Risk of Hydrates in High-p ressure Natural Gas Transport Lines
mixed phases cool, hydrates form and plug transmission lines, causing costly production shut-downs in large pipelines, sometimes for as long as months, while the hydrates are dissociated.16
• The hydraulic removal method is based on the dissociation of the hydrate plug by depressurisation from one or both sides of the hydrate plug. This method seems promising given the porous structure of the gas pipeline plugs.8,20
However, depressurisation
The marine transportation of natural gas in compressed form using ocean-going ships is an evolving technology.
Fortunately, the hydrate stability temperature and pressure range are predictable to within experimental accuracy using modern thermodynamic (TI) programmes usually based upon the Gibbs free energy extension18
from one side should be performed carefully as the hydrate may get dislodged and travel along the line at high speed and cause severe damage to the pipe.
• Thermal methods consist of delivering a local heat flow towards the plug through the pipe wall in order to raise the system temperature above the hydrate-formation point. This method is possible for onshore pipelines but unsuitable for subsea equipment.11
• Mechanical methods, such as pipeline pigging, can be used to prevent hydrate plugs. Pipeline pigs are inserted into the pipe and travel throughout the pipeline, driven by fluid flow. These projectiles then remove the obstacles or deposits they encounter.8
of the van der Walls and Platteeuw (1959) method. Commercial multiphase programmes, such as SPT OLGA and SimSci- Esscor® (and many others), can predict the pipeline conditions leading to free hydrate formation. In a pipeline, hydrates form and accumulate at the hydrocarbon–water interface. Once the hydrates accumulate enough to plug the pipeline, they hinder the flow through the pipeline. This is a major safety and economic concern to the industry. Unfortunately, low temperatures (such as deep-sea floor temperature of 271K) and the mandates of high pressure for economic energy densities place many pipelines well within the hydrate-formation region.
Combating Hydrate Formation
Gas hydrates may be of potential benefit both as an important source of hydrocarbon energy and as a means of storing and transporting natural gas. However, they pose severe operational problems, as the crystals may deposit on the pipe wall and accumulate as large plugs that can completely block pipelines and lead to shutting down production.11,14
Methods of preventing hydrate solids development in gas production systems have been of much interest for a number of years,19 include the following methods.
and
• Chemical methods, which can be used either to prevent or to remove plugs, consist of injecting additives in the pipe that act differently on hydrate agglomeration according to whether the inhibitors are TI, kinetic (KI) or dispersant (DI).
1. Brill JP, Mukherjee H, Multiphase Flow in Wells, Monograph
volume 17, SPE Henry L Doherty Series, Society of Petroleum Engineers Inc., Richardson, Texas, 1999.
2. Svandal A, Kvamme B, Grànàsy L, Pusztai T, The Influence of
Diffusion on Hydrate Growth, J Phase Equilibrium Diffusion,
2005;26(5):534–8.
3. Buanes T, Kvamme B, Svandal A, Computer simulation of CO2 hydrate growth, J Cryst Growth, 2006;287(2):491–4.
4. Imen C, Delaheye A, Fournaison L, Petitet JP, Benefits and drawbacks of clathrate hydrates: a review of their areas of
interest, Energ Convers Manag, 2005;46(9–10):1333–43.
5. Holder GD, Enick RM, Solid Deposition in Hydrocarbon System-Kinetics of Waxes, Aspaltens and Diamondoids, Final Report, Gas Research Institute, 1995.
6. Lee S, Liang L, Riestenberg D, West OR, et al., CO2 Hydrate Composite for Ocean Carbon Sequestration, Environment
Science Technology, 2003;37:3701–8.
7. Englezos P, Clathrate Hydrates, Ind Eng Chem Res, 1993;32:
108
1251–74.
8. Sloan ED, Clathrate Hydrates: The Other Common Solid Water
Phase, Ind Eng Chem Res, 2000;39:3123–9.
9. Tanasawa I, Takao S, Low-Temperature Storage Using Clathrate Hydrate Slurries of Tetra-n-Butylammonium Bromide: Thermo-physical Properties and Morphology of Clathrate Hydrate Crystals. In: Fourth International Conference
on Gas Hydrates, Yokohama, Japan, 2002;963–7.
10. Fournaison L, Delahaye A, Chatti I, Petitet JP, CO2 Hydrates
in Refrigeration Processes, Ind Eng Chem Res, 2004;43(20):
6521–6.
11. Sloan ED, Clathrate Hydrate of Natural Gases, 2nd edn., Cap. 2,
Marcel Dekker: New York, 1998;455–64.
12. Wilkens RJ, Flow Assurance. In: Fluid Flow Handbook, Chapter 29, Saleh J ed., McGraw-Hill: New York, 2002.
13. Jadhawar P.S, Inhibition Methods: Gas Hydrate Problems in
Pipelines, Chem Eng World, 2003;38(6):65–8.
14. GPSA Engineering Data Book, 11th Ed., Gas Processors Suppliers
EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 1
• A combination of mechanical and hydraulic methods may also be used to remove hydrate plugs. Hydrates may form at the flowpath connecting the subsea production trees and the main production manifolds. The manifold is usually isolated by the subsea tree wing valves. Therefore, the trees remain pressurised up to the wing valve and depressurisation is not possible from
The ultimate remedy for a hydrate problem is to
remove or reduce the water content of the gas using gas-dehydration processes.
both sides. In such circumstances, work-over drilling rigs may be called to penetrate the tree through coiled tubing and help to depressurise upstream of the wing valve to dissociate the hydrate plug. Obviously, this will be a costly solution.
• The ultimate remedy for a hydrate problem is to remove or reduce the water content of the gas using gas-dehydration processes. However, in practical field operations, water can be economically removed only to a certain vapour pressure and residual water vapour always exists in a dry gas.21 Gas dehydration reduces the risk of rapid hydrate formation and deposition. ■
Association: Tulsa, OK, 1998.
15. Hammershmidt EG, Formation of Gas Hydrates in Natural Gas Transmission Lines, Ind Eng Chem, 1934;26(8):851–5, .
16. Sloan ED, Fundamental principles and applications of Natural Gas Hydrates, Nature, 2003;426:353–9.
17. Lysne D, An Experimental Study of Hydrate Plug Dissociation by
Pressure Reduction, PhD thesis, Norwegian Institute of Technology, University of Trondheim, 1995.
18. Ballard A, Sloan ED, The Next Generation of Hydrate
Prediction: An Overview, Proc. of the 4th International Conference Gas Hydrates, 2002.
19. Bufton SA, Ultra Deepwater will Require Less Conservative Flow Assurance Approaches, Oil Gas J, 2003;101(18):66–77.
20. Kelkar SK, Selim MS, Sloan ED, Hydrate Dissociation Rates in
Pipeline, Fluid Phase Equil, 1998;150(15):371–82.
21. Denney D, Enhanced Hydrate Inhibition in an Alberta Gas
Field, Natural Gas Technology, 2005;74–5.
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