--- III. itti A. on_ coo. & EP Department DOCUMENT NO: WOC-HSE-ST Section H005 Hydrates

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1 --- III itti A. on_ coo WOC LP & EP Department WOC HSE Standards Manual DOCUMENT NO: WOC-HSE-ST-0001 Section H005 Hydrates REV ISSUE PREPARED BY REVIEWED BY APPROVED BY ISSUE DATE 044 /- 6 WO Executive 0 Issued for Use W. McIntosh LP & EP Manager Vice President Operations June 2009

2 ft,-,,- *Atli _,Laik of, IL c Hydrates WOC-HSE-ST June of 10 PURPOSE To provide guidelines for the proper identification and safe removal of gas hydrates. APPLICATION All WAHA plant and field facilities. POTENTIAL HAZARDS Caution must be exercised when a line where hydrates are suspected, is being depressured and opened; the hydrates may plug the line and trap pressure as well as dissolve and release hydrocarbons and toxic gases after depressuring. NOTE : Hydrated lines should never be totally depressured. Pressure should only be reduced to a point below the hydrate curve. REQUIREMENTS FOR HANDLING HYDRATES Caution: Do not attempt to remove hydrates by trying to force them to move through increasing or decreasing pressure on one side of the plug. Moving ice plugs will rupture pipes and vessels. Partially hydrated systems (a plug is forming but has not yet completely blocked the line) can be treated with methanol or glycol once the condition that caused the hydrate is removed. 5. PROCEDURES 5.1. Identification Static pressure spiking on pipeline systems is the first indication that hydrates are beginning to form. A drop in flow should not be solely attributed to liquid hold-up; hydrates must be considered as the possible cause. The same applies to flowing wells, particularly if the wellhead temperature and pressure are dropping A pipeline system should not be run when hydrates are forming, unless methanol is being injected or the flowing temperature is being increased over a short period of time.

3 1,1 4 0, c "..,,..._.,. Hydrates WOC-HSE-ST June of Removal Prior to removal, a site-specific procedure must be for the safe handling of hydrates Site specific procedure must include, but not be limited to the following: All hydrates procedure must be considered to be safety hazards, Supervisor or designate must be notified when a hydrate is identified All piping/equipment used for depressuring will meet WAHA Piping specifications, all threaded piping is to be no more than IA " NPT. Depressuring valves must be needle or globe valves. 6. DEFINITIONS 6.1. Hydrates are a mixture of water and gas that will form a solid "ice plug" under appropriate conditions of temperature and pressure. The water for the hydrate comes from free water in the gas stream, as is the case when the gas stream is at or below its water dew point An unusual characteristic of hydrates is that their formation is not strictly temperature dependent and will form at high pressures when the temperature of the flowing gas is well above the freezing point of water. Because hydrates produce a restriction to the normal flow of gas, there will be a differential pressure across the hydrate, which contributes to further growth of the hydrate until it, completely blocks normal flow Hydrates present a very real hazard to people and equipment if they are not removed correctly. Hydrates can disintegrate and pose a real threat to both people and equipment. As the plug dissolves gases are released and velocities will reach approximately 400 m/sec., creating a shock wave and behaving as a solid projectile. If large differential pressures are created across the plug in order to force it to move (possibly coupled with the addition of methanol to dissolve the plug), tremendous forces are released once the plug starts to move and disintegrate at the same time. Pipe rupture can happen one of two

