Transient Codes in Use

Although there are a large number of engineers and scientists working in the area of transient two-phase flow the number of commercial software packages is limited to just a few.

The most widely used is the OLGA code developed by IFE and SINTEF in Norway. OLGA was the first commercial general transient two-phase flow package, which has enjoyed wide use by the 6 members of the consortium that sponsored the development. The early versions of the code required considerable expertise to create input data and to interpret the results. As a result many of the oil companies using the code found it difficult to use and very costly. Some of them have developed their own versions of OLGA for their specific needs.

In competition to OLGA is the PLAC code developed by Harwell laboratories which was only commercialised in the autumn of 1992, but had been used for consultancy for more than two years previous. was developed with support from BP and is the transient simulator most widely used in-house by BPX.

There are several other transient two-phase flow codes presently under development. Total are working on a transient version of the mechanistic program called TACITE, which will feature a slug tracking model. Shell have a code called TRAFLOW which is under development as an on-line simulator and analysis code.

There is a great deal of activity in the development of transient programs. However most of this is being carried out in-house by oil companies and research centres, hence little is known about them. For this reason the discussion in this Section will be limited to the codes that BPX have had direct experience with. It is however expected that competition in the market place will increase when some of the other codes are commercialised.

A description of the BP developed BPREAKL code for estimating release rates from volatile oil pipelines is given in Section 9 of the Multiphase Design Manual.

OLGA

OLGA is a dynamic, one dimensional modified two fluid model for transient two-phase hydro-carbon flow in pipelines and networks, in which processing equipment can be included. The code has been developed by the two-phase flow project which commenced in 1984 and was based on the computer program OLGA 83. This was originally developed for

by IFE in 1983.

The two-phase flow project was aimed at improving OLGA by expanding an experimental data-base from a high pressure 8” large scale test facility run by SINTEF at Tiller in Norway.

Extensive testing was carried out by IFE and by the projects member oil companies which included Conoco Norway, Esso Norge, Mobil Exploration Norway, Norsk Hydro, Petro-Canada, Saga Petroleum, and Texaco Exploration Norway.

The basic models in OLGA contain three separate mass conservation equations for the gas, the continuous liquid and the liquid droplets, these are coupled with the interphase mass transfer terms. Momentum conservation is applied to the gas-droplet field and to the continuous liquid, hence giving two-equations. The mixture energy equation is written in conservation form, accounting for total energy balance in the system. OLGA predicts as a function of time the pressure, temperature, mass flow of gas and liquid, the holdup and the flow pattern. Closure laws are required for the friction factor and the wetted perimeters of the phases and these are

Section 7. Transient Flow

flow regime dependant. The droplet field also requires a entrainment and deposition model.

The two basic flow regimes adopted are distributed and separated flow. The former contains bubble and slug flow and the latter stratified and annular flow. The transition between the two regime classes are determined according to a minimum slip concept.

Due to the numerical solution scheme the original versions of OLGA are particularly well suited to simulate slow mass flow transients. The implicit time integration applied allows for long time steps which is important for the simulation of very long transport lines, where typical simulation times are in the range of hours to days.

The necessary fluid properties (gas/liquid mass fraction, densities, viscosities, enthalpies etc.) are assumed to be functions of temperature and pressure only, and have to be supplied by the user as tables in a specific input file. Thus, the total composition of the two-phase mixture is assumed to be constant both in time and along the pipeline for a given branch. The user may specify a different fluid property table for each branch, but has to ensure realistic fluid composi-tions if several branches merge into one.

In 1989 Scandpower acquired marketing rights to the OLGA program and commercial use of the code increased considerably. The early versions of OLGA had a simple data input and output file format which made the setting-up of problems very time consuming. Later versions have user friendly interfaces which greatly enhances the use of the code, is used for input data generation and is used for post processing. Fluid properties are generat-ed using a package callgenerat-ed PVTOL.

