WHITE PAPERAn Integrated Approach to ModelingPipeline Hydraulics in a Gathering andProduction SystemBy Product Marketing, Aspen Technology, Inc.

Executive SummaryIn recent years, as oil and gas fields become less accessible and their hydrocarbon quality lower andmore variable, maintaining or increasing production levels has emerged as a key field developmentgoal. One of the most pronounced challenges in meeting this goal is managing thecomplex hydraulics of pipelines used in gathering systems and to transport theoil and gas from wells to processing facilities. As these pipelines get longerin new fields, deeper in offshore environments, or simply older in agingimplementations, E&P companies face critical problems for which theyneed better performance predicting and troubleshooting tools. From abusiness standpoint, solving these technical challenges is an increasingpriority, since the capital expenditures involved in constructing andretrofitting gathering systems are a high proportion of development costs,but pale in comparison to the possible loss in field profitability due to flowinterruptions.This paper provides an overview of a new pipeline hydraulics modelingcapability that eliminates the need to employ separate third-party tools forpipeline hydraulics. There are many benefits to modeling the entire gathering andproduction system (be it offshore, onshore, topside, etc.) within one tool, including not onlybeing able to optimize the design from a capital and energy perspective, but also ensure the overallsafety of the system.Aspen HYSYS has been widely used to model many facets of the oil and gas production fields,including separation systems, environmental control systems, gas dehydration, H2S and CO2removal, and more. It is the tool of choice to determine the heat and material balance, separationperformance, and regulatory compliance, among other key performance criteria (Gulbraar, 2011).In the past, third-party hydraulics packages were employed in conjunction with or separately fromAspen HYSYS to address that specific aspect of the design problem. This data then needed to beincorporated in the HYSYS model, which led to an inherently inefficient approach. Now the entiregathering system and production system can be addressed in an integrated fashion.Aspen Technology offers two tools within Aspen HYSYS to accurately and rigorously modelcomplex pipeline hydraulics: Aspen HYSYS Pipe Segment Model and Aspen HYSYS UpstreamHydraulics. With these two products, companies can model simple pipelines or a complex networkof pipelines; they can simulate the dynamics of the multiphase flow through the pipe, and implementflow assurance measures to reduce erosion, corrosion, deposit formation, and slugging. Thesetwo products allow producers to simulate pipelines from wellhead to processing facility, startupto shutdown, from the beginning to the end of the field lifecycle, in steady state or dynamically,reducing the design time from several months to three days or less (2. Genesis Oil and Gas, 2011).This paper demonstrates the solution and its capabilities.2An Integrated Approach to Modeling Pipeline Hydraulicsin a Gathering and Production System 2015 Aspen Technology Inc. 11-7579-0615

Need for Accurate Modeling of HydraulicsOne of the key expectations in modern energyproduction is reliability. In industrializedsocieties, the need for reliable energy sources isparamount, and therefore the need for reliablesourcing is fundamental. This reliability startswith the exploration phase of oil and gas fields.Since the formation of these fields is a naturalphenomenon, the composition of the materialextracted is unknown. However, there are toolsExploration and Production (E&P) companiescan use to reliably extract these resources, evenwith unknown compositions.The design of any gathering system andproduction network must contend with avariety of ever-changing business prioritiesand engineering constraints—with reliabilitybeing the constant objective. The system mustachieve maximum uptime and performanceto support the expected productionthroughput efficiently. A secondobjective is to supportchanging hydrocarboncompositions, whichnaturally differ overthe lifetime of aproduction field,but also changeunpredictablydue to new andunconventionalproduction flows beingfed into the gatheringnetwork.While the typical production involves anoil phase, a gas phase, and a water phase,sometimes solids such as sand and gravel alsoget mixed into the flow, depending on the fieldgeology and its age. All of these factors need tobe taken into consideration when an extractionstrategy and the pipeline gathering networkare developed. The design of the pipelinenetwork is fundamental to ensure the flowfrom the field is consistent and steady, so thatdownstream processing can be equally steadyand uninterrupted. The integrated consideration3An Integrated Approach to Modeling Pipeline Hydraulicsin a Gathering and Production Systemof the pipeline network, its hydraulics, and theseparation and production systems is critical toensure that safe operations are correctly builtinto the design.The pipeline network must be designed inaccordance with industry standards such asAPI, ANSI, and ASME as well as environmental(EPA), safety (OSHA), and any national, state,regional, or local regulations. The engineeringdesign must take into account the envelope ofexpected temperatures, pressures, and volumesof the mixture going through the pipeline,as well as the entire geography it covers totransport the products, and environmentalconditions and restrictions along its path.With a growing number of fields being locatedoffshore or in hard-to-reach areas, pipelineshave become increasingly longer and thesurrounding environment more diverse,creating the need for better simulation of thesenew characteristics. It is also important toconsider possible flow composition changesalong the lifecycle of the field, depositions ofimpurities on the pipeline walls such as wax andasphaltene, as well as the corrosion of the pipesand slugging. With ageing pipelines all overthe world, these considerations have becomevery important when deciding how to bestextend the lifetime of a field, as well as whethermaintenance is enough or if the pipeline needsto be replaced.For the pipeline to be reliable, anotherimportant factor is flow assurance, or ensuringthat the multiphase mixture containingoil and gas is constantly flowing to assureproduction and avoid damage to downstreamequipment such as pumps and valves. This isespecially difficult with deepwater wells, asany maintenance necessary in the pipeline iscostly and incurs significant production losses.Taking all these factors into consideration,it is easy to see why correct pipeline designis so important. In fact, the pipeline usuallyaccounts for the second largest component ofthe capital cost forecast for deepwater field 2015 Aspen Technology Inc. 11-7579-0615

