked with an *. However, they can be included in the input for subsequent dynamic simulations.

The following keywords are defined:

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BITNOZZLE Defines pressure loss through the drill bit nozzles.

CHECKVALVE* Defines name, position and allowed flow direction for a check valve.

COMPRESSOR* Defines name, position and operating characteristics of a compressor.

CONTROLLER Defines name, type, controlled variable(s) and characteristic data for a controller.

HEATEXCHANGER Defines name, position and characteristic data for a heat exchanger.

LOSS Defines name, position and values for local pressure loss coefficients.

PLUG* Defines name, starting position and characteristic data for a pig or a hydrate plug.

PUMP* Defines name, type and characteristic data for a pump.

SEPARATOR Defines name, position and characteristic data for a separator.

TOOLJOINT Defines internal and external pipe upsets in the flow path, resulting in correction factor for the wall roughness.

VALVE Defines name, position and characteristic data for a choke or a valve.

SETPOINTVARIABLE Alternative way of defining controlled variables.

2.1.2.7 Output options

The user can control the output by the use of the following keywords.

OUTPUT Defines variable names, position and time for printed output. PLOT Defines variable names and time intervals for writing of data to the

OLGA viewer file.

PRINTINPUT Specifies the printing of input data. In addition to printing of different input data in an edited form, fluid properties and compressor data may be printed.

PROFILE Defines variable names and time intervals for writing of data to the profile plot file.

TREND Defines variable names and time intervals for writing of data to the trend plot file.

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2.1.3 Input Data Dependency

The keyword statements with corresponding key values may be given in an arbitrary order with the following basic limitation. Any reference to information given in another keyword statement requires that this keyword statement has already been specified.

For example, if the keyword UNIT is specified, all keyword statements containing dimensioned quantities using the units specified in a UNIT statement must be given after the UNIT statement.

There are some exceptions to this limitation, e.g., a specific GEOMETRY name can be referred to in a BRANCH statement before it is defined, and a CONTROLLER name can be referred to in the INTEGRATION statement before it is defined. Any keyword statement appearing before a RESTART statement will not be recognised. Any change in input data in a restart run must be specified after the RESTART statement.

The keyword statements that belong to the geometrical system definition cannot be changed in a restart run.

Further dependency between input data are specified in section 3. The following table lists keywords that are always required (R), optional (O) or not allowed (-). Keywords that are optional both with and without restart are not listed.

KEYWORD STATEMENT WITHOUT RESTART WITH RESTART Case information and execution

modes ENDCASE FILES

INTEGRATION RESTART R R R O** R O* R R Geometrical system definition NODE

BRANCH GEOMETRY MATERIAL PIPE WALL R R R O R O - - - - - -

Boundary conditions BOUNDARY R O

Output options OUTPUT R O

* If a PVT table file is not specified in a restart, OLGA will apply the same file as used in the simulation restarted from.

** For definition of time points for writing to restart file.

2.1.4 Description of Input file 2/ Fluid Properties

OLGA requires the necessary fluid properties, defined in Chapter 4, to be given as pre-calculated tables in a special input file, or as component data in a feed file where fluid properties are calculated internally in the code. The latter option, compositional tracking, is described in section 2.3.13, while the rest of this section describes the option with pre-calculated tables. The default extension of a file with precalculated tables is .tab, and .ctm for a feed file.

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In the case of a pipeline network, different fluid properties are allowed in each of the pipeline branches. In this case, however, the user must specify fluid property data for a realistic composition in the merging branch.

The fluid properties required for each phase, gas, oil and water (if 3-phase): - densities

- partial derivatives of densities w.r.t. pressure and temperature - viscosities

- heat capacities at constant pressure and composition - enthalpies

- thermal conductivities

- entropies (optional for critical flow model) Also required are:

- gas mass fraction

- water vapour mass fraction in the gas phase - equilibrium free water mass fraction

- surface tension between each pair of phases

The program itself accepts any fluid provided the tables conform to the specified format in Chapter 4. For example, typical low pressure laboratory air water experiments may be simulated by attaching the proper air-water data file. In this case a different correlation for the void fraction in liquid slugs should be applied and this is achieved by using the OPTION-keyword setting the key parameter SLUGVOID = AIR.

