The coiled tubing unit has evolved into a portable, compact and efficient unit that eliminates the problem of making and breaking connections going in and out of the hole. It uses a continuous length of variously sized tubing that is stored on a reel.
The length is usually sufficient to reach the well’s total depth or plug back depth. The reel of tubing does not move the pipe, as is commonly thought by those unfamiliar with a coiled tubing unit; rather it is a tubing injector/extractor.
The origins of continuous-tubing technology (coiled tubing) can be traced to work pioneered by Allied engineering teams during the Second World War. Project PLUTO, Pipe Lines Under The Ocean, was a top secret Allied invasion project involving the deployment of pipelines from the coast of England to several points along the coast of France. The continuous-length 3” ID pipe was wound upon massive hollow conundrums (floating spools) and was designed to be sufficiently buoyant with a full spool of pipeline to be towed behind cable-laying ships. At the completion of Project PLUTO, the Allied forces deployed a total of 23 pipelines, of which 17 pipelines were laid across the English Channel to provide a continuous supply of fuel to sustain the Allied invasion for the liberation of Europe. Some 172,000,000 gallons (651088.8 m3) of petrol were delivered to the allied armies through PLUTO pipelines at a rate of more than one million gallons (3785.4 m3)per day.
The development of the most common coiled tubing injector in use today can be traced back to G. H. Calhoun, et al (U.S. Patent No. 2,567,009). This apparatus provided the ability to insert, suspend, and extract strings of elongated cylindrical elements (such as tubing, cables, etc.) for high-pressure well services. A modified version of this device was developed for use in the U.S. submarine fleet to enable vessels to deploy radio communications antennae to the ocean surface while still submerged. Using the Calhoun et al concept, Bowen Tools developed the vertical, counter-rotating chain tractor device called the A/N Bra-18 Antenna Transfer System, designed to deploy a 5/8” (15.88 mm) OD polyethylene encapsulatedbrass antenna from as deep as 600 feet (182.9 m) beneath the water. Fabricreinforced phenolic saddle blocks were installed down the middle section of both sets of chains and they secured the antenna within blocks that were grooved to match the outer diameter of the tube. The antenna was stored on a spool beneath the antenna transfer system for ease of deployment and retrieval. The pressure seal was provided by a strippertype element that allowed the antenna to penetrate the hull of the vessel. The basic principles of this design concept aided in the development of the prototype coiled tubing injector systems in use today.