The subsea control system plays an essential role in subsea production. The integrity of the supporting subsea cables is therefore vital. To assess integrity of these cables the conventional method uses 500VDC test in accordance with ISO-13628-5/API 17E. This test is typically performed by connecting a downline from a support vessel to the subsea cable. The test set-up has issues associated with Humidity, Downline Cable Performance, Schedule, and subjective Interpretation. In addition, there is a risk of operator error and very low levels of insulation resistance are difficult to measure.
A new low voltage measurement technology, has been trialed during a 2015 campaign as an alternative to the conventional method. This new method was expected to address at least some of the identified issues.
The low voltage meter uses just 3.3VDC with the insulation resistance measuring range of 1kΩ to 10GΩ. The unit can be deployed by the ROV or diver without a downline cable. An offshore trial was performed on four standard subsea cables within an existing umbilical. The umbilical length of was 18 km and two conductors with sizes of 10mm2 and 6mm2 were tested.
The test results were compared with a measurement by a conventional 500VDC meter with a downline, and a similar test from the far end of the cable at the host facility with the same 500VDC meter. The results of the conventional 500VDC offshore test showed a wide range of sometimes illogical readings, whereas the Low Voltage test at 3.3VDC appears to show more consistent and rational results. As such, it is concluded that the Low Voltage test is actually more reliable than the conventional method.
The added benefit of the Low Voltage meter is that it can pick up low insulation values under 2MΩ comfortably, where the 500VDC meter struggled and failed to register a result.
These low Insulation Resistance values are still well above the operating threshold of the subsea control system, and as such, being able to quantify values between 500KΩ and 5 MΩ is an added benefit to confirm the integrity of the poor insulation subsea cables.
A test on a similar cable of 20km under factory conditions showed a good correlation between 500VDC conventional testing and the 3.3VDC testing with the Subsea Low Voltage meter.
Offshore testing can be significantly simplified with this metet, and overall schedule reduced. This in turn would save substantial costs during offshore inspection and fault finding campaigns. The result of the test by the new technique is more reliable, consistent and cost effective method to confirm Insulation Resistance in a Subsea Cable System.