A History of Coiled Tubing
Early coiled tubing (CT) technology can be traced to project PLUTO (Pipe Lines Under The Ocean)— a top-secret effort to install pipelines across the English Channel during World War II.1 In June 1944, Allied engineers deployed several pipelines to provide fuel for D-day invasion forces. Most of the lines were fabricated from 40-ft [12-m] joints of 3-in. inside diameter (ID), 0.212-in. wall thickness steel pipe welded together to form 4,000-ft [1,220-m] sections.
These larger pipe sections were welded end-to-end, spooled onto 40-ft diameter fl oat- ing drums and towed behind cable-laying vessels. Successful deployment of 23 pipelines ranging in length from 30 to 70 miles [48 to 113 km] set the stage for future development and use of coiled tubing in oil and gas wells. Elements of modern CT injector heads can be found in a device developed by Bowen Tools during the early 1960s for deploying radio antennae to the ocean surface from sub- marines submerged as deep as 600 ft [183 m]. The antennae were stored on a spool beneath the injector for easy extension and retrieval. These basic concepts aided in the design of CT units and injector systems. The fi rst such unit, built by Bowen Tools and the California Oil Company in 1962, included an injector rated for surface loads up to 30,000 lbm [13,608 kg] that ran a continuous string of 1.315-in. outside diameter (OD) pipe. The unit’s 9-ft [2.7-m] diameter storage reel included a hub with a rotating fl uid swivel to allow contin- uous pumping down the coiled tubing. However, low yield-strength steels and the
numerous end-to-end, or butt, welds required to fabricate continuous tubing could not with- stand repeated bending cycles and high tensile loads. Weld failures, equipment breakdowns and fi shing operations to retrieve lost coiled tubing caused operators to lose confi dence in this technique.
From the 1960s through the 1970s, manu- facturing companies, including Bowen Tools, Brown Oil Tools, Uni-Flex, Inc., Hydra Rig Inc. and Otis Engineering, continued making
improvements in CT equipment and injector heads. These changes allowed larger coiled tub- ing sizes to be used at greater working depths, improved coiled tubing performance and reliability, and reduced the number of surface equipment failures. Unfortunately, an overall poor success rate and a reputation for limited reliability continued to plague CT operations. The late 1970s and early 1980s represented a turning point for coiled tubing, which up to that time was milled, or formed, in 1,500-ft [457-m] sections. In 1978, improved manufacturing quality and continuous milling allowed fabrica- tion of 11
-in. OD pipe. In 1980, Southwestern Pipe introduced 70,000-psi (70-ksi) [483-MPa] high-strength, low-alloy (HSLA) steel for coiled tubing. The early 1980s saw the introduction of 11
⁄4 ⁄2 -in. and 13 ⁄4 -in. OD coiled tubing. In 1983, Quality Tubing Inc. began using
3,000-ft [914-m] sheets of Japanese steel to reduce the number of required welds by 50%. Later in the 1980s, Quality Tubing introduced bias welding to eliminate butt welds. This process involved cutting flat steel strips diag- onally to enhance coiled tubing strength and life by spreading the heat-affected weld zone spirally around the tube. In addition, a better understanding of coiled tubing fatigue enabled improvements in reliability and pipe performance. In 1990, the fi rst string of 2-in. coiled tubing was milled for a permanent well completion. Soon after that suppliers began manufactur- ing 23
⁄ 8 -, 25 ⁄ 8 -, 27 ⁄ 8 -, 31 ⁄ 2 - and 41 ⁄ 2 -in. OD sizes for
well-servicing applications. Today, coiled tub- ing is manufactured from steel with high yield strengths of 90, 100, 110 and 120 ksi [620, 689, 758 and 827 MPa], as well as corrosion- resistant alloys. Higher strength steel, larger diameters and the need to reduce costs were key factors behind the CT revolution of the 1990s, and subsequently accounted for the extraordinary increase in concentric, or through-tubing, well-intervention work.
1. Wright TR Jr and Sas-Jaworsky II A (eds): World Oil’ s Coiled Tu bing Handbook. Houston, Texas, USA: Gulf Publishing Co. (1998): 7.
tubing at faster rates has improved well cleaning, eliminated the need for friction-reducing chemi- cal additives and reduced overall job times. In the new CT unit design, the current and future success of this technology can be attributed to platform designs tailored to CT requirements. To date, all of the targeted effi- ciency gains have not been realized on the Valhall platform, but with each campaign the team moves closer to those goals. The need for effi cient CT technology is not lim- ited to offshore operations. Schlumberger developed the CT EXPRESS rapid-deployment coiled tubing service for intermediate-depth onshore wells (next page, top). This system com- prises two trucks— a purpose-built CT unit and combination nitrogen and liquid pump— operated by three people. It provides the same capabilities as conventional units with fi ve-person crews. The combination pumper includes a liquid-
nitrogen tank and liquid-additive systems, and provides electrical and hydraulic power. This unit is designed for applications involving relatively low pump rates, moderate pressures and contin- uous operations for long periods. Tubing remains stabbed in the injector head during transportation, and the bottomhole assem- bly (BHA) can be assembled and pressure tested prior to arrival on location. A drop-in-drum tubing reel and innovative BOP pressure-test stand facil- itate unit mobilization. For rig-up safety and efficiency, no hydraulic or electric connections have to be made on location.
The unit operator controls the reel, injector head and BOP stack from a cyber-based control cabin, which utilizes available personnel more effectively and improves wellsite communi- cation. There are also separate stand-alone control panels for operation of individual equipment components. Statistics from CT operations show that inac- tion or incorrect actions contribute to at least one-third of all failures. About 83% of the failures were triggered by a downhole event, resulting in forces that exceeded safe CT working limits. To address this problem, the Schlumberger IIC Intelligent Injector Control, which is compatible with both conventional and new CT SEAS units, provides automated control of CT conveyance. In conjunction with CoilCADE coiled tubing design and evaluation software, IIC technology ensures that CT operations remain within specifi ed job parameters. This system performs automated injector load, or pull, tests and controls speed, applied load, depth and other parameters while running in or out of a well.
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Oilfi eld Review
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