Flow Assurance Challenges
Flow assurance analysis is a recognized critical part of the design and operation of subsea oil/gas systems. Flow assurance challenges focus mainly on the prevention and control of solid deposits that could potentially block the flow of product. The solids of concern generally are hydrates, wax, and asphaltenes. Sometimes scale and sand are also included. For a given hydrocarbon fluid, these solids appear at certain combinations of pressure and temperature and deposit on the walls of the production equipment and flowlines.
The solids control strategies used for hydrates, wax, and asphaltenes include the following:
- Thermodynamic control: Keep the pressure and temperature of the entire system out of the regions where the solids may form.
- Kinetic control: Control the conditions under which solids form so that deposits do not form.
- Mechanical control: Allow solids to deposit, but periodically removing them by pigging.
Flow assurance has become more challenging in recent years in subsea field developments involving long-distance tie-backs and deepwater. The challenges include a combination of low temperature, high hydrostaticpressure for deepwater and economic reasons for long offsets. The solutions to solids deposition problems in subsea systems are different for gas versus oil systems.
For gas systems, the main concern of solids usually is hydrates. Continuous inhibition with either methanol or mono-ethylene-glycol (MEG) is a common and robust solution, but low-dosage hydrate inhibitors (LDIs) are finding more applications in gas systems. The systems using methanol for inhibition are generally operated on a once-through basis. The methanol partitions into gas and water phases and is difficult to recover. Systems using MEG on the other hand normally involve the reclamation of MEG. If a hydrate plug forms, the remediation method may be a depressurization.
For oil systems, both hydrates and paraffins are critical issues. In the Gulf of Mexico (GoM), a blowdown strategy is commonly used. The strategy relies on the insulation coating on the flowline to keep the fluids out of the hydrate and paraffin deposition regions during operation. During start-ups and shutdowns, a combination of inhibition, depressurization, and oil displacement is performed to prevent hydrate and paraffin deposition. Wax is removed by pigging. The strategy is effective, but depends on successful execution of relatively complex operational sequences. If a hydrate plug forms, it is necessary to depressurize the line to a pressure usually below 200 psi for a deepwater subsea system and wait for the plug to disassociate, which could take a very long time in a wellinsulated oil system.
Flow Assurance Concerns
Flow assurance is only successful when the operations generate a reliable, manageable, and profitable flow of hydrocarbon fluids from the reservoir to the end user. Some flow assurance concerns are:
- System deliverability: Pressure drop versus production, pipeline size and pressure boosting, and slugging and emulsion.
- Thermal behavior: Temperature distribution and temperature changes due to start-up and shutdown, and insulation options and heating requirements.
- Solids and chemistry inhibitors: Hydrates, waxes, asphaltenes, and scaling.
 R.C. Reid, J.M. Prausnitz, T.K. Sherwood, The Properties of Gases and Liquids, third ed., McGraw-Hill, New York, 1977.
 K.S. Pedersen, A. Fredenslund, P. Thomassen, Properties of Oils and Natural Gases, Gulf Publishing Company (1989).
 T.W. Leland, Phase Equilibria, Fluid, Properties in the Chemical Industry, DECHEMA, Frankfurt/Main, 1980. 283–333.
 G. Soave, Equilibrium Constants from a Modified Redilich-Kwong Equation of State, Chem. Eng. Sci vol. 27 (1972) 1197–1203.
 D.Y. Peng, D.B. Robinson, A New Two-Constant Equation of State, Ind. Eng. Chem. Fundam. vol. 15 (1976) 59–64.
 G.A. Gregory, Viscosity of Heavy Oil/Condensate Blends, Technical Note, No. 6, Neotechnology Consultants Ltd, Calgary, Canada, 1985.
 G.A. Gregory, Pipeline Calculations for Foaming Crude Oils and Crude Oil-Water Emulsions, Technical Note No. 11, Neotechnology Consultants Ltd, Calgary, Canada, 1990.
 W. Woelflin, The Viscosity of Crude Oil Emulsions, in Drill and Production Practice,, American Petroleum Institute vol. 148 (1942) p247.
 E. Guth, R. Simha, Untersuchungen u¨ber die Viskosita¨t von Suspensionen und Lo¨sungen. 3. U¨ ber die Viskosita¨t von Kugelsuspensionen, Kolloid-Zeitschrift vol. 74 (1936) 266–275.
 H.V. Smith, K.E. Arnold, Crude Oil Emulsions, in Petroleum Engineering Handbook, in: H.B. Bradley (Ed.), third ed., Society of Petroleum Engineers, Richardson, Texas, 1987.
 C.H. Whitson, M.R. Brule, Phase Behavior, Monograph 20, Henry, L. Doherty Series, Society of Petroleum Engineers, Richardson, Texas, (2000).