The fluid properties represent a key input to OLGA. They define a significant part of the model of the system to be simulated. Unphysical fluid properties are often the reason for unrealistic simulation results.

The fluid property tables that are supplied as input to OLGA are usually generated from a PVT package. Different programs that determine the fluid properties from equilibrium calculations of a given hydrocarbon composition often differ in results and they may even fail to converge to the correct solution at certain conditions. For this reason the user should carefully examine the fluid property tables to ensure consistency and avoid unphysical values. Among properties that frequently cause wrong simulation results when they are out of range are viscosities, phase densities and phase density partial derivatives. The latter are important in dynamic cases since they determines the "stiffness" of the system and thereby the tendency to develop terrain slugs and the size and frequency of such slugs. As a rule of thumb the pressure derivative of the densities of each phase should correspond to typical values of the inverse of isothermal sound velocities squared.

Another rough consistency check is that the heat capacity at constant pressure should not deviate too much from the change in enthalpies with the temperature in the tables. Another important aspect is the occurrence of a single phase region for at least a part of the pressure and temperature range. It should be checked that properties of the non-existing phase have physically realistic values. The reason is that the non-existing phase may actually occur for such pressure and temperature combinations due to slip effects or phase separation. It is recommended to make 3-D plots of the fluid properties in order to check if something is very wrong with the data.

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When running OLGA, the PVT-data are, to some extent, checked for unrealistic values and if necessary modified before the simulation starts. If such modifications are made, a warning will be written to the print out file and for standard output. Enthalpies can have different zero point depending on the PVT-tool used for the calculations. To avoid problems with negative enthalpy values the tables are checked, and if negative values are encountered a constant is added to all enthalpy values (for all phases) so that the table values become positive. A message about this is written to the output file.

2.1.5 Input file 3/Restart

The default extension of this file is .rsw. The program will always dump the values of the variables necessary to continue the simulations except the fluid property tables, to the restart file. By default this is done at each printout time as specified with the keyword OUTPUT. The file is by default rewound before any write operation, so it contains the values at one particular time, only. Optionally, the user can specify timepoints for writing data to the restart file and turn off the rewinding of the file before writing data. In a restart case the program reads the restart file and execution may continue where the previous case stopped. Optionally, the user can specify from what timepoint to continue a previous case. This is done by specifying the keyword RESTART; the complete state from a previous run is recovered and the run may continue with modifications given in the keyword statements that follow after the RESTART keyword.

The RESTART feature may be used in three different ways: to continue an incomplete previous run, to intentionally subdivide a large job into steps or to simulate a series of major events such as a start, stop, and restart of production.

2.1.6 Input file 4/Compressor Data

If a compressor is simulated, a compressor data file is required. The file contains tables of the compressor characteristics as a function of reduced rotational speed RPM and reduced inlet massflow, see chapter 4. The reason for giving the characteristics as function of reduced values is to obtain one table for several inlet conditions (pressure, temperature) instead of having one table for each inlet condition.

2.1.7 Input file 5/Pump Data

If a pump is simulated, a pump data file can be given. The pump data file contains the pump characteristics and is described in more detail in section 2.2.10 and in chapter 4. The code also contains a default set of pump data tables that can be used if the pump data file is not given.

2.1.8 Input file 6/Wax Data

If wax deposition is simulated, a wax data file is required. The wax data file contains a table with the properties of the wax forming components and is described in more detail in chapter 4.

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2.1.9 Input file 7/Hydrate Curve Data

If the possible formation of hydrate is simulated, a hydrate curve file is required. The hydrate curve consists of pairs of temperatures and pressures. An example is shown in section 3.3.25.

2.2 Process Equipment

The following is a description of the models of the process equipment available in OLGA. The available components are: two and three phase separators, critical and subcritical chokes and valves, compressor with controlled bypass, pumps with recycle and bypass flow, heat exchanger, check valve, controlled mass source, leak, and pig/plug.

The main purpose of including process equipment has been to give more realistic boundary conditions for multi-phase transport lines. A particular motivation has been to study the influence of a process system (and its control algorithms) on particular multi-phase phenomena such as terrain slugging in the pipeline-riser system.

