Development of DC deep water submarine cables that enable installation along optimal routes

Japan, there are many deep-water areas. By enabling cable laying in deep-water areas, it is possible to shorten routes, reduce construction time, and lower costs.

In this study, the design of a DC cable optimal for laying at a depth of 1500 m is examined.

First, a prototype DC 500 kV submarine cable equipped with double steel wire armour was manufactured, designed with consideration for deep-water installation. Its mechanical and electrical properties were evaluated through a tensile test, a sidewall pressure test, a tensile bending test, a load cycle test, and an impulse voltage test. Furthermore, based on the results of the mechanical properties evaluation, the optimal armour design was investigated through structural and installation analysis.

For the tensile test, a maximum tension of 1000 kN, equivalent to a depth of 1500 m, was applied to the prototype cable, and axial stiffness and conductor share rate were measured.

These results were fed back into the structural analysis.

Next, a tensile bending test was conducted to evaluate the effects of sidewall pressure. A tension of 800 kN was applied to the prototype cable, and the sheave diameters were set to 8 m and 5 m. There were no visible abnormalities in the cable after the test, and the test was completed successfully. Subsequently, part of the sample was subjected to electrical testing, and the load cycle test at DC 925 kV for 30 days and the subsequent impulse voltage test were completed without insulation breakdown. These evaluations confirmed that the prototype cable had sufficient mechanical and electrical performance as a DC 500 kV deep-water submarine cable. Additionally, structural analysis was conducted by feeding back the evaluation results of the prototype cable to achieve a more optimal armour design. Based on the tensile test result, the contact parameters were calibrated and applied to the analysis of other armours configuration.

The analysis targets were steel round wire and three types of flat wire. The structural analysis results showed that the structure that could reduce conductor stress the most was the flat wire

H6.1×W10 mm. This is considered that flat wire is better at reducing the conductor share rate compared to round wire, and the cross-sectional area of the armour is larger compared to other flat wires.

In this study, it was confirmed that the prototype cable had sufficient mechanical and electrical performance considering laying at a depth of 1500 m. Furthermore, it was found that applying flat wire armour makes the structure safer and more optimal for the conductor. These findings can increase the options for laying routes of DC submarine cable and contribute significantly to cost reduction.