 L.N. Mohinder (Ed.), Piping Handbook, seventh, ed., McGraw-Hill, New York, 1999.
 L.F. Moody, Friction Factors for Pipe Flow, Trans, ASME vol. 66 (1944) 671–678.
 B.E. Larock, R.W. Jeppson, G.Z.Watters, Hydraulics of Pipeline Systems, CRC Press, Boca Raton, Florida, 1999.
 Crane Company, Flow of Fluids through Valves, Fittings and Pipe, Technical Paper No. 410, 25th printing, (1991).
 J.P. Brill, H. Mukherjee, Multiphase Flow in Wells, Monograph vol. 17(1999), L. Henry, Doherty Series, Society of Petroleum Engineers, Richardson, Texas.
 H.D. Beggs, J.P. Brill, A Study of Two Phase Flow in Inclined Pipes,, Journal of Petroleum Technology vol. 25 (No. 5) (1973) 607–617.
 Y.M. Taitel, D. Barnea, A.E. Dukler, Modeling Flow Pattern Transitions for Steady Upward Gas-Liquid Flow in Vertical Tubes, AIChE Journal vol. 26 (1980) 245.
 PIPESIM Course, Information on Flow Correlations used within PIPESIM, (1997).
 H. Duns, N.C.J. Ros, Vertical Flow of Gas and Liquid Mixtures in Wells, Proc. 6th World Petroleum Congress, Section II, Paper 22-106, Frankfurt, 1963.
 J. Qrkifizewski, Predicting Two-Phase Pressure Drops in Vertical Pipes, Journal of Petroleum Technology (1967) 829–838.
 A.R. Hagedom, K.E. Brown, Experimental Study of Pressure Gradients Occurring During Continuous Two-Phase Flow in Small-Diameter Vertical Conduits, Journal of Petroleum Technology (1965) 475–484.
 H. Mukherjce, J.P. Brill, Liquid Holdup Correlations for Inclined Two-Phase Flow, Journal of Petroleum Technology (1983) 1003–1008.
 K.L. Aziz, G.W. Govier, M. Fogarasi, Pressure Drop in Wells Producing Oil and Gas, Journal of Canadian Petroleum Technology vol. 11 (1972) 38–48.
 K.H. Beniksen, D. Malnes, R. Moe, S. Nuland, The Dynamic Two-Fluid Model OLGA: Theory and Application, SPE Production Engineering 6 (1991) 171–180. SPE 19451.
 A. Ansari, N.D. Sylvester, O. Shoham, J.P. Brill, A Comprehensive Mechanistic Model for Upward Two-Phase Flow in Wellbores, SPE 20630, SPE Annual Technical Conference, 1990.
 A.C. Baker, K. Nielsen, A. Gabb, Pressure Loss, Liquid-holdup Calculations Developed, Oil & Gas Journal vol. 86 (No 11) (1988) 55–59.
 O. Flanigan, Effect of Uphill Flow on Pressure Drop in Design of Two-Phase Gathering Systems, Oil & Gas Journal vol. 56 (1958) 132–141.
 E.A. Dukler, et al., Gas-Liquid Flow in Pipelines, I. Research Results, AGA-API Project NX-28 (1969).
 R.V.A. Oliemana, Two-Phase Flow in Gas-Transmission Pipeline, ASME paper 76- Pet-25, presented at Petroleum Division ASME Meeting, Mexico City, (1976).
 W.G. Gray, Vertical Flow Correlation Gas Wells API Manual 14BM (1978).
 J.J. Xiao, O. Shoham, J.P. Brill, A Comprehensive Mechanistic Model for Two-Phase Flow in Pipelines, SPE, (1990). SPE 20631.
 S.F. Fayed, L. Otten, Comparing Measured with Calculated Multiphase Flow Pressure Drop, Oil & Gas Journal vol. 6 (1983) 136–144.
 Feesa Ltd, Hydrodynamic Slug Size in Multiphase Flowlines, retrieved from http:// www. feesa.net/flowassurance.(2003).
 Scandpower, OLGA 2000, OLGA School, Level I, II.
 Deepstar, Flow Assurance Design Guideline, Deepstar IV Project, DSIV CTR 4203b– 1, (2001).
 J.C. Wu, Benefits of Dynamic Simulation of Piping and Pipelines, Paragon Technotes (2001).
 G.A. Gregory, Erosional Velocity Limitations for Oil and Gas Wells, Technical Note No. 5, Neotechnology Consultants Ltd, Calgary, Canada, 1991.
 American Petroleum Institute, Recommended Practice for Design and Installation of Offshore Platform Piping System, fifth edition,, API RP 14E, 1991.
 M.M. Salama, E.S. Venkatesh, Evaluation of API RP 14E Erosional Velocity Limitations for Offshore Gas Wells, OTC 4485 (1983).