The process equipment in OLGA is not intended as a tool in the design of a process system; for example the effectiveness of the separators must be given and the heat exchangers are idealised heat sinks.

The setting of control parameters such as amplification and integral time might be improved with better knowledge of the conditions in the transport line. For example, it was found that the amplification of a flow controller had to be reduced when the liquid flow rate was increased and many droplets were created in the riser. Severe velocity oscillations developed, otherwise.

Technically the program accepts process equipment anywhere along the pipeline. The user must, however, assure a sensible coupling of the process equipment as interactions may lead to instabilities.

The user does not need to consider time step changes due to process equipment, the time step is automatically adjusted if any strong transient is occurring in the process equipment, in particular in connection with controller actions.

2.2.1 Separator

The process equipment includes single train and multi train separators. They are modelled as (fictitious) pipe sections (one per separator) each separator with a user defined length and inner diameter. The single train separators follow the flow in one of the separator outlets, whereas the multi train separators follow all the outlets in different pipelines. The two separator types are described in more detail below.

Single train separator:

Please observe that for a network case, the single train separator should not be positioned in the first section of the branch immediately downstream of a junction node.

The single train separators are equipped with normal and emergency drains. The normal drain opening is governed by the control system while the emergency drain just opens and closes. Both the normal drain valve and the emergency drain valve have finite opening and closing time, given by the user. However, if the separator

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level drops below a critical value, all drains close immediately to prevent situations that OLGA cannot handle numerically.

Model description and user guidelines:

Fig. 2.2 shows the schematic of a three phase single train separator in OLGA. 1. Separator type:

The separator may be horizontal or vertical, two-phase or three-phase. If the water option is not used, only two phase separator is allowed.

2. Pipeline downstream of single train separator:

The pipeline may continue from the gas side of the separator or from the normal oil drain. It is also possible to mix the gas and oil downstream of the separator. The actual option is determined by the keyword TRAIN. In the latter case the two pipes between the separator and the mixing point are treated (numerically) as singular flow restrictions and they are not defined within the regular pipe model (i.e. they have no volume).

PC LC LC Gas Oil Water Gas Oil Water

Fig. 2.2 An illustration of a three-phase train separator

3. Valves:

The separator has the following types of valves attached (not all of them are used for all input modes):

(1) Normal oil drain valve with a subcritical flow model (2) Emergency oil drain with a subcritical flow model (3) Water drain with a subcritical flow model

(4) Gas outlet: The valve model is used with either subcritical or critical flow The valves can be referred to (defined in keyword VALVE), or be defined in the SEPARATOR keyword directly (in which case the valve models are the same as in VALVE).

The valve openings (areas) are controlled by the controller system. Therefore specifications of controller reference labels for the normal oil drain valve, normal water drain valve and the gas outlet valve are required. If the valves are defined through the keyword VALVE, the controllers’ labels are specified

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in VALVE. There is no controller for the emergency valve (only governed by the levels specified below).

If valve sizing tables are used, the liquid pressure drop equation (eq.(2.73)) is used for oil drain, emergency oil drain and water drain. The gas pressure drop equation (eq. (2.74)) is used for the gas outlet.

REMARKS: There can be no gas valve if a compressor is located at the

separator gas outlet (the boundary succeeding the separator section).

4. Levels used by a single train separator:

A single train separator needs specification of 3 different levels if it is a two phase separator, and 5 if it is a three phase separator since separate water and oil drains are used.

These levels are given either as phase fractions or as level heights. The levels are:

Keyword Description

HHOILHOLDUP The maximum liquid level allowed before the oil HHOILLEVEL emergency drain starts to open. The opening

time is given in the input as STROKETIME.

RESETHOLDUP The liquid level below which the oil emergency drain RESETLEVEL starts to close. The closing time is given in the input as

STROKETIME.

LLOILHOLDUP The liquid level below which the oil drain rate starts to

LLOILLEVEL decrease.

LLWATHOLDUP The water level below which the water drain rate starts

LLWATLEVEL to decrease.

HHWATHOLDUP The water level above which water will be drained HHWATLEVEL together with oil.

LLOILHOLDUP/LLOILLEVEL and LLWATHOLDUP/ LLWATLEVEL do not affect the specified valve openings, but controls the drain rates from the separator directly. That is, the drain rates are scaled down in order to avoid too low holdup (see remark below) in case the controllers are not properly defined.