development at 30%, second only to drilling and completion of subsea wells. The cost to build thepipelines is high, around 10 million per well, but miniscule compared to the billions of dollars inpotential production losses incurred from incorrect designs over the lifecycle of the field. (John& MacFarlan, 2008) Safety and controllability are also important considerations when designingpipelines. Transient operation—including changes in temperature, composition, and pressure insideoil and gas pipelines—is common, and must be taken into account. In addition, startups, shutdowns,and downstream production requirement changes to the steady-state conditions must be consideredin a dynamic model.All of these requirements make pipeline hydraulics an extremely important step in field developmentplanning. Therefore the accurate modeling of the pipeline and the connection between the pipelineand the equipment for midstream and upstream processing is fundamental in assuring optimizedflow and production throughout the lifecycle of the field as well as safe operations of the entirenetwork. This model can be used to optimize new production, and revamp or add new assets to anexisting process.Overview of Pipeline Modeling SolutionsTo meet these industry needs, Aspen Technology makes available two approaches for modelingpipeline hydraulics, both within Aspen HYSYS. Both have similar capabilities, and both support theuse of dynamics for transient flow conditions with the supported pipe model, but one uses simplifiedsolvers for quicker results, while the other offers more rigorous modeling for complex pipelinedesigns.The Pipe Segment Operation is the recommended solution for modeling a single pipeline that canbe divided into multiple parts. Usually, these segments are determined by terrain or environmentalchanges along the path of the pipeline, where different flow correlations must be used to moreaccurately model the pipe. The second approach, Aspen Hydraulics— which is part of AspenHYSYS Upstream—is designed to model a network of pipelines, such as offshoreproduction. In this scenario, multiple wells with different characteristics areexplored, and the resulting network of pipes with multiphase flow mixingrequires more rigorous modeling.Both approaches support the use of dynamic simulation, with AspenHydraulics supporting more rigorous calculations for pipe networks. Theintegration of both approaches with Aspen HYSYS makes it possible tomodel not only the pipeline, but the interface between the pipes and theprocessing equipment at midstream and upstream facilities. Coupled withthe dynamic simulation to make the process safer and to adapt as the fieldand pipeline age, these tools are crucial for the operation of pipelines all overthe world.4An Integrated Approach to Modeling Pipeline Hydraulicsin a Gathering and Production System 2015 Aspen Technology Inc. 11-7579-0615

