Whilst 'Waiting on Weather' it might be necessary to hang the drill string off from the wellhead or BOPs. In severe cases, it might also be necessary to disconnect the riser from the BOP stack. The decision to 'hang-off should be taken to allow sufficient time for the bit to be pulled safely into the casing shoe. A natural tendency for the driller to pull up too quickly could perhaps induce swabbing and a possible kick situation later.
The preferred method to hang-off the string is to use a special hang-off tool which supports the pipe in the wellhead. It consists of a fluted hanger that is designed to land on the wear bushing, casing hanger or seal assembly, a slick joint around which the pipe ram(s) can be closed and an upper sub containing both a coarse left hand thread and a drill pipe thread box. Each hang-off assembly should be appropriate to the particular wellhead (BOP) in use so that once it is in place and disconnected the pipe ram(s) can be closed round the slick joint and the shear rams can be closed above the top sub.
For some wellhead systems, only one fluted hanger is needed and can be run whatever size wear bushing or casing hanger is in the wellhead. Other systems require the hanger to be changed according to the size of the last casing string run. Various spacer subs appropriate to the distance between the hang-off point and the shear rams might also be required.
By the time step 4 is reached, the rig could be heaving quite a lot so if possible the Kelly/Top Drive should be used to release the running sub. It is good practice of course to have the hang-off assembly (appropriate for the wear bushing size) permanently made up and stood back in the derrick ready for use.
In the event that there is pressure under the rams, this could be caused by gas swabbed into the well whilst pulling to the shoe and which had migrated whilst waiting on weather. If so, it might be possible to safely remove it by circulating under the ram — down the kill line and up the choke line. The alternative would be a 'top kill' or stripping operation.
Barite settling out from the mud and causing a reduction in hydrostatic can also be a problem during the prolonged periods of waiting on weather.
In the event that a very severe storm blows up without warning, there might not be sufficient time to pull the bit into the casing and use the hang-off tool. If this emergency arises, the bit should be pulled just the distance from the rotary table to the stack so that one of the drillpipe connections can be broken and made up again only chains tong tight. The string is then run again and the loose tool joint landed carefully on the upper pipe ram. With the weight slacked off, the string is turned to the left to back it of at the loose connection. The other pipe rams, shear rams and ram locks (if applicable) can then be closed.
Note that in this case, it might be better not to run an inside BOP since it is very likely that the string, being hung off in open hole, will be found to be differentially stuck and wireline backoff operations could be necessary. If the string does have to be hung-off in open hole, then the hang-off tool should not be used at all.
As an alternative to a special hang-off tool, it is possible to use a joint of drillpipe with a square shouldered upset on it which can be landed on the pipe rams. Such a method is satisfactory but is not preferred to landing the string in the wellhead.
When running long strings of casing, it is always advisable to have a suitable casing/drillpipe crossover available to enable the hang-off tool to be used to hang-off the casing string if necessary.
Another method of hanging-off the string is to support it from a retrievable packer set in the casing just below the wellhead. To do this requires more time than if a hang-off tool were used but in some circumstances, it might be the most suitable means available.
In addition to the retrievable packer (which seals off the annulus) a special storm valve is used in the string. This is made up into the packer and serves both as means of releasing the running string and sealing off the bore of the drill pipe. The valve, which is a sleeve type operated by left hand rotation.
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