In 1993 acquired the Tiller facility for 5 years and embarked on a substantial R and D effort to improve OLGA, areas under development include:

l Improved mechanistic modelling and closure laws

l Slug flow modelling

l Three phase flows

l Compositional tracking

These areas also indicate where there are deficiencies in the present code.

In addition, OLGA has been interfaced with the D-SPICE multi-compositional dynamic process plant simulator to improve the modelling of equipment, however this interface only pro-vides a dynamic exchange of data between the codes and is hence not a fully integrated model.

Some of the other original two-phase flow project sponsors have also developed OLGA for their own use, notably Conoco who have developed improved thermal modelling and pigging simu-lation in their version called

The Analysis Code (PLAC) was originally developed from the nuclear reactor safety code TRAC (1986) under a four year programme sponsored by BP Exploration and the UK Offshore Supplies Office. The major parts of that work were to remove the redundant reactor specific components and to convert from steam/water physical properties to multi-component

Section 7. Transient Flow

hydrocarbon properties. More suitable models for interfacial friction and flow regimes were also introduced. The first release of PLAC was available for use by AEA Petroleum Services and BP Engineering in mid 1990. of the code was not realised until the autumn of 1992 however BP and AEA conducted numerous validation tests on the code with good and bad experiences. PLAC is the transient simulator used most widely in-house by XFE to model transient two-phase pipeline flows. Like OLGA the original versions of the code were cumber-some to use and the ease of use has been greatly improved by the development of interactive pre and post processors.

PLAC solves mass, momentum and energy equations for each phase using a one-dimensional finite difference scheme. Six equations are solved, gas and liquid mass and momentum con-servation, total energy concon-servation, and gas energy conservation. Unlike OLGA, PLAC does not have a separate equation incorporating a droplet field in the gas stream. Appropriate flow pattern maps and constitutive relationships are provided for wall and inter-facial friction and heat transfer, and a model for multi-component phase change is included. has flow regime maps for vertical and horizontal pipes and switches to vertical flow if the angle of inclination is above 10 degrees. The horizontal flow pattern map is based on the method of Taitel and Dukler (1976).

The fluid physical properties are calculated from a user supplied mixture composition using an internal PVT package, however this generates a table of properties similar to OLGA, and hence still relies on simple equilibrium phase behaviour predictions. PLAC can only use one composi-tion in a network. The current version of the code has the capability to handle PIPES, TEES and VALVES and is able to predict a range of phenomena including and/or thermal transients, severe slugging, shutdown/restart problems and pipeline depressurisation. The boundary conditions are specified by FILL and BREAK components.

TRANFLO is a three phase flow transient program developed by the mathematical sciences group at BP Research, The program is based on the one-dimensional averaged equations of motion for the conservation of mass and momentum for each phase and does not include the effects of gas entrainment. The equations for each phase are transformed into evo-lution and algebraic equations. The resulting system of equations is analogous to that for motion of liquid down a open, inclined channel. The program can generate waves on the inter-face due to physical instabilities. These evolve non-linearly into a train of periodic waves whose fronts propagate as shocks or hydraulic jumps. The waves can touch the top of the pipe lead-ing to the formation of slug flow.

TRANFLO was under development at on the VAX computer and showed some promise in the simulation of sloshing in dips, slug tracking, and water puddling in pipelines. The program development ceased in 1993 and was handed over to AEA Petroleum Services,

where its future is unclear, however it may be incorporated into PLAC or ported onto a PC. The version at BP is limited to isothermal cases without phase change, and is also poor for vertical flow. It is an R&D tool and is not suitable in its present form for modelling complete pipeline systems, but may be useful for investigating three phase effects in simple specific geometries. The three phase version of is expected to make TRANFLO obsolete in the future.

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Section Transient Flow BP Design

is a general purpose simulation package capable of solving coupled, ordinary differ-ential equations. It is used within BP to examine transient conditions in process facilities such as advanced control of reactors and distillation columns, the interaction of slugging oil and gas production lines with facilities, compressor control and surge, stream fuel gas and relief sys-tems etc..

is also capable of addressing a wider range of problems. It can perform steady state calculations, dynamic simulation, optimisation, parameter estimation and data reconciliation. It is also a flowsheeting package and data is organised in terms of streams of connecting units.

