A downhole gauge is only as reliable as the cable that reaches it, and the cable is the component most likely to be damaged before it ever gets downhole. Tubing encapsulated cable protects the conductor inside a welded steel tube, but the decisions that determine whether it survives are made long before deployment: profile shape, tube wall, bend radius and termination. Getting those four right is most of the engineering, and none of them are visible once the cable is clamped to the tubing.
Round Versus Square Encapsulation Profile
Encapsulation acts as a bumper, absorbing the handling damage that occurs while the cable is being deployed. The profile choice is not cosmetic. In a square encapsulated cable it is very difficult to keep the conductors concentric within the section, so the wires are stressed unevenly and the pressure rating varies around the cable; a round encapsulation gives a much higher and more consistent rating for the same amount of material. Round also saves weight: for an 11 mm jacket, the round profile reduces weight by up to 11 percent and material by about 13 percent compared with a square profile, which matters when several thousand feet of cable has to be spooled, shipped and handled at the wellsite.
Tubing Encapsulated Cable Tube Material and Collapse
The outer tube is welded from steel strip and then drawn and heat treated to final size. Two materials are standard, 316L stainless steel and Incoloy 825, and the published selection tables run from 0.125 inches to 0.625 inches outside diameter with wall thicknesses from 0.028 inches upward. Collapse strength is calculated from a minimum yield strength of 90,000 psi and ranges widely with wall thickness - roughly 29,000 psi on a 0.125 inches tube at 0.028 inches wall, rising to about 32,300 psi on a 0.25 inches tube at 0.065 inches wall, and falling again as diameter increases at constant wall. Choosing on diameter alone is the common error; wall thickness is what sets the collapse margin, and published ratings for the smaller sizes reach 25,000 psi.
Bending Radius and Handling Limits
Minimum bending radius is published per size and is the constraint that most often governs installation, because the cable has to be bent around the tubing joint, over the spool and through the wellhead. Published values run from 0.75 feet on the smallest tube to 3.75 feet on the largest. Exceeding them rarely breaks the cable immediately; it kinks the tube, which then damages the conductor inside and presents weeks later as a gauge failure with no obvious cause. Where flexibility during installation matters, increased strand count and spiral shielding improve performance without changing the tube or the pressure rating.
Tubing Encapsulated Cable in Flatpack Assemblies
Flatpack construction consolidates several lines into a single flat profile:
A power cable for an electric submersible pump
A monitoring TEC line for downhole pressure and temperature gauges
Control line tubing and chemical injection lines
Stranded galvanised steel bumpers for mechanical protection
Running all of them as one assembly simplifies installation in offshore and deviated wells, where every additional clamped line is another opportunity for damage during running. The trade-off is clearance: the flat bundle has to be sized against the completion rather than added to it afterwards.
Conductor, Filler and Termination Choices
Six factors drive the final specification: conductor, filler, diameter, crimp pin, end shell and cable size for the application. Insulation and jacket materials are selected against temperature, with polypropylene at the low end and fluoropolymer compounds extending the working range toward 300 degrees Celsius on the highest-rated builds, while general-purpose designs are commonly rated to 150 degrees Celsius. Termination is where field failures concentrate - whether pins are crimped or soldered, what outer diameter the back-shell allows, and how far the encapsulation is stripped back all decide whether the connection survives. Supply format matters too, with coil lengths commonly in the range of 5,000 to 35,000 feet depending on size.
The cable is the cheapest component in the system and the most common cause of gauge loss. Tubing encapsulated cable specified on collapse strength, bend radius, material temperature and termination - not on diameter alone - removes most of the failures that get attributed to the gauge itself.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
