Nowadays, several of the world’s oil and gas regions are in a mature development phase, with many installations experiencing a decline in hydrocarbon production. Still, they offer the potential for significant reserves from untapped, bypassed or previously noncommercial pockets if there were an economical way to access the reserves.

Many existing well completion designs also are nearing the end of their design life or are no longer suitable for the current production rates. They require workovers, and, ultimately, will have to be abandoned. Production challenges are also compounded by the fact that many of the installations no longer have serviceable rig sets or their cranes have reduced liftweight and height-reach capacity.
The latest generation of Hydraulic Workover (HWO) Units addresses these needs by providing lightweight, self-erecting modular systems capable of working under live or dead well conditions. These units can incorporate automated pipe handling and auxiliary services, such as fluid pumping, mud treatment and cutting handling, to reduce crew size and help improve safety.  With this evolution in HWO services, operators can now employ cost-effective sidetrack drilling as a replacement for drilling rig sets, especially in areas previously designated as uneconomical to drill.

History of HWO services
The evolution of HWO services can be traced back to the 1920s. Halliburton hydraulic workover operations began in 1929 when Mr. H.C. Otis, Sr. designed, patented and built the world’s first unit to run or pull pipe under pressure.
That’s when cable-operated rig-assist (RA) devices (Figure 1) were first introduced to the industry to enable “snubbing” of pipe velocity strings under live well conditions.

These units utilized the drilling rig block to “push” the pipe via an arrangement of pulleys and cables – and later chains – into the well. The cable was attached to a travelling head which contained a set of inverted slips to grip the pipe. A set of stationary slips attached to the blowout preventer (BOP) stack, then held pipe when the travelling head was repositioned (Figure 2).

As the rig block pulled up, the cable was pulled through the pulley and pulled the pipe downward.  Once the pipe had reached the heavy state, where the pipe’s buoyant weight is greater than the force generated by the well’s pressure exerted on the pipe’s area, the block would be converted back to normal elevator operations. The BOP stack consisted of a number of pipe rams that sealed against the pipe and could be manipulated to allow the collars to pass without losing well integrity. An internal pipe seal was achieved with check valves or removable plugs.

It was forty years later, in the 1960s, when the first generation of HWO using hydraulic cylinders to replace the cable and pulley system was introduced to the industry. These units had the advantage of not requiring the drilling rig to be present and provided the first standalone HWO services (Figure 3).  HWO units typically have a jackwith an eight- to 10-foot stroke. A workbasket (Figure 4) positioned at the top of the hydraulic jack assembly provided the work area for the crew to make up the pipe and control the jack and BOP stack.

Standalone HWO units typically do not have the facility to rack back pipe and can only run pipe in singles.

This style of unit has evolved over the years to include telescopic guides between the stationary and travelling head to prevent buckling of the pipe during high snub forces. Powered rotary heads are also installed on the travelling head to provide rotation of the pipe.
The range of HWO operations can be expanded with the introduction of a work window between the stationary slips and BOP stack. The window provides means to deploy and attach control lines and electric submersible pump (ESP) power lines to the work string without risking damage as when going through the slips and jack. It also allowed deployment of BHAs or completion components, such as hangers, which have a larger outside diameter than the through bore of the jack. With this development, HWO units could compete directly with conventional workover rigs for completion workovers. During these operations, singles are normally run with trip speed constrained by the time to torque up the joints, and attach control lines, ESP power lines clamps, to the completion string.
Tower systems were later introduced to (Figure 5) remove the need for guide wires to maintain HWO unit vertically and allowed the complete assembly to be skidded between wells in offshore applications. Towers also improved the primary access and escape routes through built-in stairs.
Increased activity in underbalanced drilling in the late 1990’s led HWO units to return to their rig-assist roots. Though hydraulic cylinder jack systems now replaced the cable systems previously used to achieve the rig assist,
These various developments have allowed modern HWO units to perform a wide range of operations both in dead and live well conditions: 

• Scale and fill removal
• Fishing and milling
• Perforating
• Acidizing and washing
• Drilling – sidetrack, underbalanced drilling, Managed Pressure Drilling
• Completion and ESP installation and changeouts
• Plug and abandonment
• Well control.

