Introduction to Coiled Tubing
CT can be used as an effective tool for numerous pipeline applications, including: Transportation of inspection tools
- Removing organic deposits and hydrate plugs
- Removing sand or fill
- Placing a patch or liner to repair minor leaks
- Setting temporary plugs text
Land-based CT operations in pipelines are similar to CT operations in horizontal wellbores with a few notable exceptions. First, the injector head must supply all the force required to RIH with the CT. The lack of a vertical CT section means that none of the CT weight is available to push on the CT ahead of it. Second, the injector head must be oriented horizontally at the entrance to the pipeline. This requires a special mounting frame and an injector that will operate efficiently while lying on its side. Finally, the injector head will be required to snub the CT into the pipeline during the entire RIH operation, and thus the weight measuring device (weight cell) must be configured for accurate measurement of snubbing forces.
CT operations in pipelines from an offshore platform are similar to operations in extended reach wellbores that kickoff at a shallow depth. The primary difference is that the path of the CT between the injector and the conduit on the sea floor may include several short radius bends. These bends impart a high drag force, and increase the snubbing force requirement on the CT injector. Since the injector may have to snub the CT into the pipeline during most of the RIH phase of the operation, the CT weight measuring device (weight cell) must be configured for accurate snubbing force measurement.
Regardless of the operational environment (land vs. offshore), post-helical buckling lockup of the CT is typically the primary limiting factor for pipeline operations. Lockup limits both the CT's horizontal reach into the pipeline, as well as the maximum available force that can be transmitted by the CT.
In addition, the radial clearance between the CT and the pipeline conduit is usually much larger when compared to standard wellbore operations. This effectively reduces the downhole critical force required to helically buckle the CT. Also, oil pipelines typically have an internal coating of highly viscous oil or wax that significantly increases the CT sliding friction coefficient. This excessive drag against the CT can also reduce the length of CT that can be pushed into the pipeline prior to buckling.
Overcoming Pipeline Drag Limitations
A common approach to reducing CT drag in conventional operations is the use of liquid friction modifiers to reduce the coefficient of friction between the CT and wellbore. This technique can be used in pipeline operations, and may provide a sufficient operating window to perform the desired operation.
Another typical approach utilized to overcome CT drag in pipelines is the addition of "skates" to the CT string at regular intervals. A skate resembles a rigid centralizer or stabilizer, with a roller on the end of each arm (blade). The skates serve to support the CT and prevent it from dragging against the inside of the pipeline. This effectively converts the drag from sliding friction to rolling friction. It's not uncommon for this approach to reduce the effective friction coefficient by 75%, which allows the CT to be pushed much farther into the pipeline without experiencing lockup.
In addition to the use of skates, hydraulic "thrusters" can be used to successfully reach the desired length inside the pipeline. A thruster consists of jets aimed in a direction opposite to the desired CT movement direction. This action applies a tensile force to the CT, and act to literally pull the CT string along the pipeline. Providers of the thruster service claim that the combination of thrusters and skates can enable CT operations to be successfully performed in horizontal pipelines up to five miles from the injector head point.