LOAD TESTING
Purpose
The design of the metal-jacketed power cable is for a conventional ESP cable to be installed inside an outer metal jacket. The metal jacket is then laser welded, in a controlled environment to ensure the quality of the weld process, Fig. 9a. The ESP cable is fed into the formed jacket prior to welding, Fig. 9b. After welding, the metal jacket is drawn down onto the ESP cable, creating an interference fit. Load testing was performed to determine the optimal amount of interference between the ESP cable and metal jacket required to prevent loss of support between the two components during service.
Testing Setup and Procedure
Several 10 ft samples of the metal jacket were drawn to different diameters to provide different interference fits between the jacket and the cable. A restraining force was then applied to the metal jacket while the cable inside was loaded until it began to move and was pulled from the jacket, Fig. 10. The process was repeated a number of times to provide an average retention force.
Load Test Results
An optimal restraining force was found to be 500 lb/ft. This provided sufficient restraint of the ESP cable while preventing undue stress on the cable system.
BENDING CYCLE/FATIGUE TESTING
Purpose
The metal-jacketed power cable is designed to enable deployment of ESP equipment using modified CT equipment.
During installation, the power cable is subjected to low-cycle fatigue, similar to that experienced by a CT string. The bend/ fatigue test was to determine the failure mechanism of the cable and to estimate the service life of the cable — the cable is expected to be reused.
Testing Setup and Procedure
Two 8 ft long samples were prepared for tests to establish a baseline fatigue life of the Incoloy? 825 tube. The samples were placed into a bending jig with a 36” bending form radius. One test sample was oriented with the longitudinal weld positioned intrados; the other test sample was oriented with the longitudinal weld positioned neutrados. An internal pressure of 500 psi was applied to each sample. Each sample was cycled until failure occurred, as indicated by loss of internal pressure.
Bending Cycle/Fatigue Results
Under the testing conditions, the bending fatigue test resulted in an average fatigue life for the cable of 557 cycles, which is in excess of 275 deployments and retrievals. Both samples failed with a normal pinhole occurring in the form of a transverse crack that started from the outside diameter and propagated by fatigue, Fig. 12. The pinhole from Test 1 was associated with a small, shallow external dent. None of the pinholes started at the seam weld.
The outside diameter measured during the fatigue test was close to the nominal value. The ovality and ballooning detected were very low — ovality of 0.1% to 0.4%; ballooning of 0.001” to 0.009”. It is noteworthy that these results indicate the dimensional changes due to low-cycle fatigue for the Incoloy 825 tube, under the testing conditions, were below the maximum acceptable for normal CT operations.
Some displacement of the solid conductors and their insulation was observed at the sample’s ends after the bending fatigue test. This could be the result of the movement of the internals while the cable was being bent during the test.
No metallurgical abnormalities were associated with any of the pinholes. Additional external and internal fatigue cracks were observed, which is normal from a fatigue test, e.g., once the maximum fatigue life is reached, fatigue cracks start at different locations.
ELECTRICAL INTEGRITY AND CABLE THERMAL GROWTH TESTING
Purpose
Downhole temperatures are significantly higher than the temperature at which the metal-jacketed power cable is fabricated. The electrical testing is designed to validate the cable’s electrical characteristics at these elevated temperatures. Thermal cycling of the power cable was also used to validate the thermal growth of the system.
Testing Setup and Procedure
For testing the electrical characteristics at an elevated temperature, an 8 ft sample of the power cable was prepared. Each conductor within the sample was tested phase to phase and phase to ground before and during controlled durations of heat up to 30℉. The test was repeated five times and the results were compared. Samples were measured for change during the heat cycle, and a detailed examination of the individual power cable components was completed after the test.
A 10” sample was prepared for the thermal growth test to allow for measurement of the cable and conductors at 50℉increments between 150℉and 300℉.
Results
The results of the electrical characteristics testing at elevated temperatures confirmed no degradation to the conductors in either phase-to-phase resistance or phase-to-ground resistance. Resistance was in excess of 11 giga-ohms for all tests.
A detailed analysis of the armor, elastomeric cable jacket and conductor insulation was made, and no damage was observed.
Results of the thermal growth test confirmed that the thermal growth was within the material’s expected coefficient of thermal expansion (CoE). The measured CoE was within -6.5% to 0.6% of the reference data for the special metals Incoloy alloy 825.
SHEAR TESTING
Purpose
Should an unplanned well control incident occur during deployment or retrieval of the metal-jacketed power cable, a blowout protector (BOP) must be activated to prevent the flow of hydrocarbons. In those circumstances, the BOP would shear the cable. A shear test of the cable was conducted to ensure the BOP could perform as designed.
Testing Setup and Procedure
A combination ram shear/seal BOP was filled with water above the rams. A sample piece of the metal-jacketed power cable was installed in the BOP. The shear/seal rams were activated to shear the cable, and the force and time required to sever the cable were recorded. The time to sever the cable should be quick to minimize the unplanned release of hydrocarbons. Once the cable is severed, pressure testing of the BOP and hydrostatic testing will determine if the BOP is capable of sealing afterward under both low-pressure conditions — at 200 psi to 300 psi — and high-pressure conditions — at 15,000 psi.
Results
The activation of the combination ram shear/seal BOP quickly severed the cable, taking less than 30 seconds to cleanly shear the cable into two pieces. Subsequent hydrostatic testing at 300 psi and 15,000 psi was successful as well. Inspection of the cable and the shear rams revealed a clean shear of the cable without damage to the rams.
CONCLUSIONS
A rigorous evaluation and test program resulted in development of a novel high H2S rated, cable deployed ESP system that can improve the speed and efficiency of ESP deployment. The metal-jacketed power cable system improves deployment and retrieval safety and reduces risks in comparison to conventional rig deployed ESP systems. A laser welded unannealed 825 nickel alloy was found to meet required standards of resistance when subjected to high H2S and chloride levels. The metal-jacketed cable also met the physical and electrical requirements, including cable support and grip, bending fatigue, cable shear, and electrical continuity and resistance.
Due to the failure of the lower cost materials investigated for use in lower H2S wells, new material will be sourced and further testing performed. Additional work is being performed to develop these alternate materials with testing parameters being revised to simulate less extreme well conditions. It is hoped these efforts should identify cable material with the mechanical strength and corrosion resistance required, at a lower cost. Further refinement in the design has involved reviewing reductions in cable weight to allow for longer deployment lengths for deeper ESP setting depths.
Field trials of the 825 nickel alloy cable deployed ESP system is expected to occur in the second half of 2016.
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