The units in turn are models taken from commercial or user generated libraries.

The program is generally used by the dynamic simulation group to investigate the response of process plant to multiphase pipeline transients such as severe slugging and normal slugging.

This is handled by a Dynamic Slugging Model that has been incorporated as a module within and allows and integrated model of the pipeline and facilities to be used to assess the effect of slugging on the entire process plant. The model simulates the deceleration of the slug as it enters the riser (due to the increase in the hydrostatic head) and the subsequent acceleration of the slug as it exits the riser and enters the facilities. The slug produc-tion phase is followed by a period of gas as the pressured gas bubble behind the slug depressures into the facilities. The Dynamic Slugging Model assumes a two-phase system (liquid and gas) and requires the size and frequency of the slugs to be input. It does not allow slugs to break-up or to amalgamate.

WELLTEMP was designed to examine temperature transients caused by flow in wells. It can also be used to examine temperatures in pipelines. It handles heat flow through tubing, cas-ings, cement and formation. various operations can be such as injection, pro-duction, forward and reverse circulation, shut-in and cementing. It assumes steady state flowrates; although different flow periods can be entered (steady within each period). It can han-dle dual or even triple completions (and co-bunhan-dled flowlines). In dual completions it would allow injection down one string and production back up the other.

It can be used to examine surface and tubing temperatures during drilling or simulat-ed treatments. This is useful when considering fluid properties, cement characteristics etc. It can also help when considering corrosion, wax deposition or hydrate formation. It can be used to examine cool down times in pipelines. It could also be used to refine the temperature profile in the and during gas lift operations.

WELLTEMP is marketed by Enertech Engineering and Research Co of Houston and has been primarily used by XFE to investigate the warm-up of wells and flowlines. WELLTEMP can usu-ally run much quicker than and has been used to investigate sensitivities before perform-ing a detailed PLAC simulation. The simple composition treatment in WELLTEMP is a limitation to detailed transient two-phase flow analysis.

The University of Tulsa through the TUFFP JIP have developed a number of transient codes over the years which have been made available to BP. One of the earliest codes was produced

by M W Scoggins in 1977 and was based on a one-dimensional two-phase flow model with slip. The model is based on the existing steady state Eaton method for holdup and the Dukler two-phase friction model. The treatment of phase behaviour and physical properties is based on hard wired black oil correlations and the simulation is isothermal only. This program has been ported onto a PC by XFE and is called SCOGGINS, however the code suffers from numerical problems and its inherent limitations mean that it is little used.

The latest transient analysis code from TUFFP was developed by Minami in 1991 who imple-mented a simplified model proposed by Taitel. He also developed a new pigging model that was coupled to the transient model. The sets of equations are discretized using a lagrangean grid system for the pigging model. The resulting set of equations are solved with a semi-implicit scheme and an approach based on the stability of the slug flow structure is used to predict the flow pattern transition boundaries.

The version of this code released to BP is only capable of simulating conditions in the TUFFP 1400 feet long 3” test facility and is hence of little use. Comparison of the predictions with recent data from transient tests on the Tulsa facility indicate that the code is inferior to

Flowmaster is a computer program designed primarily for single phase fluid flow analysis and pipe network design, and has various analysis options including steady state hydraulic analysis, flow balancing, and pressure surge analysis. The network is constructed from a pallet of com-ponents and the design data input for each item. XFE primarily use FLOWMASTER for single phase pressure surge analysis, however there are a number of features which can facilitate a simple surge analysis for two-phase flows including a bubbly pipe component, which allows the speed of sound to be modified to account for a homogeneous gas fraction, and a primer com-ponent, which can simulate liquid filling a gas space.

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Section 7. Transient Flow