REMARKS: If TRAIN = GAS the separator will be treated as a normal section if the liquid holdup becomes larger than 0.995. That is, the gas and liquid fraction in the flow through the gas outlet is determined by the gas mass to liquid mass ratio in the separator. The oil flow (liquid flow if two-phase separator) through the oil drain is set to zero when the oil level drops below LLOILLEVEL or the bulk liquid fraction is less than 0.001. For the water drain, the water flow is set to zero when the water level drops below LLWATLEVEL or when the bulk water volume fraction is less than 0.001.

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If TRAIN = OIL the separator will be treated as a normal section if the liquid holdup drops below 0.005. That is, the gas and liquid fraction in the flow through the oil outlet is determined by the gas mass to liquid mass ratio in the separator. The flow through the gas outlet is treated as described for TRAIN = GAS when the liquid holdup becomes larger than 0.995.

If TRAIN = MIX the gas and oil are mixed downstream of the separator. In this case the gas outlet is treated as if TRAIN = GAS and the oil outlet as if TRAIN = OIL. In this case the pipe model must include at least one complete OLGA PIPE downstream the separator.

5. Separation efficiencies

a) Liquid carryover in gas outlet.

The gas-liquid separation efficiency is defined as one minus the volume fraction of the liquid droplets in the gas outlet stream. By default, the gas-liquid separation efficiency is equal to one, that is, no liquid carryover in the gas outlet. The user can, however, specify a constant gas-liquid separation efficiency by the key EFFICIENCY in the keyword SEPARATOR. The liquid droplet volume fraction in the gas stream is then equal to one minus the value assigned to EFFICIENCY.

For three phase flow the liquid droplet volume fraction is distributed to water and oil droplet volume fractions according to the water and oil volume fractions in the settled liquid in the separator.

b) Oil in water drain.

The oil volume fraction in the water drain is determined by the following relation for separation efficiency:

rsp so

o

_T

K

eff

= 1

−

(2.1)

where Kso is the time constant for separating oil from water and Trsp is the residence time which is defined as the separator liquid volume / liquid volume flow into the separator.

The oil volume fraction in the water drain is then 1 - eff0. c) Water in oil drains.

The water volume fraction in the oil drains is determined by an equation similar to the one above:

rsp sw w

T

K

eff

= 1

−

(2.2)

where Ksw is the time constant for separating water from oil.

If the water level is above a certain limit, HHWATHOLDUP or HHWATLEVEL, the water above this limit is assumed to be drained together with the oil and the separation efficiency for separating water from oil is modified as follows:

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(

_of

)

(

_o

)

_of rsp sw w

_T

H

eff

H

K

−

+

−













−

=

1

1 eff

(2.3)

where Hof is the ratio of the water layer height above the limit to the liquid height above the same limit.

The water volume fraction in the oil stream is then 1-effw. 6. Separator controllers

Guidelines for choosing controller parameters are given in section 2.2.3. The separator normal drain valves are governed by the control system. Two applications of controllers are recommended:

a) In a situation with no capacity limitation of the separator normal drain, a single controller for separator level is recommended. Liquid level in the separator is then the input variable to the controller.

b) In a situation with capacity limitation of the separator normal drain, an override controller is recommended.

- One subcontroller controls the separator level, where liquid level in the separator is the input variable to the subcontroller.

- The other subcontroller controls the liquid flow through the normal drain. Normal drain flow is the input variable to the subcontroller.

At normal operation, the drain valve opening is controlled by the level controller, whereas in situations where the liquid flow through the normal drain exceeds the capacity of the normal drain, the drain valve opening is controlled by the drain flow subcontroller.

If liquid hold-up in the separator exceeds HHOILHOLDUP or HHOILLEVEL, the emergency drain starts to open and remains open until liquid hold-up decreases below RESETHOLDUP or RESETLEVEL.

Multi train separator:

The multi train separators are equipped with predefined outlets, and all the outlets are followed in different pipelines. In order to simulate more than one outlet flow, the separator must be positioned in the last section of an incoming branch to a split node. The split node must have the same number of outgoing branches as the number of separator outlets.