The Basics of Flow SimulationWhen modeling a pipeline, there are several important factors to consider in the design, themost significant of which are the pressure drop, flow rate, flow geometry, and changing flowpatterns. When modeling any flow through a pipe, the basic necessary inputs are inlet and outletpressure, and mass or molar flow. A large number of variables affect pressure drop, which makesit a complex yet very important variable when considering pipeline calculations. These variablesinclude thermodynamic properties of all components of the mixture traveling through the pipe,as well as their interactions with each other and the pipe material at the interface. There are alsoconsiderations for compressibility, difference in density, and the spatial arrangement of the differentcomponents during flow.Flow CorrelationsThe large number of variables involved makes it practical to use flow correlations when modelingpipelines using Pipe Segment in Aspen HYSYS or Aspen Hydraulics. Both products offer a varietyof industry-recognized flow correlations that can be used for different flow characteristics. Thesecorrelations can be either empirical (made to fit experimental data using dimensionless parameters)or mechanistic (developed to model particular flow pattern). Neither empirical nor mechanisticcorrelations can be used in all conditions, since they are developed for a particular set of flowcharacteristics. Therefore, when choosing a correlation model, it is important to know the mostappropriate one depending on flow direction, and the necessity to model a flow map and liquidholdup, as shown in Table 1.The last model on the table was created by the Tulsa University Fluid Flow Projects (TUFFP), acooperative Industry-University research group supported by member companies and governmentagencies. (University of Tulsa, 2013) The group was formed to research and develop solutions forchallenges encountered by the member companies pertaining to multiphase fluid flow in pipelines.Among other activities, this consortium has constructed physical test beds in which they testthe behavior of oil and gas flow in pipeline segments on an ongoing basis. Aspen Technology is amember of this group and, as such, has access to the most up-to-date results obtained in the group’songoing research, using results to provide a more accurate flow correlation for multiphase pipelines.Aspen Technology includes the latest TUFFP correlations within the Hydraulics modeling capabilityin Aspen HYSYS.5An Integrated Approach to Modeling Pipeline Hydraulicsin a Gathering and Production System 2015 Aspen Technology Inc. 11-7579-0615

ModelAziz, Govier & FogarasiHorizontal FlowVertical FlowLiquid HoldupFlow MapNoYesYesYesBaxendell & ThomasUse with CareYesNoNoBeggs & Brill (1973)YesYesYesYesBeggs & Brill (1979)YesYesYesYesDuns & RosNoYesYesYesGregory, Aziz, MandhaneYesNoYesYesHagedorn & BrownNoYesYesNoHTFS HomogeneousYesYesNoNoHTFS Liquid SlipYesYesYesNoOLGAS 2-PhaseYesYesYesYesOLGAS 3-PhaseYesYesYesYesOrkisewskiNoYesYesYesPoettman & CarpenterNoYesNoNoTulsa 99NoYesYesYesTUFFPYesYesYesYesTable 1: Pipe Flow Correlations Available in the Aspen HYSYS Pipe Segment ModelDynamic ModelingMultiphase flow is intrinsically unstable, especially when traveling long distances over differentterrains in a pipeline that may be corroded or eroded, or may contain obstructions. Therefore, it isimportant to design not only a steady-state model for the pipes, but also a dynamic model that takesinto account changes to the flow as it moves through the pipeline as well as changes to the pipelineitself as it and the field age. This transient model is important not only for these variations, but alsofor startup, shutdown, and changes in production.Dynamic simulation is possible using both Aspen Hydraulics and Pipe Segment Model in AspenHYSYS. The only difference is that the dynamic modeling performed within Aspen Hydraulics is morerigorous, and therefore, should be applied to more complicated scenarios such as multiple producingfields that are served by one network of pipelines with convergences. The Aspen HYSYS dynamicmodel uses the same physical property packages as the steady-state model, and the steady-statemodel is easily converted to a dynamic model. The software features a Dynamics Assistant toensure an easy transition with no under- or over-specification. This approach maximizes the valueof the steady-state model and reduces the expense and complexity associated with building andintegrating a separate dynamic model. The engineer should always consider employing DynamicModeling to achieve the best design to ensure flow assurance and network reliability.6An Integrated Approach to Modeling Pipeline Hydraulicsin a Gathering and Production System 2015 Aspen Technology Inc. 11-7579-0615

