hydrate plug removal

8/10/2019 Hydrate Plug Removal

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Hydrate Plug RemovalIn 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 tosafety of personnel.

Safety issuesHydrates 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 . [1]

The most common way to remove a hydrate plug from a flow channel is by depressurization. Flow is stopped, and the lineis slowly depressurized from both ends of the plug. At atmospheric pressure, the hydrate stability temperature is invariablyless than that of the surroundings, so heat flows from the environment into the hydrate plug. The plug melts radiallyinward, 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 thesurrounding fluid, so at the line velocity, the plug momentum is twice that of the surrounding fluids. When the hydrateprojectile encounters an obstruction or change in flow direction, such as a pipe elbow, bend, or valve, the resulting impactor pressure increase frequently causes line rupture, equipment damage, fire, and potential injury or loss of life.

Fig. 1.1a Hydrate safety(hydrate projectiles frequently erupt through pipes)(after King e t a l . )[1]
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Fig. 1.1b Hydrate safety(hydrates dissociate rapidly upon heating, and large pressure increases cause pipe ruptures).

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 pipefitter was attempting to remove a hydrate plug by heating an exposed pipeline with a torch. The gas pressure from adissociated mid-hydrate plug rose rapidly, perhaps being confined by the plug ends. The pipeline exploded, and, in theresulting 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-plugremoval 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 containthe pressure.

Depressurizing a plug gradually from both ends is recommended as a safer alternative to single -sideddepressurization. However, it may be impossible to depressurize from both sides, as when only one plug end isaccessible or when a very long time is are required to depressurize a large upstream volume. In such cases, verycareful single-sided dissociation may be done by experienced personnel.


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Removal recommendations

There are several recommendations regarding hydrate-plug removal, for example::

Monitor the sys tem 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 thepipelines. 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 . [1] 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 anoption. In some cases, the fluid hydrostatic pressure must be removed from the face of a plug to enabledepressurization; 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-sidedhydrate dissociation, to determine the size of the annulus between the plug and the pipe, for both dissociation andinhibitor injection past the plug.

Unlike some cases with wax plugs, hydrate-plugged lines have always been freed from obstruction. However, safetyconcerns, 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 ma ke 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. W hen hydrates form in a vertical portion of a channel, such as a riser or well string, it may be possible toinject 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 . [2]

Emerging technology for hydrate-plug detection and removalThe emerging methods are divided into plug detect ion and plug removal.

Plug detection

There are several methods of determining the temperature and pressure along various points in a flow line . [3] Theseinvolve 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 sensorsto determine the low temperature caused by the endothermic heat of dissociation. (See Fig. 1 [2])
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Fig. 1 Selective intelligent completion schematic facilitating controlled gas injection from AW/AX reservoir to the overlyingAV reservoir gas cap (completion schematic courtesy of Brunei Shell Petroleum).

Plug removal

Coiled tubing represents the primary mechanical means of freeing the hydrate plug, but the maximum coiled -tubingdistance is currently approximately 5 miles. Coiled tubing may be used to remove a substantial liquid hydrostatic head atthe 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.

References

1. 1.0 1. 1 1.2 King, R. et al. 1994. CAPP Guidelines for the Prevention and Safe Handling of Hydrates . Calgary,

Canada: Canadian Assn. of Petroleum Producers.2. 2.0 2.1 Sloan, E.D. 2000. Hydrate Engineering , Vol. 21, Ch. 2, 44, 46-47. Richardson, Texas: Monograph Series,

SPE.3. 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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