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 [[Tubing injector for CT unit|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 for CT operations|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.