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Downhole Instrument Cable: Round vs. Square Encapsulation

May 22, 2014 at 7:48 PM

GEO PSI Downhole instrumentation cable

1. Introduction

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

2. Summary

Round cable outperforms square cable in weight, flexibility, and crush strength. There are also issues with manufacturing and 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.

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

3. 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.

4. 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 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. The pictures below this can clearly be an issue (figure: 1).

GEO PSI Downhole instrumentation cable

Figure 1: Cross-Section of 11mm Square Cable

GEO PSI Downhole instrumentation cable

Figure 2: Cross-Section of 11mm Round Cable

5. 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.

GEO PSI Downhole instrumentation cable

Figure 3: Round cable = 0.08lb/ft (total)

GEO PSI Downhole instrumentation cable

Figure 4: Square cable = 0.09lb/ft (total)

6. 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 cable is 34-40% more flexible than square.

GEO PSI Downhole instrumentation cable

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

GEO PSI Downhole instrumentation cable

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

GEO PSI Downhole instrumentation cable

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

7. Crush Strength

Crush strength simulations performed using simulation can be used as an indicator to the performance under a given situation. 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. This simulation assumes that the wire is concentric in the encapsulation. Simulations on a cross-section of the actual Square cable (Figure: 10) show forces are higher face of the wire. It has been observed that, in practice, it is difficult to keep the wire concentric with the square cross-section encapsulation. See manufacturing section (4).

GEO PSI Downhole instrumentation cable

Figure 8: ~7100PSI on face @ 500lbs applied

GEO PSI Downhole instrumentation cable

Figure 9: ~7000PSI on face @ 500lbs applied

GEO PSI Downhole instrumentation cable strength test

Figure 10: ~9500PSI on face @ 500lbs applied

8. 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, even if it twists along the axis will not increase volume on the reel or frictional forces.

GEO PSI Downhole instrumentation cable

Figure 11: Square Cable twisted down axis.

GEO PSI Downhole instrumentation cable

Figure 12: Round Cable twisted down axis.

9. 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.

10. 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.

Tags: downhole instrumentation Oil & Gas Oil Production Downhole Drilling Downhole Pressure Tools Oil Production Tools Oil Production Techniques Encapsulated Cable Oil production best practices round encapsulated downhole cable square encapsulated downhole cable Finite Element Analysis
Category: Technology

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