Flow AssuranceThe key to a reliable pipeline is ensuring continuous flow from the production site to the processingsite at as close to design-flow throughput as possible. Flow assurance encompasses the thermalhydraulic design and assessment of multiphase production fluids through pipelines, as well as theprediction, prevention, and remediation of flow stoppages. The purpose is to ensure successful andeconomical flow of multiphase fluids from reservoir to point of sale. Flow interruptions can causethe disruption of production and damage to the processing equipment, which combined can cost arefinery millions of dollars in lost revenue. In fact, the cost of lost production due to flow assuranceissues can dwarf the installation costs for the entire pipeline. Therefore, it is important to considersuch factors when designing the production pipeline.Flow interruptions may be caused by the corrosion or erosion of the pipe materials, the unstablenature of multiphase flow, or the formation of blockages in the pipes—either physically or due tochemical reactions, such as hydrate formation. The instability of multiphase flow, usually referredto as slugging, can cause significant pressure differences along the pipeline due to the interactionsbetween the gas and liquid phase, which is accentuated due to terrain changes. Blockages in thepipes are caused by the deposition of solids found in the flow, the most common of which are wax,hydrates, and asphaltene. These depositions occur under certain temperature conditions that canbe minimized, and remediation usually includes production stoppage, so it is important to use thecorrect dynamic tools to design the pipeline to minimize these issues.Aspen HYSYS V8 for UsabilityAlthough hydraulics modeling capabilities were introduced earlier than Aspen HYSYS V8 (releasedin December 2012), it is highly recommended that engineers use Aspen HYSYS V8.0 or later whenperforming the modeling described in this document. The later versions of Aspen HYSYS introducea completely redesigned user environment, with an Office 2010 “ribbon” paradigm and a workflow/workspace that makes this capability more accessible to new users.Figure 1: Example of a pipe segment in an Aspen HYSYS V8 flowsheet. The segment can be selectedfrom the palette on the left and has selections for Flow Assurance and Dynamics on the right.7An Integrated Approach to Modeling Pipeline Hydraulicsin a Gathering and Production System 2015 Aspen Technology Inc. 11-7579-0615

Aspen Pipe Segment ModelDeveloped specifically for basic pipeline design, Aspen HYSYS includes a tool called Aspen PipeSegment Model. This tool can be used much like the other unit operations available in Aspen HYSYS;it is available in the “common” object palette and can be added to the main flowsheet (see Figure 1).It is used primarily in modeling single pipelines without mixing, and uses simple solvers for fastercalculations.With pipe segments, the entire pipeline is divided into smaller pipe sections depending on thecharacteristics of each segment to ensure more accurate modeling. Each pipe segment can have adifferent flow correlation, for example, to adapt to different geographical characteristics (see Figure 2).When using the Pipe Segment Model, you need to specify just two of the following: inlet pressure,outlet pressure, or mass/molar flow rate; once two variables are specified, the third can becalculated. Other specified inputs include the length of the segment, elevation change, outer andinner pipe diameters, pipe material, and the number of increments. The design also includes modelsfor heat loss based on heat loss through pipe, outlet temperature, and heat transfer coefficients.Depending on the characteristics of the simulation, it might be helpful to disable some segments ofthe pipe while solving the rest of the flowsheet, and the Pipe Segment Model allows for that with an“Ignore” option.Figure 2: Multiple correlation selection for horizontal, vertical, and inclined pipe flow in a Pipe Segment.8An Integrated Approach to Modeling Pipeline Hydraulicsin a Gathering and Production System 2015 Aspen Technology Inc. 11-7579-0615

The Pipe Segment Model also performs dynamic calculations for flow, to account for changes alongthe pipeline as well as for pipeline aging. These calculations are also important when conductingFlow Assurance analysis, using specific tools within Aspen HYSYS that examine CO2 Corrosion, PipeErosion, Slug Analysis, Wax Deposition, and the formation of Hydrates (see Figure 3). These toolsare important when assessing conditions that could permit interruptions to flow, especially as thefield (and pipeline) ages, and then minimizing the risk of that occurrence.The simulations using the Aspen Pipe Segment Model can be used by Engineering and Constructioncompanies designing field production systems, to predict the behavior of the fluids in the well asthey progress through the pipeline, over distance as well as over time. The models also empowerthe operator of the field to predict and avoid possible interruptions in flow, along with schedulingmaintenance on the pipeline, in order to improve production and extend the lifetime of the field.Figure 3: The Flow Assurance tab (right) for a Pipe Segment (left) showing erosioncalculations related t