Next-generation HWO Units
While the use of jack style HWO units has been steadily growing, their perceived limitations – pipe-trip speed, ease of rig-up and crew size – have been obstacles to wider adoption in preference to reinstating platform rig sets. Similarly, these concerns have prevented current HWO units from filling the shortage of small- to medium-class land rigs resulting from high drilling activity. The next generation of HWO units developed by Halliburton, addresses these constraints while maintaining portability and flexibility of use features.
The primary constraint to trip speed with cylinder jacks is the stroke length which requires three to four stroke operations to trip a single joint of pipe. A full single long stroke capability provided by a rack and pinion technology overcomes the multiple stroke concern.  A series of hinged rack segments, which are contained within a lightweight narrow mast, is driven by a set of pinion motors at the base of the mast (Figure 6).
The system can support either a power-swivel, double-slips rotary head, (15,000 ft/lbm at 94 RPM) or a topdrive (25,000 ft/lbm at 150 RPM) depending on the type of operation. The mast has a stroke of 55 feet and can accommodate Range III pipe with a tripping speed of 45 joints per hour in power-swivel configuration and 30 joints per hour with the topdrive.
The drive mechanism is extremely compact and has the ability to push (125,000 lbm), as well as to pull (300,000 lbm). All pipe loads are transferred to the motors, requiring the mast only to provide lateral support and counter the torque generated by rotation. As such, a rack and pinion mast structure is typically at least 20 percent lighter than a conventional ”A” frame wire-rope-driven derrick structure.
The mast has been incorporated into a modular design such that the maximum lift of any of the system components is under 10 Tons. A specially designed pivot allows the mast to be erected using a limited capacity crane with a jib reach of only 35 feet (Figure 7).

When constructed, the unit has a small footprint  of 18.4 feet by 15 feet. The complete mast structure can be erected and operational offshore in less than 36 hours (Figure 8).
State-of-the-art PLCs control and automate the unit and auxiliary equipment functions. A crew of only three can operate the unit when performing workovers and a crew of five works sidetrack drilling operations. Pipe is lifted to the drill floor by a pipe handling table, which can store up to 15 joints of five-inch drillpipe. The individual single are then loaded individually up to and from the floor. The topdrive can then latch the pipe and makeup without any crew-handling.
This unit has been designed to incorporate a 10-foot stroke cylinder-style rig-assist jack, providing extra pull capacity up to 600,000 pounds and snub of some 300,000 pounds. The jack’s travelling head is mounted on a passive bearing which allows the topdrive or power swivel to rotate the pipe. The jack design allows a full through bore of 37-1/2 inches.  (Figure 9).

The unit can be provided with a purpose built modular mud system, typically 210 cubic metres, and cuttings handling package which has been designed to be controlled and monitored from the driller’s cabin. The modular design allows easy expansion or reduction to meet the needs to a particular operation and production facility. The fluid pumping package is similarly remotely run from the cabin. For drilling operations, the unit can be mounted on a substructure which has integrated Xmas tree and BOP stack trolleys for ease of access.
Principle features:

• Modular construction, each component 10,000 kilograms or less
• Efficient construction; self-erecting
• 300,000-pound pull capacity with an optional integrated 600,000- pound rig-assist jack
• 125,000-pound push capacity with an optional integrated 300,000-pound rig-assist jack
• Ability to handle Range I, II and III
• Power swivel torque of 15,000 ft/lbm at 94 RPM
• Topdrive rotary capacity, 25,000 ft/lbm at 150 RPM
• Skid mounted, with ability to skid between multiple well slots
• Reduced staffing levels – three per shift workover, five per shift sidetrack drilling 
• 45 jts/hr power swivel and 30 jts/hr topdrive trip speed
• BOP and wellhead handling device
• Integrated modular pump and fluids system.

By Eriend Corrigall, Product Manager, Well Interventions Division, Eastern Hemisphere- Halliburton.