Two-phase separators have a gas outlet, a liquid outlet and an emergency outlet. For three-phase separators the liquid flow is divided into an oil outlet and a water outlet. In addition, an optional outlet for flare is available for both separators, see Fig. 2.3:

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flare outlet (optional) oil outlet w ater outlet em ergency outlet gas oil w at gas outlet

Fig. 2.3 An illustration of a three-phase multi train separator.

Fig. 2.4 shows how the multi train separator is modelled in OLGA

S E P

Fig. 2.4 The multi train separator is positioned at the last section of an incoming

branch.

Model description and user guidelines:

1. Separator type:

The separator may be horizontal or vertical, two-phase of three-phase. If the water option is not used, only two-phase separators are available.

2. Pipelines downstream of multi train separators

The multi train separators have a set of predefined outlets, and the flow out of each outlet is followed in different branches.

Branches must be assigned to the following outlets for a two-phase separator • Gas outlet

• Oil outlet

• Emergency outlet • (Flare outlet)

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For a three-phase separator, the following outlets are defined • Gas outlet • Oil outlet • Water outlet • Emergency outlet • (Flare outlet) 3. Valves

The multi train separators have no internal valves. All valves must be defined on the outgoing branches, using the VALVE keyword. Note that a valve may be positioned at the first section boundary of an outgoing branch from a split node.

4. Levels used by a multi train separator

The separator levels are controlled by the valves and controllers in the outlet branches. Moreover, the water level limit for when the water will be drained together with the oil can be specified in the separator keyword.

HHWATHOLDUP or HHWATLEVEL

The other level keys LLWATLEVEL, HHOILLEVEL, LLOILLEVEL and RESETLEVEL (and the corresponding HOLDUP keys) are not allowed for the multi train separators. The keys INITOILLEVEL and INITWATLEVEL are used to calculate initial oil and water level in the separator.

5. Separation efficiencies

The models for separator efficiencies are the same for single train and multi train separators. See above description for single train separators.

6. Separator controllers

Guidelines for choosing controller parameters are given in section 2.2.3. The multi train separator has no internal controllers, as the controllers are connected to valves, and all valves are defined outside the separator.

2.2.2 Compressor

The compressor is described by compressor characteristics that give the pressure and temperature increase over the compressor as a function of flow through the compressor and the rotational speed of the compressor. In the model the compressor is represented as a flow dependent and rotational speed dependent pressure jump and energy source. Any recirculation around the compressor is treated by a source into the section upstream of the compressor, and a sink out of the section downstream of the compressor, as OLGA cannot handle recirculation directly.

The compressor characteristics and the surge volume flow are given in the form of tables. The compressor surge volume flow is the lowest volume flow the compressor can operate on without being unstable. Compressor data needed for the model are found by linear interpolation in the compressor tables. There must be one set of tables for each compressor.

Pressure increase and derivatives of pressure increase are calculated from the pressure characteristics and are used for setting new coefficients in the momentum

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equations. Temperature increase is calculated from the temperature characteristics and is used for setting new coefficients in the energy equation.

The compressor speed and the recirculation around the compressor are governed by the control system. In addition, the compressor speed is limited by a user specified range. The surge volume flow calculated from the compressor tables is used as set point for the controller that controls the recirculation around the compressor.

It is assumed that during operation the control system keeps the compressor within the bounds of validity of the characteristics.

Compressor pressure step evaluation

The compressor pressure characteristics give compressor pressure ratio as a function of reduced rotational speed and reduced mass flow.

      Θ Θ = = 0.5 ₀_.₅ 1 2 _f _G _, n p p

δ

π

(2.4) where

π

= (-) is the pressure ratio p1 = (N/m2) is the inlet pressure p2 = (N/m2) is the outlet pressure G = (kg/s) is the mass flow

δ θ δ = ⋅ − = − p

Pa is normalised inlet pressure T

K is normalised inlet temperature G

kg s is reduced mass flow

n

r is reduced rotational speed 1 5 1 0 5 0 5 1013 10 288 . ( ) ( ) ( / ) ( / min) . . Θ Θ

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Fig. 2.5 Compressor characteristic diagram

The pressure increase over the compressor is calculated from the compressor pressure characteristics.

