Application of hydraulic workover units to North Sea operations
Authors: Brown J.D. 
Published: December 2007

Introduction
As more fields are brought into production, North Sea operators will be faced with an increasing need for well maintenance and for deciding how best they are to be tackled. Reservoir pressure decline faces all operators as production continues, which may or may not be avoided production continues, which may or may not be avoided or stabilized by stimulating the drive by gas injection or water flooding programmes. Should a field approach the point where pressure decline fast makes redundant the point where pressure decline fast makes redundant the use of conventional workover/kill fluid due to fluid losses and resulting potential formation damage, then, routine workovers can become a problem. Several techniques are available today such as the adoption of diverting agents; uni-beads; retrievable bridge plugs, (limited by tubing/ casing sizes) etc. to plug off or isolate the perforations so allowing the well to be circulated to a kill fluid and consequently serviced safely.
However, it is the intention of this paper to consider the hydraulic workover unit, a recent arrival to North Sea operations, with its ability to work over both oil and gas wells under pressure apart from performing most of the operations normally conducted by medium to heavy conventional workover rigs.
The technique of working on live wells, or 'snubbing' as it is often yet erroneously referred to generally, is by no means new and was first developed out of necessity in the USA sometime back in the thirties,
The operation of snubbing refers to the need to restrain the pipe or tubing from being blown out of the well whilst stripping it in or out of the well and occurs only at the stage when the hanging weight of the string is equal to - the balance point - or less than the resulting upward thrust created by the wellbore pressure acting upon the blanked off cross-sectional area of the tubing. Below this 'balance point' and for the greater part of the well the tubing is simply 'stripped' in or out, the blowout preventers containing the well pressure.
For many years, however, operating on live wells was only considered as an emergency measure and it has been only in the last decade or so, with the advance in more reliable and sophisticated equipment that it has gradually gained acceptance by a great part of the oil industry as a routine method of tackling certain well workovers. In the USA the economic and practical benefits of such operations have been appreciated for some years now and it is no surprise that hydraulic workover units should make an appearance here in the North Sea.

HYDRAULIC W.O. RIGS
These units were developed out of the demand for mobile compact yet powerful units to operate in isolated land areas or offshore on small platforms unable to support a conventional rig and where tender supported, or jack-up type rigs would prove costly.
Basically, the outfit is a hydraulic jack type rig where the draw-works are replaced by a jacking incorporating a set of (reversible) travelling slips and a double acting stationary slip assembly. A telescoping gin pole dual winch system handles the tubing so eliminating the need for a mast.
The jack unit comprises of four hydraulic cylinders that form the support columns of the jack assembly. At the top of the piston rods is a base plate supporting a travelling slip bowl assembly and a built in hydraulically powered rotary. This complete assembly will stroke to a powered rotary. This complete assembly will stroke to a height dependant upon the length of the hydraulic pistons, usually 10–12 ft. From the top of the cylinders is supported the work basket with a floor level just below the rotary with room for two to three operators with access to all the rig controls including the BOP system and pipe handling winch. The rotary table offers a limited drilling capability suitable for packer milling operations etc. Below the jack unit are the stationary slip assemblies, the upper set of which are inverted to restrain the pipe during snubbing operations; the lower set being used for conventional operations. The complete outfit is mounted directly onto the wellhead via the BOP stack when rigged up for major well servicing.
All rig functions are hydraulically motivated with a system working pressure of up to 3,000 p.s.i. power being developed from the outfit's own prime movers making them independant of platform or installation services except for lighting if and when required.
Two H.W.O. rigs arrived recently in the UK to operate in the North Sea and Europe, one owned by Baker Oil Tools and the other by Otis. Both units are high performance and represent the top of the range at present, as regards maximum pipe size handling and present, as regards maximum pipe size handling and pulling/snubbing ability although it is believed that even pulling/snubbing ability although it is believed that even larger units are being planned. The basic specifications of both units are as follows:
Baker Model HRS-340K
Maximum pull......................... 340,000 lbs. Maximum snub capacity................ 188,000 lbs. Maximum stroke....................... 10 ft. Tubing range......................... 2–3/8—7-5/8 in. o.d. Rotary torque........................ 3,500 ft, lbs.(20–150 rpm) Hydraulic system working pressure........................… 3,000 p.s.i. (max.) Total unit weight (excluding BOP's)............................. 28,200 lbs.