## 6 Joint Testing of the EFSOS System
### Drilling Test in Ice Well  
During the ice well tests, the RECAS prototype carried out six drilling operations (Figure 9), with each hole reaching a depth of 10-11 m. As the sonde was drilled downward, the lower melting tip was heated to full power, and the lateral heaters were gradually activated as the sonde entered the water. To compensate for the current fluctuations caused by the on/off switching of the downhole heating elements, the surface input voltage was adjusted. This was done to keep the downhole voltage within the 600-650 V operating range. The average rate of penetration (ROP) achieved was 1.76-2 m/h, with a heating power of 6.45 kW at an ice temperature of \(-\)16 \({}^{\circ}\)C. At a constant input power, the borehole diameter had an inverse relationship with the penetration rate, ranging from 185 to 195 mm under the aforementioned drilling parameters. The weight on bit (WOB) was automatically controlled within the preset range of 20-50 kg for most of the time, and occasional peaks exceeding the limit were within \(\pm\)10 kg.  
For the upward drilling tests, a layer of transparent lake ice was placed above the ice well to facilitate the observation of the drilling process. Initially, the RECAS sonde was suspended at approximately 1 m below the ice surface in the borehole. Crushed ice at \(-\)20 \({}^{\circ}\)C was then injected into the hole to expedite the closure of the borehole. Once the borehole above the sonde was completely frozen, a sealed vessel containing transparent lake ice was placed and frozen on top of the ice hole. Five upward drilling tests were carried out using this method. An average ROP of approximately 2 m/h was achieved at an input heating power of 6.3 kW.  
During the upward drilling process, the temperature of the upper melting tip is crucial to ensure the safety of the cable passing through the central hole. In the initial upward drilling test, the cable broke near the upper melting tip before reaching its rated load. This happened when a portion of the tip extended beyond the water surface, even though the temperature control of the tip had been activated. This occurred because the temperature sensor at the tip (measuring the temperature of the part submerged in water) detected a lower temperature than the part above the water surface, leading to misjudgment and causing the central temperature of the upper melting tip to exceed the cable's maximum operating temperature.  
After the temperature control system disconnected the power supply to the upper melting tip, the temperature in the central hole continued to rise. However, when the upper-temperature limit of the melting tip was set at 80 \({}^{\circ}\)C, the maximum temperature stayed below 100 \({}^{\circ}\)C.  
Five sensors were placed at different positions inside the hole. Their purpose was to monitor the temperature within the central hole of the upper melting tip. The temperature distribution was measured when the tip was fully submerged in water and when it was half-submerged in water. The results are presented in Figure 10. When the tip was fully submerged in water, the temperature within the central hole was lower than that recorded by the sensor inside the melting tip. This indicated that the cable could operate safely without any problems. However, during the initial phase of the \"ice melt out\" process when half of the tip was exposed to air, the temperature within the central hole increased significantly compared to the temperature inside the tip.  
Figure 10: Temperature measurement of upper melting tip: (**a**) whole tip is in water; (**b**) half the tip is in air.  
Figure 9: Drilling testing in ice well: (**a**) RECAS sonde; (**b**) drilling downward in ice well; (**c**) drilling upward in transparent lake ice above the ice well.