For calculation of the operating point, the compressor speed is necessary. The speed is governed by the control system and is limited by a user specified range. The range is normally determined by the speed range in the compressor tables.

n = (nmax - nmin) u + nmin (2.5) where n is the compressor speed, and u is the signal from the control module. u is in the range from 0 to 1, where u equal to 1 means that the compressor speed is at its maximum.

Compressor temperature calculation

In order to calculate gas temperatures, the power supplied by the compressor, PWC, is added as an enthalpy source to the enthalpy balance for the pipe section following a compressor boundary. The temperature resulting from this balance is used for calculating fluid properties, while the compressor outlet temperature is only used for informative purposes.

Polytropic compression is assumed. The enthalpy source due to the compressor is:

z

W

PW

H

_c _c

∆

=

i (2.6)

where W is the mass flux through the compressor and ∆z is the section length of the section downstream the boundary where the compressor is located.

For an inlet at pressure P1 and a density of ρ1, the power required for compression to an outlet pressure p2 is:

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











−













−

=

−

1

1 1 2 1 1 n n c

p

n

p

PW

ρ

(2.7)

The relation between pressure ratio, temperature and the polytropic exponent n is: n n

p

T

1 1 2 1 2 −













=

(2.8)

The compressor temperature characteristics are also given in the form of tables, and the temperature ratio is found by linear interpolation.

The polytropic factor (n-1)/n is calculated from pressure ratio and temperature ratio using equation (2.9).

(

)

(

₂2 1₁

)

ln ln 1 p p T T n n− ₌ (2.9) Calculation of surge volume flow

The surge flow is in the form of tables, with reduced surge mass flow as a function of reduced compressor speed, see Fig. 2.5. The surge volume flow is used as set point for the anti surge controller (ASC) that controls the recirculation around the compressor, preventing unstable compressor operation. Reduced surge mass flow as a function of compressor speed is found by linear interpolation.

For a compressor located at boundary j the surge volume flow is calculated as follows: 1 5 . 0 1 5 1 5 . 0 ₂₈₈ 10 013 . 1 − − −         ⋅       Θ = j j j surge surge T p G Q

ρ

δ

(2.10)

Recirculation flow modelling

Recirculation flow around a compressor is modelled as a set of negative and positive sources, since OLGA cannot handle recirculation directly. The flow is controlled by a choke with the choke opening governed by the control system.

The recirculation is between two neighbouring sections with a compressor on the common boundary, see Fig. 2.6. Only gas is allowed to flow in the recirculation loop. The recirculation flow is treated as a source into the section volume ahead of the compressor boundary, and a source out of the section volume after the compressor boundary.

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Fig. 2.6 Recirculation loop

The pressure drop across the restriction is equal to the pressure difference between the section downstream and upstream of the compressor.

The recirculation flow is restricted by the critical pressure difference. If the pressure difference between the section upstream and downstream of the compressor is higher than the critical one, the critical one is used.

The pressure drop over the restriction is:

g d ch

W

D

C

p

ρ

2 4 0 2

5 .

0 _











=

−

∆

(2.11) Solving for W:

(

)

0.5 0 2 2       ∆ ± = D D C p W

ρ

_g _ch _d (2.12)

where the positive sign relates to a positive source, and the negative sign relates to a negative source. W is the mass flux, rg is the gas density in the section it is flowing out of (section after the compressor), D is the diameter of the section with the source, and Do is the orifice diameter of the controlled choke.

For subcritical flow through the controlled choke the pressure difference between the section upstream and downstream of the compressor is used in equation 2.12. For critical flow through the controlled choke, the critical pressure difference is used for calculating the recirculation flow. Critical pressure difference is based on single phase gas flow with constant specific heat ratio, γ. Specific heat ratio of 1.3 is used.

1

2

1

−











 +

=

γ γ

γ

π

crit crit

p

(2.13)

























+

−

=

−

=

∆

_, −1

1

2

1

γ γ

γ

p

_ch_crit _crit (2.14) where

γ is specific heat ratio _      v p c c

πcrit is critical pressure ratio

p is the pressure in the section it is flowing out of ∆pch, crit is critical pressure difference

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The energy leaving the section downstream of the compressor and entering the section upstream of the compressor through the recirculation loop is calculated as follows.

z

W

h

H

_rec_g _g recg

∆

=

, , (2.15)

hg is the specific gas enthalpy in the compressor downstream volume, Wrec,g is the mass flux based on the section area of the section with a source and ∆z is the section length of the section with a source.

