Abstract

This case study is drawn from a conceptual study carried out for an independent UK based Oil and Gas company.  The flowline was sized by combining an integrated reservoir/flowline simulator with simple discounted cash flow economics.  The operability of the flowline was also an important issue as the speed at which it could safely alter its production rate was stipulated in the Gas Sales Contract (GSC).  The study demonstrates how many conceptual issues in gas-condensate systems can be studied using steady state simulators, thus allowing transient simulation to be deferred until the later phases of design.

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Case Study No. 1 - Optimising Long Distance Gas-Condensate Flowlines

Case Study No. 2 - Optimising a Tieback to Existing Facilities

Abstract

This study was carried out for a US oil company operating in the central North Sea.  It shows that even short conceptual studies can benefit tremendously from an integrated reservoir–flowline approach to the analysis.

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Case Study No. 3 - Optimising an Extension to an Existing North Sea Asset

Abstract

This study was carried out for a UK based oil company operating in the North Sea wishing to extend one of its existing subsea production systems.  The case study shows how as flowline systems grow they can become as much of a restriction to flow as the wells or reservoir.  Therefore, detailed evaluation of the overall deliverability is best achieved using an integrated reservoir model coupled to a representative well, flowline and riser network model.  Such evaluations are now commonplace using the latest Life of Field simulation tools.

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Case Study No. 4 - Life of Field Stability of a North Sea Oil Development

Abstract

Transient operability issues can significantly affect the NPV of subsea developments.  Severe slugging, for example, can dictate major design parameters such as flowline routes and diameters and hence affect the overall CAPEX.  It can also affect revenue by reducing the operability of flowline-riser systems.  Consequently, such issues should be analysed at the earliest possible opportunity during design.  By automating the analysis of the output from transient multiphase flow simulators, it is possible to gain a greater understanding of the operability of such systems during conceptual design.  This case study describes a method employed by Feesa on several previous Flow Assurance operability studies.

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Case Study No. 5 - Life of Field Cooldown in a Deepwater Development

Abstract

Transient multiphase flowline simulators can help engineers understand many different Flow Assurance issues and have proved to be a useful tool during the design of many oil and gas developments.  However not all transient issues benefit from being studied using such complex and costly tools, either because the problem is straightforward or an accurate solution is not required or the simulator is not best suited to model the physical phenomenon of interest.

Cooldown of subsea flowlines can be one such issue.  This case study was motivated by experience gained in the design of a large West African deepwater development.  It shows how the requirement for expensive transient simulations can be minimised when assessing the key Flow Assurance issue of cooldown, leaving time to study more important aspects of the system.  Rather than focusing on the minutiae of gas-liquid distribution and cooldown time, it is suggested that time is better spent on the bigger picture in front-end design, answering important questions such as how does operability affect the NPV of the project?

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Case Study No. 6 - Life of Field Corrosion of a Flowline System

Abstract

This corrosion study was carried out in conjunction with a large engineering company as part of a FEED for a deepwater development.  A method was employed to aid communication between the Flow Assurance and Corrosion disciplines.  This interdisciplinary approach removed the need to identify an arbitrary corrosion design case.

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Case Study No. 7 - Hydrodynamic Slug Size in Multiphase Flowlines

Abstract

Hydrodynamic slugs are understood to be initiated by the instability of waves on the gas-liquid interface in stratified flow under certain flowing conditions.  When hydrodynamic slugs are numerous, the pipeline is said to be operating in the slug flow regime.  The slug flow regime is characterised by complex gas-liquid flows often exhibiting chaotic behaviour.

Commercially available one-dimensional transient flow simulators rely on correlations to model the complex three-dimensional multiphase phenomena within a slug.  These correlations can be tuned to field data giving good a posteriori predictions for operating facilities.  However, their ability to make a priori predictions of slug flow (including slug size and frequency) remains limited.  Hence, good design practice still requires the application of healthy design margins.

This note considers the subject of hydrodynamic slugging and qualitatively addresses the accuracy of the prediction methods.

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Case Study No. 8 - Non-Newtonian Flows in Pipelines

Abstract

There are many analytical methods that can be employed by Flow Assurance engineers to great benefit and saving.  One such example is for the analysis of Non-Newtonian fluids, such as gelled oils.  A method has been developed and successfully employed on such diverse systems as a deepwater flowline containing a gelled oil and the effluent stream from a pharmaceutical reactor.  Gelled oils in pipelines present a particularly difficult problem as their rheological behaviour can be a function of temperature and the history of how the gel was formed.  This leads to considerable uncertainty associated with the fluid properties.  Given these uncertainties, a fit-for-purpose model is described which predicts the flows of gels in pipes.  This model provides a cost-effective and inexpensive alternative to expensive add-ons to transient simulators.

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