Introduction to Coiled Tubing
There are multiple permanent CT installations that go beyond the pipeline applications discussed in the previous section. These applications include flowlines, velocity strings, and control lines. The largest market for composite CT is for flowline and pipeline installations. Velocity strings are the final resting place for many used CT strings. And, control lines are the largest market for small diameter CRA tubing.
In permanent installations, CT may be used as a flowline between offshore structures. The CT installation costs in this application are normally much less than for conventional barge-lay installations of welded line pipe. Prior case studies have documented savings in excess of 50 percent. In addition, the lower internal surface roughness of CT flowlines provides for lower frictional pressure loss than equivalent size jointed pipe. A 15-20% reduction in pressure loss (pump horsepower) has been reported with CT flowlines. This provides additional economic benefit in the form of lower operating and maintenance costs.
The largest CT flowline installed through year 2000 is 4 1/2 in. OD. However, CT suppliers have produced short lengths of tubing up to 6 5/8 in. OD for this application.
Individual sections of flowline can be connected mechanically (use of slip-type connectors) or by welding, with the latter being more common. "Full bore" socket connections can be installed and inspected at the CT mill. During installation, the socket requires only a single weld and inspection for each new reel added to the flowline. This approach reduces installation time, and the full bore does not disrupt the flow or future pigging operations.
The use of velocity strings is a common practice, especially in depleted gas wells. The objective of this permanent installation is to decrease the available production surface area within the wellbore such that the produced gas has sufficient energy to carry any produced liquids to surface. Small diameter (OD < 2 in.) velocity strings are often constructed from used CT work strings. A downhole hydraulic simulator is often used to estimate the performance of the string over a range of expected operating conditions. This modeling can help design a velocity string that maximizes well production. However, for these depleted wells, the choice of CT size and installation hardware may be heavily dependent on the price/availability of used CT strings.
CT is often used as the hydraulic control line connection between production facilities and subsea equipment. Each installation typically requires multiple control lines. As a result, multiple lines are normally bundled into a single line (umbilical) to reduce installation costs and to make the system more robust. The DEEPSTAR joint industry project developed the CT umbilical control line shown in Figure 18. It consists of four separate 3/4 in. CT strings surrounded by insulation, and further protected by two layers of armor wire. Approximately 33,000 ft. of this control line was installed in the North Sea in 1995.