A heat exchanger may also be included in the recirculation loop. The aim of the heat exchanger is to extract energy to obtain a desired heat exchanger outlet temperature. In this case, the temperature of the recirculation source entering the section upstream of the compressor has to be specified. The heat exchanger is modelled as an ideal heat loss.

The energy source entering the upstream section in a situation with a heat exchanger in the recirculation loop is calculated as:

(

)

sp g g hex g rec hex g g rec

h

z

W

h

H

−

=

∆

−

=

, , (2.16)

h_gsp_{is specific gas enthalpy based on the desired heat exchanger outlet} temperature and pressure in the section where the source is entering. ∆h_hex is specific enthalpy decrease in the heat exchanger. The energy extracted through the heat exchanger is limited by the heat exchanger capacity.

The orifice opening of the controlled choke is governed by the control system. The control module also takes care of the stroke time of the controlled choke. The orifice is calculated as: (2.17) 5 . 0 max , 0 0 D u D =

where u is the signal from the control module, and is in the range from 0 to 1. u equal to 1 means that the controlled choke is fully open.

Restrictions and assumptions

Only gas flows in the recirculation loop. Critical flow calculations are based on single phase gas flow with a constant specific heat ratio of 1.3. If the pressure in the section upstream of the compressor exceeds the pressure in the section downstream of it, the recirculation flow is set to zero.

The compressor may be positioned at any section boundary except at the outlet boundary of the pipeline. In that position the controlled bypass is unrealisable since it is defined as going from the downstream section to the upstream section.

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It is recommended that a separator is located upstream of the compressor in order to avoid liquid flow through it. Numerically, the compressor model works with liquid phase present but the results make no sense.

Compressor minimum rpm, MINRPM, and maximum rpm, MAXRPM, are specified by the user. These two parameters determine the operating range of the compressor and they must be within the rpm range of the compressor tables.

The anti surge security factor, SECURITYFACTOR, determines the anti surge con-trol line. To protect the compressor against surge conditions, select always security factor > 1. A typical value for security factor is from 1.1 to 1.3. A security factor of 1.2 means that the control valve in the recirculation loop starts to open at a compressor inlet flow that is 20% higher than the surge flow specified in the compressor tables.

The ASC should be configured as a PI controller. The ASC is a kind of non-linear controller that have two amplification factors, AMP1 and AMP2. AMP1 is used if the inlet flow to the compressor is less than the surge flow, and AMP2 if the inlet is greater than the surge flow. Both AMP1 and AMP2 have to be negative, and the absolute value of AMP1 should be higher than the corresponding absolute value of AMP2 in order to rapidly open the recycle valve and to impose an inertia to close it again. A short stroke time for the recycle valve should also be selected.

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2.2.3 Controllers

The control system has been included to simulate the actions of a real control system as applied on a multiphase pipeline and typical downstream processing equipment.

The main function of the control system is to maintain process parameters within specified bounds by controlling process equipment parameters like valve settings and compressor speed. Typically, the control system plays an important role during situations like:

- Start-up/shut-in of production - Closing/opening of wells - Changing production levels - Receiving pigs

- Damping of terrain slugging - Pressure release

Operating modes of the kind mentioned above typically require units for: - Level control

- Flow control - Pressure control

- Anti surge control (for compressors) - Compressor speed control

- Emergency shutdown system - Manual operator actions - Pressure release system

These requirements were taken into account when the types of controllers implemented in the control system were chosen.

There are three main types of controllers available for the control of process equipment:

- Proportional-Integral-Derivative (PID) controllers - Manual controllers

- ESD and PSV controllers

A controller can be switched between these types and sub-types by setting the controller mode (subkey MODE).

Proportional-Integral-Derivative (PID) Controller

A PID controller may be mathematically formulated as:

bias dt de edt e K u t _d i c +      + + =

_∫

τ

0 1 (2.18)

(

x

stp

)

e

=

−

(2.19) where:

x = input process parameter (pressure, level, etc.) = time constant

τ