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Hydrate plug removal
In addition to the more immediate operating safety hazards, such as plugging blowout preventers, blocking drillstrings, and collapsing casing and drilling annuli, there are less obvious but very important safety hazards for removing hydrate plugs from flow channels. Frequently, improper removal of hydrate plugs results in damage to equipment and threats to safety of personnel.
Hydrates cause safety problems for two reasons (both of which are shown schematically in Figs. 1.1a and 1.1b):
- Upon removal, when hydrate plugs are depressurized improperly, with large pressure gradients across the plug, hydrate projectiles frequently erupt from pipes
- When hydrates are heated, large confined pressure increases cause pipe ruptures.
The most common way to remove a hydrate plug from a flow channel is by depressurization. Flow is stopped, and the line is slowly depressurized from both ends of the plug. At atmospheric pressure, the hydrate stability temperature is invariably less than that of the surroundings, so heat flows from the environment into the hydrate plug. The plug melts radially inward, detaching first at the pipe wall.
Any pressure gradient across the detached plug causes it to act like a projectile, as shown in Fig. 1.1a,  with measured plug velocities up to 180 miles/hr for short distances. The hydrate has the density of ice, almost twice that of the surrounding fluid, so at the line velocity, the plug momentum is twice that of the surrounding fluids. When the hydrate projectile encounters an obstruction or change in flow direction, such as a pipe elbow, bend, or valve, the resulting impact or pressure increase frequently causes line rupture, equipment damage, fire, and potential injury or loss of life.
Example 1: A common plug-removal hazard
A Siberian incident, in February 2000, illustrates the second common plug-removal hazard, shown in Fig. 1.1b. A pipe fitter was attempting to remove a hydrate plug by heating an exposed pipeline with a torch. The gas pressure from a dissociated mid-hydrate plug rose rapidly, perhaps being confined by the plug ends. The pipeline exploded, and, in the resulting fire, one man died; four others were badly injured.
Safely removing hydrate plugs
Hydrate-plug dissociation should always be done slowly and with great care. Rules-of thumb for safe hydrate-plug removal may be summarized as:
- Always assume multiple hydrate plugs; there may be pressure between the plugs.
- Attempting to move hydrate (or ice) plugs can cause ruptures in pipes and vessels.
- While heating a plug is not normally an option for a buried or submerged pipeline, heating should always be done with great care from the ends of the plug. Heating should be done only with assurance that the plug ends will not contain the pressure.
- Depressurizing a plug gradually from both ends is recommended as a safer alternative to single-sided depressurization. However, it may be impossible to depressurize from both sides, as when only one plug end is accessible or when a very long time is are required to depressurize a large upstream volume. In such cases, very careful single-sided dissociation may be done by experienced personnel.
There are several recommendations regarding hydrate-plug removal, for example::
- Monitor the system from early hydrate warnings, such as slush in pigging returns; changes in water rates and fluid compositions at the separator; pressure-drop increases; and acoustic signals (pinging) of hydrates hitting the pipelines. Before the line plugs inject methanol or glycol to prevent full flow blockage.
- Pigging partially plugged lines and backpressuring plugged lines should be used with care because plug compaction or “snowplow” accumulation may occur.
- Locate the hydrate-plug midpoint through pressure cycles, monitoring the rate of change of upstream pressures upon reduction or increase of the downstream pressure.
- Slow depressurization from both sides of hydrate plugs is the preferred method of removal. One-sided depressurization should be done very slowly and cautiously and, then, only if two-sided depressurization is not an option. In some cases, the fluid hydrostatic pressure must be removed from the face of a plug to enable depressurization; this may be done using coiled tubing as indicated in the following subsection.
- Hydrate plugs melt radially upon slow, two-sided depressurization. It is possible to predict the time for two-sided hydrate dissociation, to determine the size of the annulus between the plug and the pipe, for both dissociation and inhibitor injection past the plug.
- Unlike some cases with wax plugs, hydrate-plugged lines have always been freed from obstruction. However, safety concerns, time, and patience to wait days or weeks are required for hydrate dissociation after depressurization. Attempts to make hourly changes are ineffective.
- Some solutions, such as attempting to “blow the plug out of the line,” can make the situation worse with a larger, compacted hydrate plug.
- Methanol or glycol injection is usually ineffective because of the necessity of having the inhibitor contact the hydrate-plug face. When hydrates form in a vertical portion of a channel, such as a riser or well string, it may be possible to inject glycol or to place a heater at the plug face to promote hydrate dissociation.
Hydrate-plug-removal case studies are detailed in Appendix C of Ref. 2.
Emerging technology for hydrate-plug detection and removal
The emerging methods are divided into plug detection and plug removal.
There are several methods of determining the temperature and pressure along various points in a flow line. These involve sophisticated methods using fiber optics, Raman spectroscopy, Brillouin backscattering, Bragg grating pressure sensors, and acoustic hydrophones. To date, these methods have been demonstrated only under research conditions. For hydrates in lines above the water, it is possible to locate the hydrate plug on depressurization, using infrared sensors to determine the low temperature caused by the endothermic heat of dissociation. (See Fig. 1)
Coiled tubing represents the primary mechanical means of freeing the hydrate plug, but the maximum coiled-tubing distance is currently approximately 5 miles. Coiled tubing may be used to remove a substantial liquid hydrostatic head at the hydrate face to enable depressurization. Coiled tubing may also be used to inject methanol or glycol at the face of a hydrate plug, when density is insufficient to drive the inhibitor to the plug face.
- King, R. et al. 1994. CAPP Guidelines for the Prevention and Safe Handling of Hydrates. Calgary, Canada: Canadian Assn. of Petroleum Producers.
- Sloan, E.D. 2000. Hydrate Engineering, Vol. 21, Ch. 2, 44, 46-47. Richardson, Texas: Monograph Series, SPE.
- Kullik, R. and Allen, J. 1998. Flow Assurance Instrumentation. Presented at the Offshore Technology Conference, Houston, Texas, 4-7 May. OTC-8733-MS. http://dx.doi.org/10.4043/8733-MS
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