Defining Requirements for Thermo-hydraulic Analysis in Support of Design and Operation of Hydrocarbon Production Systems
Authors: Lockett Timothy James , Hill Trevor 
Published: April 2012

Abstract
Many aspects of the design and operability of a system to gather and conveyunprocessed hydrocarbons depend on the output from thermo-hydraulic modeling—todefine the physical system, to guide the assessment and reduction of risk, andto establish the operating constraints within which the system can be safelyand reliably operated. Thermo-hydraulic modeling is typically undertaken usingcommercially available and widely used software tools for steady state andtransient modeling. These tools reflect a significant Research and Developmentinvestment by the industry over 20 years.
Uncertainties in the accuracy of the results still remain, particularly asparameters are used outside of the range for which validation has beenundertaken. It is also vital to note that the validity of the results alsodepends on the competency and skill of the user to define a range ofappropriate cases, to interpret the results with due regard for the capabilityand limitations of the tools, and to communicate those results to relateddisciplines.

Introduction
For many aspects of engineering there are well-established industry codes andstandards. Operators may define the requirements for the way their engineeringis to be conducted by reference to these documents. However, there is a lack ofsuch standards in the area of flow assurance, the discipline encompassingmultiphase flow and aspects of production chemistry in the transportation ofunprocessed reservoir fluids.
The purpose of this paper is to describe key parts of the requirements in thearea of modeling multiphase flow, and to illustrate, through examples, areaswhere the modeling process may need to be improved.

The Value of Reliable Modeling
A significant contribution to the success of the oil industry in recent decadeshas been derived from the increased use of multiphase flow transportationsystems, typically to gather production from remote wellhead locations into acentral processing facility. In the case of several currently operatingmulti-field deep-water developments the projects would not have been economicon the basis of having processing facilities over each reservoir, and the useof multiphase gathering systems is therefore a key enabler.
The answers which are derived from modeling the multiphase flow through suchsystems therefore matter both at the level of individual design decisions andalso, potentially, at the level of justifying the viability of a majordevelopment.
Multiphase flow is complex - typically three ‘phases’ (gas, oil and water)flowing together in the production system which itself will typically includepipelines with changes in orientation (uphill, downhill, and horizontalsections, luted sections at spools, risers), and subject to an overall declinein pressure with length and typically a reduction in temperature with length. The changes in pressure and temperature cause the phase properties (density, viscosity) to change, and the relative quantities of the phases to change asgas expands and liquid evaporates or condenses.