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Service reel for CT unit

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The service reel serves as the coiled tubing(CT) storage apparatus during transport, and as the spooling device during CT well-intervention and drilling operations. Figs. 1 and 2 show the side view and front view of a typical service reel.


The inboard end of the CT may be connected either to the hollow segment of the reel shaft (spoke and axle design), or to a high-pressure piping segment (concave flange plates), both of which are then connected to a high-pressure rotating swivel. This high-pressure fluid swivel is secured to a stationary piping manifold, which provides connection to the treatment-fluid pumping system. As a result, continuous pumping and circulation can be maintained throughout the job. A high-pressure shutoff valve should be installed between the CT and reel shaft swivel for emergency use in isolating the tubing from the surface pump lines. The reel should also have a mechanism to prevent accidental rotational movement of the drum when it is required to remain stationary. In any event, the reel supporting structure should be secured to the deck or surface grade on location to prevent movement during operations.

In addition to the fluid-pumping service of the reel, electric wireline may be installed within the CT string to provide a means for conducting logging and downhole tool manipulation operations. The wireline is run inside the CT, and is terminated at the reel shaft within a pressure bulkhead on the CT manifold. The single or multi-conductor cable is run from the pressure bulkhead to a rotating electric connection (slip collector ring) similar to that found on electric wireline units. On reels equipped for electric-line service, this electric connection may be located on the reel shaft opposite the rotating fluid swivel or at the pressure bulkhead adjacent to the inboard swivel piping.

Service reel operation

In preparation for initial installation, a wing union is typically welded onto the end of the CT to be hooked up to the high-pressure piping within the reel (typically referred to as the “reference” end). The mechanical connection is inserted through a slot in the reel core drum, and made up to the high-pressure piping. Once the connection has been properly terminated, the tube is bent over a preset guide to create a reasonably smooth bend transition to the outer surface of the core drum.

The initial layer of the tubing is spooled across the core drum until the tubing wrap reaches the opposing flange. Then, the tubing is spooled back over the base layer, resting in the recesses between the tubes on the previous layer. This wrapping process is continued through the remaining successive layers until the desired amount of tubing is spooled onto the reel. The manner in which the tubing is wrapped onto the reel allows the tube to be supported within the space formed by the previously wrapped tubing and offers a unique stacking geometry.

The core radius of the service reel defines the smallest bending radius for the tubing. For CT used repeatedly in well intervention and drilling applications, the core radius should be at least 20 times the specified outside diameter (OD) of the CT. This factor may be less for CT that will be bend-cycled only a few times, such as for permanent installations.

The rotation of the service reel is controlled by a hydraulic motor, which may be mounted as a direct drive on the reel shaft, or operated by a chain-and-sprocket drive assembly. This motor is used to provide a given tension on the tubing, thereby maintaining the pipe tightly wrapped on the reel. Back-pressure is kept on the reel motor during deployment, keeping tension on the tubing between the injector and service reel. This tensile load applied to the tubing by the reel motor is commonly called “reel back tension,” requiring the injector to pull the tubing off the reel. The amount of reel back tension required increases with an increase in CT OD, yield strength (increased bending stiffness of the tubing), and distance between the service reel and injector. In addition, the required load on the reel drive system increases as the size of the core radius increases. Note that this tension results in an axial load imposed onto the tubing guide arch and creates a bending moment that is applied to the top of the injector. Therefore, it is critical that the injector is secured properly so that the bending moment is not translated to the well-control stack components or wellhead.

During operations, the reel back tension also prevents the tubing from “springing.” Although the CT stored on a service reel has been plastically deformed during the spooling process, the tubing still has internal residual stresses that create a condition for potential unwrapping and outward springing of the tubing from the reel if the back tension is released. To prevent the CT from “springing,” the free end of the tubing must always be kept in tension. When not in operation, the free end of the CT must be restrained to prevent springing.

The reel drive system must produce the tension required to bend the CT over the tubing guide arch and onto the reel. When CT is retrieved from the wellbore, the hydraulic pressure in the reel motor circuit is increased, providing the torque needed to allow reel rotation to keep up with the extraction rate of the tubing injector. Also, the reel drive system should have enough torque to accelerate the reel drum from stop to maximum injector speed at an acceptable rate. The torque should be capable of handling a fully loaded reel drum with the tubing full of fluid.

Reel brake

Additional safety items should also be included in the reel package to provide for an ancillary remote-activated braking system. The primary function of the reel brake is to stop drum rotation if the tubing accidentally parts between the reel and injector and limit tubing-reel rotation if a runaway condition develops. This braking system is not intended to halt the uncontrolled dispensing or retrieval of tubing in a runaway mode but only to offer resistance to slow down the reel rotation. The brake can also minimize tubing on the reel from springing in the case of loss of hydraulic pressure and, thus, the loss in reel back tension. When the reel is being transported, the brake should be engaged to prevent reel rotation. Many units incorporate a device in their hydraulic power systems to impose backpressure at the motor to slow the reel down. Other units employ a caliper-type or friction-pad braking system, which is hydraulically or mechanically applied onto the outer diameter of the reel flange to aid in slowing the reel rotation down.

Levelwind assembly

The tubing is typically guided between the service reel and injector using a mechanism called the “levelwind assembly,” which properly aligns the tubing as it is wrapped onto or spooled off the reel. The levelwind assembly spans across the width of the service reel drum, and can be raised to any height, which will line up the CT between the tubing guide arch and the reel. Generally, a mechanical depth counter is mounted on the levelwind assembly, which typically incorporates a series of roller wheels placed in contact with the CT and geared to mechanically measure the footage of the tubing dispensed through it. The levelwind must be strong enough to handle the bending and side loads of the CT. During transportation, the free end of the CT is usually clamped to the levelwind to prevent springing. The levelwind may also be equipped with a hydraulically or pneumatically operated clamp, which can be manipulated to secure the CT at the crossbar of the levelwind frame.

Lubrication and treatment

In many cases, the service reel is equipped with a system for lubricating the outside of the CT. This tube lubricating system acts to protect against atmospheric corrosion and reduces the frictional loads encountered when deploying the tubing through an energized stripper assembly.

The high-pressure rotating swivel and treatment fluid plumbing must have a working pressure rating greater than the maximum anticipated pressure for the specified job. Special consideration should be given to cases in which the swivel and piping may come in contact with native wellbore fluids. These components must be suitable for the type of service and fluids encountered (e.g., H2S, high temperature, etc.). At least one high-pressure isolation valve should be incorporated between the high-pressure swivel and the surface-treatment piping.


Noteworthy papers in OnePetro

External links

See also

Downhole CT tool connections

Tubing injector for CT unit

Coiled tubing fatigue

PEH:Coiled-Tubing Well Intervention and Drilling Operations