Snubbing/use of a hydraulic work-over unit (HWU) is a rig-less ‘heavy duty' technique used when lighter deployment methods such as wire-line and coiled tubing are not workable. Snubbing can be described generally as using created force to snub the pipe string into a well against the pressure in the wellbore. The pipe string consists of the pipe itself and a bottom-hole assembly (BHA) of tools, very similar to the set-up used in drilling operations. The snubbing intervention technique does not require removal of the christmas tree or the wellhead, hence it can be done on a live well.
HWUs come in different sizes with various strength ratings. The majority of the units used in the GCC are the 340K type (K here means pull capacity), although some are 460K. On average between 7 to 10 personnel per shift are required to operate a snubbing unit.
Generally, snubbing is used in heavier applications and/or complete well work-over jobs. Snubbing technology offers a more cost-efficient approach to well intervention when compared to drilling/work-over rig options. The footprint of the snubbing unit is compact, hence it can be installed on an existing platform, subject to load-bearing capacity, or deployed using a lift boat / jack-up barge, when done offshore. In onshore applications, the snubbing unit is track-mounted, so it may require a tug truck to transport it.
A number of safety risks are associated with this intervention approach, so a comprehensive desktop study must be done prior to using this deployment method. The rig-up process is quite complex, which sometimes renders this intervention method uneconomical. Yet, this technology is still much more cost-effective than a drilling/work-over rig.
As a rule of thumb, an HWU is used when there is both a high-producer well and a high oil price, unless no other option is available owing to technical reasons (e.g. space/access constraint).