4 0k 71 " W#.tii 14'4 oft '0.-"-' Hydrates WOC-HSE-ST June of 10 ways. Firstly as a result of impact (the plug or pieces of it striking the pipe wall or protrusions from the pipe wall) or secondly by overpressure from a shock wave. 7. Special consideration for pipelines 7.1. Hydrate formation is a significant problem in pipelines that carry wet gas. Pockets of water will form in low points of the line, and hydrates can form downstream of that pocket of water particularly if the pipe transitions through a change in temperature. Apart from temperature change of the pipe, the gas temperature itself will decrease as a result of travelling through the pocket of water (pressure drop) and the once saturated gas now has free water associated with it at a reduced temperature. For pipelines that carry wet gas and traverse changing elevations, hydrates will never form in only one place, they will form in every place that has a change in elevation because at each of these places there will be pockets of water. See following illustration. GAS SATURATED WITH WATER HYDRATE COOLED GAS RELEASES FREE WATER FLOW FREE Vy'ATER 144 PRESSURE DROP --11P-1 -produces temperature drop i n gas st ream 7.2. For longer pipelines where there can be more than one plug, do the following: Isolate a section of line, preferably below and above a change in elevation (this is the most likely location for a hydrate - see illustration) and install a pressure indicator at

5 Hydrates WOC-HSE-ST June of 10 each end. Slowly de-pressure one end of the isolated section about 10% of the line pressure. If both pressure indicators read the same and decrease at the same rate, the plug is not in that section. Continue to move along the pipeline and isolate the shortest possible sections, testing as above, until the section containing the plug is located. Be aware that there may be more than one plug. Doing this test in the shortest possible sections reduces the distance a plug could travel once it disintegrates Once the section of pipe containing the hydrate is located, inject methanol at one end of the line while slowly depressuring the other end Don't fully de-pressure one side but do reduce the pressure below the point of hydrate formation, close the depressuring valve and allow time for the hydrate to disintegrate. If the plug dissolves enough to start moving, it won't move far against a closed valve At the end of the line where methanol was injected, the methanol will vaporize and move through the line by diffusion, the same way as gas line antifreeze works when put in the gas tank of your car Watch the pressure indicators at each end of the line. A sudden change in either of them indicates that a plug is moving. Immediately stop or dramatically reduce the flow at the depressuring point. Control the movement of the plug(s) by controlling the rate of depressurization To test for disintegration of the plug or to return the line to service: Close the valve nearest to the downstream side of the plug. The hydraulic effect of trying to move against a closed valve will prevent the plug from moving.

6 oi, c 09 + Hydrates WOC-HSE-ST June of SLOWLY re-pressure the line. Both gauges should always read the same if the plug has dissolved. If the downstream gauge does not respond at the same rate as the upstream gauge, the plug has only partially dissolved, allow more time Once you are sure that the plug has disintegrated, be sure that the condition that created the hydrate in the first place is no longer present and then SLOWLY reestablish flow through the process. Low initial flow rates will assure that there is not enough energy for portions of any remaining plug to cause a problem. 8. HYDRATE FORMATION IN WELLHEADS AND IN TUBING 8.1. Once a hydrate has formed and plugged the wellhead, it can be mechanically removed or disintegrated by heat, depressurization, or alcohol injection. Following are cases in which these methods are used: 8.2. If the hydrate is small, increase the flow rate slowly in an attempt to dislodge the plug If serious, and suspected to be below the master valve or in the tubing near the surface, shut the wing valve and master valve and depressurize the top of the tree. Remove the unibolt or hammer cap of the tree and fill the body of the wellhead above the master valve with methanol. Replace the cap and open the master valve, flow should return to normal. This can be very time consuming and sometimes require pressuring up with methanol or working with wireline equipment Caution is required when dealing with hydrates in the wellhead. It is necessary to know the characteristics of all wellhead valves such as the number of turns required when closing or opening them fully. If this is unknown, there is a danger that a hydrate plug might prevent full closure of the valves thus creating a very hazardous situation which may cause a release of the hydrate at any time. (i.e. The hydrate could release while person is adding methanol through the top of the wellhead.)

