travelling slips, Oil & Gas Glossary

Travelling slips are the gripping element mounted at the top of the hydraulic jack assembly on a snubbing unit, also called a hydraulic workover (HWO) unit, and they are the component that physically moves a tubular string into or out of a live well that is still under pressure. A snubbing unit works in a hand-over-hand cycle: the travelling slips grip the pipe and the hydraulic cylinders stroke up or down to move it, while a second set of stationary slips holds the string in place during the moment the travelling slips release and reset for the next stroke. Because the travelling slips are bolted to the moving carriage of the jack, every metre of pipe run in or pulled out of the hole passes through their grip. Two distinct sets are carried on the unit, and the choice between them is dictated by the force balance at the wellhead. When the pressure-area force acting on the cross-section of the pipe exceeds the buoyed weight of the string, the well is trying to eject the tubing and the operation is described as pipe-light, or snubbing proper; the light-pipe slips grip in the direction that resists the upward thrust and the jack pushes the pipe down against well pressure. Once enough pipe is in the hole that string weight overcomes wellbore force, the operation crosses the balance point and becomes pipe-heavy, where the well behaves more like a conventional stripping job and the heavy-pipe slips carry the hanging load while the jack lowers or raises it. Each set is engineered for opposite load directions and slip-insert geometries, so running the wrong set risks slip slippage, tubing damage, or a dropped string. On a typical 340,000 lbf or 600,000 lbf snubbing unit the slip bowls and inserts are matched to the tubular size in use, from 60.3 mm (2 3/8 in) tubing through 177.8 mm (7 in) casing, and the inserts are changed out as string size changes. In the Western Canadian Sedimentary Basin the travelling slips are central to live-well work on sour and high-pressure gas completions, where killing the well with fluid is undesirable because it risks formation damage or because reservoir pressure makes a conventional kill impractical. Snubbing through the travelling and stationary slip system lets a crew run completions, mill out plugs, fish, or pull tubing on a flowing Montney or Duvernay well without ever shutting it in, which preserves productivity and shortens the intervention. The slips operate inside a window assembly above the blowout preventer stack, and their reliable bite on the pipe body is what makes the entire snubbing safety case work, since a slip failure under pipe-light conditions can launch a tubular out of the hole.

Key Takeaways

Moving Grip On The Jack: Travelling slips sit on the moving carriage at the top of the hydraulic jack and physically advance the pipe with each cylinder stroke, alternating bite with the fixed stationary slips so the string is gripped at all times. The hand-over-hand sequence, grip, stroke, set the stationary slips, release, retract, repeat, is what allows continuous movement of tubing through a pressurized wellhead.
Two Sets For Two Regimes: A snubbing unit carries one set for pipe-light (snubbing) and one for pipe-heavy (stripping) service. The light set resists the upward thrust when wellbore pressure-area force exceeds string weight; the heavy set carries the hanging load once buoyed string weight wins. Using the correct set for the force balance is a non-negotiable safety control.
The Balance Point Governs Selection: The crew calculates the balance point, the depth at which string weight equals wellbore force, before and during the job. Above it the well is pipe-light; below it pipe-heavy. The travelling slip set must be swapped or reoriented as the string crosses this point, typically tracked on a snubbing tally sheet against surface pressure and pipe weight.
Insert Sizing And Bite: Slip inserts are matched to tubular outside diameter, from 60.3 mm tubing to 177.8 mm casing, and to pipe grade so the wickers bite without crushing thin-wall tubing. Worn or mismatched inserts are a leading cause of slip slippage, marked pipe bodies, and dropped strings, so inserts are inspected and dressed between jobs.
Core Of The Snubbing Safety Case: Under pipe-light conditions a slip failure can eject the tubular from the hole at speed, so redundant gripping, the travelling and stationary pair plus the BOP stack rams, is mandatory. In WCSB sour service the slip system works alongside H2S controls under AER Directive 037 well requirements and IRP 15 snubbing practice.

Pipe-Light Snubbing Versus Pipe-Heavy Stripping

The defining variable for travelling slip selection is whether the well is pushing the pipe out or letting it hang. In pipe-light snubbing, common early in a job when little tubing is in the hole, the upward force from wellbore pressure acting on the pipe cross-section exceeds the buoyed string weight, so the hydraulic jack must push the pipe downward against the well and the light-pipe travelling slips grip to resist ejection. As more string enters the hole, weight accumulates until the balance point is reached and the operation becomes pipe-heavy, behaving like conventional stripping where gravity wants to pull the string in and the heavy-pipe slips carry the load. Crews track surface pressure, pipe weight per metre, and depth on a running tally so the slip changeover is never missed, since a reversed grip at the balance point is a dropped-string hazard.

Window Assembly, BOP Stack, and Slip Maintenance

The travelling and stationary slips operate inside a window assembly positioned above the snubbing blowout preventer stack, which provides the pressure barrier while the slips provide mechanical grip on the moving pipe. The window gives the crew visual and physical access to set slips, change inserts, and clear the pipe body. Inserts are dressed or replaced when wickers round over, and bowls are inspected for cracking before each job because a fatigued bowl under pipe-light load is catastrophic. Hydraulic supply to the slip cylinders is interlocked so the travelling and stationary sets cannot both release at once, preserving continuous grip. In sour WCSB service the entire assembly is rated and function-tested against the maximum anticipated surface pressure before pipe ever moves through the slips.

Fast Facts

Snubbing predates modern hydraulic units by decades: the term comes from the practice of forcing, or snubbing, pipe into a flowing well against pressure, work so dangerous in the cable-tool era that it was reserved for blowout control. The travelling-and-stationary slip principle that makes it routine today, two grips trading off so the string is never free, is mechanically identical to the way a climber ascends a rope with two ascenders, and a modern 600,000 lbf hydraulic snubbing unit can push pipe into a well holding more than 34,000 kPa (about 5,000 psi) of surface pressure.

Travelling slips work in mechanical opposition to stationary slips, the fixed grip that holds the string while the travelling set resets, so the two are never understood in isolation. The entire mechanism is part of a snubbing unit, the hydraulic workover package that enables live-well intervention. Grip happens above a blowout preventer stack that supplies the pressure barrier, and the technique is one form of well intervention performed without killing the well, which is why slip reliability is treated as a primary safety control rather than a routine consumable.

Real-World WCSB Scenario

A Montney sour gas producer near Dawson Creek needs to pull and replace tubing on a well still flowing at roughly 31,000 kPa (about 4,500 psi) surface pressure with measurable H2S, where killing the well with brine risks both formation damage and a difficult re-start. The operator contracts a 340,000 lbf hydraulic snubbing unit at a day rate near CAD 18,000 plus crew and equipment, choosing live-well snubbing over a conventional rig kill. Early in the job, with only a few stands in the hole, the well is pipe-light, so the crew runs the light-pipe travelling slips and the jack pushes 73.0 mm (2 7/8 in) tubing down against the well while the stationary slips hold between strokes.

As the string passes the calculated balance point near 900 m, the operation turns pipe-heavy and the crew swaps to the heavy-pipe slips to carry the hanging load for the remainder of the run. The job is completed in two days under IRP 15 snubbing practice and AER Directive 037 well requirements, with no shut-in and no kill fluid, saving the producer an estimated CAD 120,000 in deferred production and avoided formation-damage remediation versus a conventional workover.