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Coiled tubing (CT) is an electric-welded tube manufactured with one longitudinal seam formed by high-frequency induction welding without the addition of filler metal. Coiled tubing can be used in well intervention, and more recently, in drilling operations.
The first step in the typical CT manufacturing process involves the acquisition of steel stock supplied in 40- to 48-in.-wide sheets that are wrapped onto a “master coil” to a nominal weight of approximately 40,000 lbm. As a result, the lengths of sheet steel will vary depending upon the wall thickness. For example, a 40-in.-wide sheet of steel having a wall thickness of 0.109 in. rolled to a length of 2,700 ft will weigh approximately 40,000 lbm. If the 40-in.-wide sheet steel has a wall thickness of 0.156 in., a 40,000 lbm-master coil will have a length of approximately 1,900 ft.
When the diameter of the CT is selected, the sheet steel on the master coil is “slit” into a continuous strip of a specific width to form the circumference of the specified tube. The flat skelp is then welded to another segment of skelp to form a continuous length of steel. The welded area is dressed off smooth, cleaned, and then x-ray inspected to ensure that the weld is free from defects. Once a sufficient length of the continuous skelp steel is rolled onto the skelp take-up reel, the tube milling process can begin.
The skelp is then run through a series of roller dies that mechanically work the flat steel into the shape of a tube. At a point immediately ahead of the last set of forming rollers, the edges of the tube walls are positioned very close to each other. These edges are then joined together by an electric welding process described as “high-frequency induction” (HFI) welding. The HFI coil generates the heat for welding by the resistance to flow of electric current. As the tube is run through the high-frequency induction coil positioned inches ahead of the forming rollers, the edges of the walls are heated to the temperature needed to create the seam weld when pressed together within the last set of forming rollers.
The weld flash exposed on the outside of the tube is removed, and the welded seam is annealed. The tube is allowed to cool in air and then within a liquid bath before passing through a nondestructive inspection station to inspect the tube body. The inspection is typically performed with an eddy-current device that creates a magnetic field around the tube body and looks for distortions in the field created by surface defects in the tube body.
The manufacturing process continues as the tube is run through a sizing mill that slightly reduces the diameter after welding and works the tubing to the required outside diameter (OD) and roundness tolerances. At this time, the tubing undergoes full-body heat treatment using induction coils. The purpose of the heat treatment is to stress-relieve the entire tube, increasing the ductility of the steel. The tube is allowed to cool, first gradually in air and then within a liquid bath. This process results in the development of pearlite and ferrite grain sizes within the steel microstructure. The final product is a high-strength CT string with ductility and physical properties appropriate for the specified yield range. The tube is then spooled onto a steel or wooden take-up reel and subjected to a hydrostatic pressure test using water treated with corrosion inhibitors.
Alternative CT manufacturing processes may require that a string be constructed by butt-welding sections of tube together. The tube-to-tube welding technique may be performed using TIG or MIG (Tungsten inert gas or Metal inert gas) welding practices, and each weld should be inspected using radiographic inspection (x-ray) or ultrasonic inspection to evaluate the quality of the weld. Note that the exterior surface of the tube-to-tube weld may or may not be dressed off and that the weld bead on the inside diameter (ID) surface is not disturbed in any way. The string of tubing is then spooled onto the service reel or shipping reel as required.
All manufactured spools of CT are given a unique identification number that is assigned at the time of manufacture. Documentation for each spool of CT should include:
- The identification number
- OD of the tubing
- Material grade
- Wall thickness(es)
- Weld positions
- Total length
A spool of CT may be manufactured from one heat or a combination of heats that are selected according to a documented procedure provided by the manufacturer. However, the steel used to fabricate the string must have a uniform material yield strength throughout. The manufacturer should maintain traceability of the CT product throughout the manufacturing and testing process. The requirements of the purchaser often include traceability to the heat of steel.
Tapered wall thickness string design
In general, tapered CT strings can be manufactured by changing the wall thickness of the tubing within the length of a spool while maintaining a constant OD. The changes in wall thickness along the string length are intended to increase the performance properties of the CT in the selected sections. The construction of a tapered CT string may be achieved in one of the following ways:
- A continuously milled string incorporating multiple single-wall-thickness skelp segments joined using skelp-end welds.
- A continuously milled string incorporating single-wall-thickness skelp segments with continuously tapered skelp segments joined using skelp-end welds.
- Continuously milled, single-wall-thickness CT segments joined to another finished tube segment of a different wall thickness using the tube-to-tube welding process.
Continuously-tapered skelp is milled having a specified wall thickness at the leading end of the steel skelp, progressively increasing in wall thickness along the length of the skelp to a second specified wall thickness at the trailing end of the skelp.
The construction of tapered CT strings conforms to the previously described manufacturing processes. Although tube segments of different wall thickness can be assembled within the string construction, it is critical that all of the segments have a uniform material yield strength . The change in specified wall thickness, t, between the adjoining CT segments should not exceed the following specified values:
- 0.008 in. where the specified wall thickness of the thicker of the adjoining segments is less than 0.110 in.
- 0.020 in. where the specified wall thickness of the thicker of the adjoining segments is between 0.110 and 0.223 in.
- 0.031 in. where the specified wall thickness of the thicker of the adjoining segments is 0.224 in. and greater.
Tapered coiled tubing can increase the operating depths and pressures where CT is applicable.
|String Type||Transition Point Defined As:|
|Tapered-wall string designs||The points along the string having a change in the specified wall thickness|
|Single-wall-thickness segments||The points where the different specified wall thicknesses are mechanically joined together|
|Continuously tapered skelp segments||The points where the ramping of the skelp occurs|
The typical design criteria for selecting transition points (desired lengths of specified-wall thickness segments) within the string includes weight and overpull loading, wellbore condition, and combined pressure loading. Note that the overpull load rating increases for the adjoining tube segment with an increased wall thickness. The effect of changes in overpull load should be applied across the entire tapered string to ensure that a given overpull load does not exceed the limits of any other tube segment within the string. Therefore, the maximum length of each CT wall thickness segment should be evaluated using overpull and combined pressure loading to confirm that the stress applied to the CT string below any point on the segment does not exceed the triaxial stress load at that point for the given safety factor.
Coiled tubing requirements
Coiled tubing is currently available in ¾- to 3½-in. outside diameter (OD) sizes. The American Petroleum Institute (API) document RP 5C7 covers materials that are high-strength, low-alloy steels with specified minimum yield strengths from 60 to 100 kpsi. A flat strip is formed into a round shape, the heat for welding is generated by the resistance to flow of electric current, and the edge is mechanically pressed together. The length of the flat strip material typically ranges from 1,000 to 3,000 ft, and a spool of coiled tubing may be in excess of 25,000 ft, depending mostly on the tubing diameter.
The chemical requirements for API coiled tubing should conform to those listed in Table 3 of API RP 5C7 . That document also includes tables showing sizes, grades, and ratings for coiled tubing. Table 1 shows tensile and harness requirements for coiled tubing.
- API RP 5C7, Coiled Tubing Operations in Oil and Gas Well Services, first edition. 2002. Washington, DC: API.
Noteworthy papers in OnePetro
H.L. Nirider, P.M. Snider et al. 1995. Coiled Tubing as Initial Production Tubing: An Overview of Case Histories, Journal of Petroleum Technology Volume 47, Number 5, 29188-PA, http://dx.doi.org/10.2118/29188-PA
Cooper, R.E 1988. Coiled Tubing in Horizontal Wells, International Meeting on Petroleum Engineering, 1-4 November, Tianjin, China, 17581-MS, http://dx.doi.org/10.2118/17581-MS