7 1, V sve :,,.,,,,,,,:- V}. - w 4._..:, orl QPa c Hydrates WOC-HSE-ST June of It is also important that the torque limits of the valves are not exceeded when attempting to open or close them when their actions may be inhibited by hydrates or freezing. NOTE: All Company Safe Operating Practices must be adhered to during critical tasks. The hydrate SOP must be reviewed on a regular basis with all affected personnel. (Minimum once a year) 9. HOW HYDRATES ARE FORMED 9.1. Liquid water is actually a loosely formed group of molecules that have spaces between them. When those spaces are filled with other molecules, crystals will form and the mixture becomes solid Gas molecules can occupy those spaces under the right conditions of temperature and pressure based on the solubility of the gas in water All gases will dissolve to some degree in water at normal temperatures and under atmospheric pressure conditions, as is the case, for example, with oxygen. If oxygen were not dissolved as a gas in lakes and streams, plants and fish would not live The amount of gas that dissolves in water to occupy the spaces between the water molecules depends on the temperature and the pressure. When the pressure is high, gas compresses and fills the spaces until they are saturated and won't hold any more. This saturation from high pressure can occur at temperatures well above the freezing point of water, which explains why hydrates occur in gas streams that are "warm". Once the water is saturated with gas, chemical bonds change and crystal growth begins leading to the eventual formation of a solid mass of water and gas Each component of natural gas has a different solubility in water and, as a result, hydrates will form at different pressures and temperatures for different compounds. Heavier gases

8 z.13-ti. ', - *, : * 4 OIL COI' g Hydrates WOC-HSE-ST June of 10 like butane and pentane are unable to occupy the spaces in the water molecule because they are too large but the lighter gases like methane and ethane do so readily The attached chart, "Hydrate Formation", demonstrates when a hydrate will form in a typical natural gas stream for different concentrations of H 2S. Continuing to operate at temperatures and pressures above the line means that a hydrate will form if the gas is not dry. Hydrates will not form when operating at temperatures and pressures below the line From the chart it can be seen that as the concentration of H 2S increases, so does the temperature at which hydrates will form for the same pressure. For example, gas in a line or vessel operating at 800 kpa will form a hydrate as the temperature drops below 0 C with no H 2S present, but a gas containing 10% H 2S will form a hydrate as the temperature drops below 10 C Hydrates, like any other obstruction in a line, can be detected by the consequences they create. Obstructions will reduce flow, increase backpressure on a system and increase the differential pressure across a part of the process.

9 . _ -, l'40,, JQP2 Hydrates WOC-HSE-ST June of 10 Hydrate Formation E 4000 a) a. 0% H2S 1% H2S II 5% H2S X10% H2S Temperature (celsius) 10. Preventing hydrates from forming Having a better understanding of how hydrates are created will help the operator prevent situations that allow them to form. Any one of the following can prevent hydrate formation: Prevent free water from being present in the gas stream dehydrate the gas elevate the temperature to put more of the water in the vapour phase routine pigging of a flow line

10 TITLE Hydrates DOCUMENT NUMBER REV DATE PAGE WOC-HSE-ST June of Increase the temperature of the flowing gas above the temperature needed for hydrate formation at the current operating pressure Decrease the pressure below the point needed for hydrate formation at the current operating temperature A restriction in a flowing line caused by a hydrate creates a pressure drop and further refrigeration thus increasing the size of the hydrate. The flow must be reduced in order to stop the growth of the hydrate and reduce the forces that will be present once the hydrate starts to move. Throttling a valve to reduce the flow will create the same auto-refrigeration effect at the valve and encourage another hydrate to form The following are options for removing a hydrate plug that has completely blocked a line. Principle: Use hydraulics to your advantage. A solid object in a gas stream cannot move when a valve at some point downstream of the plug closes the line. Stopping or reducing the flow will stop or limit the movement of the plug The most effective way to disintegrate a hydrate is to reduce the pressure equally on both sides of the plug to just below the hydrate formation point. Use a pressure indicator on each end of the line to assure that there isn't a significant difference in pressure It is the difference in pressure that gives the plug the energy to move. When the pressure has been reduced below the point at which hydrates will form, the plug will begin to disintegrate. Allow time for the hydrate to dissolve and test for completion. 11. REFERENCES Natural Gas Hydrates by John Carroll ISBN