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Well Control >  Freeze Operation  

Hot Tap Operations


Hot tapping is a procedure that is used to drill a hole through a pressure barrier using special equipment to ensure that the pressure and fluids are safely contained while to gaining access to the interior of the barrier. In other words this is a method whereby a pipeline, tank or drill pipe can be safely drilled into while it is in service and under pressure.

Experienced Safety Boss team prepares for a hot tap and freeze procedure.

Prior to starting the hot tap a clamp device is installed around the pipe to be tapped, this seals the pipe to the hot tap chamber, or in the case of a low pressure situation a flange or weld-o-let is welded on to the pipe, then a new valve is attached, then the hot tapping machine is mounted. The new hole is drilled then when the process is complete the drill is with drawn and the newly installed valve closed thereby containing the pressure and fluid. As long as there is room for the hot tapping equipment to be mounted hot tapping can be done from very small sizes( ½” and less) up to very large diameter( 36” and greater) and very high pressure situations.

Some examples where hot tapping is used are:

  • To make a new connection into a pipeline without depressuring the line or shutting down.

  • To drill through a damaged valve that will not open.

  • To re-enter an abandoned well, where no valve exists.

  • Where an obstruction or blockage in well bore tubulars exists when they are being tripped out of the hole this may result in abnormal pressures being trapped in the tube body as it comes to surface.



Freeze Operations


A Freezing operation involves establishing a frozen plug in a wellhead or pipeline so that the wellhead or pipeline can than be depressured and opened up downstream of that plug.

   



       		  

Changing out a damaged or inoperative wellhead master value is a typical example of the use of freezing plugging. A jacket containing the freezing medium is installed around the wellhead below the master value then the complete wellhead is pumped full of a thick mud solution which, given time, freezes into a solid block. Once the freeze plug is established it is pressure tested, the downstream side is depressured, and then the master valve can be removed and replaced. With the new master in place the plug is thawed and the wellhead can be placed back into service.


The freezing medium is usually dry ice (CO2) however liquid nitrogen can also be used. The length of a freeze plug is directly related to the length of the freeze jacket. Even large diameter pipelines can be freeze plugged but long jackets are required. Double freeze plugs can be used to hold pressure from both sides while a center section, between the plugs, is depressured and opened.

Experienced Safety Boss team prepares for a hot tap and freeze procedure.            Experienced Safety Boss team prepares for a hot tap and freeze procedure.

Freezing is also often done in combination with other procedures such as hot tapping and or trash plugging. For example hot tapping can be done prior to freezing to provide an access point for freeze plugging material (mud) or after freezing to inject a well kill fluid down a well below a freeze plug.

Trash plugging is a procedure that can be used to stop flow by introducing a plugging material to jam-up a leak-off area. An example would be an external leak on a cracked valve body. Trash, such as knotted rope, is introduced into the line so that it will either flow to or can be pumped to the crack outlet. The trash is sized so that it will jam in the crack thereby stopping the flow. With the flow now static a freeze plug can be placed upstream of the valve, when it is in place the down stream portion including the cracked valve can be depressured, opened and replaced.

Trash plugging can be amazingly effective. Safety Boss gained extensive experience trash plugging the explosives damaged wellheads in Kuwait. These wellheads often containing numerous and large fractures were successfully plugged by carefully sizing and staging various trash plugging materials.

This is a general description of freezing practices; the full freezing procedure is much more complicated than indicated here.

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