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Downhole Instrument Cable: Round vs. Square Encapsulation

Which is the ultimate in performance and durability?

The following study was preformed to directly compare 11mm square to 11mm round encapsulation of 4mm Downhole Cable. Various items were studied, including: Manufacturability, Weight, Flexibility, Crush Strength, Cross-Sectional Twisting, Dynamic Friction, and Deployment/Retrieval.

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Summary

The shape of downhole cable has serious implications for the success or failure of a downhole system. Round cable outperforms square cable in weight, flexibility, and crush strength. There are also issues with manufacturing, as wells as twisting of the square encapsulate down the axis of the wire during deployment and retrieval. All these factors increase the possibility of damage and wear in operation.

This study indicates that square encapsulated cable will have an overall higher cost of ownership than round encapsulated cable.

Methods

The studies on the various factors was preformed primarily using Finite Element Analysis. Simulation is a very subjective form of analysis and study outcomes can vary widely based on methods used. For this reason simulated studies should be used as an indicator to performance, not absolute fact. Further physical testing should be performed to verify simulation data.

1. Manufacturability

Extruding plastic encapsulating material over a core of other material (Steel, Copper, ect) is a mature process that has dominated cable manufacturing since the 1960’s. Although usually circular in cross-section, wire can be made in square, hexagonal, flattened rectangular, or other cross-sections for specific applications such as home electrical wiring.

Due to the stiffness and natural arc of the 4mm wire keeping it concentric (centered in the encapsulate) will be more difficult  with square encapsulation for the cable manufacturer. This points to the need to use of higher end manufacturing facilities that have the capability to produce a consistent product. This may produce a higher quality, but, will likely increase cost, lead-time, and limit manufacturing at low cost centers. Because of this, round encapsulation is more affordable to produce, and renders more consistent results.

The pictures below clearly display the issue (Figure: 1).

Square-Cable-Encapsulation-Non-Concentric-Measurement-Graphic-Icon-Downhole-Instrument-Cable-Round-vs-Square-Blog-GEO-PSI

Figure 1: Cross-Section of 11mm Square Cable

Round-Cable-Encapsulation-Concentric-Measurement-Graphic-Icon-Downhole-Instrument-Cable-Round-vs-Square-Blog-GEO-PSI

Figure 2: Cross-Section of 11mm Round Cable

2. Weight

All things being equal, material and wire, the 11mm square encapsulated cable weights ~11% more than an 11mm round encapsulated cable. This is due to the fact that there is more material in a square than a round profile. The lighter cable makes for easier handling and less overall tension on the downhole system.

Round-Cable-Encapsulation-Surface-Area-Measurement-Graphic-Icon-Downhole-Instrument-Cable-Round-vs-Square-Blog-GEO-PSI

Figure 3: Round Cable = 0.08lb/ft (Total)

Figure 4: Square Cable = 0.09lb/ft (Total)

3. Flexibility

A Round cable is more flexible than the square cable. A round cross-section has the same resistance to bending no matter which direction it is moved in. Square, on the other hand, has varying resistance to bending due to the thicker sections. Results of simulation show round encapsulated cable is 34-40% more flexible than square.

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Figure 5: Flexibility of Round Cable, 500lbs @ 90°

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Figure 6: Flexibility of Square Cable, 500lbs @ 90°

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Figure 7: Flexibility of Square Cable, 500lbs @ 45°

4. Crush Strength

In theory, if we apply a compressional load on a piece of wire with a square cross-section, the wire will deform in the middle of the face or will deform into a parallelogram. This is because the majority of the loading is directed through the face, and not out to the edges. Apply the same load to a wire with a circular cross-section and you get a very different result; the wire will deform slightly at the middle but the majority of the force is distributed around the cable. This is because when a load is applied to a circle the majority of the force travels down through the arc of a circle.

In the real world when crushing forces were applied to a section of cable, indications show that both round and square cross-sections apply about the same force directly to the encapsulated wire. However this simulation assumes that the wire is concentric in the encapsulation. Tests on a cross-section of the actual Square cable (Figure: 10) show forces are much higher on the face of the wire due to the difficulty of keeping the cable concentric during manufacturing [See manufacturing section (1)]. This reduces its overall crush strength performance of square encapsulation, and reduces the longevity and reliability of the system. In the real world, round encapsulation provides more consistent crush strength performance and can add to the reliability of the system.

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Figure 8: ~7100psi on Face @ 500lbs applied (Round)

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Figure 9: ~7100psi on Face @ 500lbs applied (Rounded Square)

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Figure 10: ~9500psi on Face @ 500lbs applied (Square)

5. Pure Axial Loading

In terms of pure axial loading, cable with a square cross section is 30% stronger in tension. However due to less material being required when producing a circular cable of equal width; the circular wire costs less and is 30% lighter.

6. Cross-Sectional Twisting

There is a tendency of the square cross-section encapsulated cable to twist along the axis of the wire when deployed and retrieved. This will increase both the volume needed on the reel as well as the frictional forces applied to the cable during deployment and retrieval. Round cross-section cable resists twisting, but even if it does twist along the axis it will not increase volume on the reel or frictional forces.

Square-Cable-Cross-Sectional-Twisting-Blog-GEO-PSI

Figure 11: Square Cable Twisted Down Axis

Round-Cable-Cross-Sectional-Twisting-Blog-GEO-PSI

Figure 12: Round Cable Twisted Down Axis

7. Dynamic Friction

Friction is a function of weight, surfaces in contact (roughness), and speed of retrieval. Each function has a bearing on the forces needed to be applied to the cable when being deployed and retrieved. If a cable weights more and has a rougher surface more forces will be applied.

Square cable has more weight and a rougher surface line (twisting down the axis) so it will create higher friction as compares to round. With more friction comes more wear, it would be expected that this will reduce the operational life of the square cable over the round cable.

8. Deployment/Retrieval

When delivered from the factory both round and square encapsulated cable lay perfectly onto the reel. After deployment and retrieval the cable can be a twisted jumble on the reel.

Twisting forces on the square cable will reduce the flexibility (manageability) of the cable as well as increase the area requires on the reel for the cable. Reels need to be big enough to compensate for the extra area it will take up when retrieved.

Round cable, even though it can still be difficult to manage, will not be effected by twisting and frictional forces to the extent the square cable.

Conclusion

In conclusion, after years of experience and extensive testing, we at GEO PSI have concluded that a round profile proves to be superior in almost every way. This is why we encapsulate all of our TEC with a round polypropylene encapsulation which maximizes durability, and usability.

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