Internal Pressure Dynamics of Mass-Impregnated HVDC Subsea Cables at Different Sea Depths

Reducing or turning off the load of a mass-impregnated non-draining cable causes its internal pressure to drop. Under certain conditions, the pressure becomes so low that harmful shrinkage voids or cavities are created in the insulation. The pressure and pressure dynamics of mass-impregnated non-draining subsea cables at external pressures corresponding to laying depths of 30, 60 and 90 m have been investigated. It appears that load changes cause rapid and large changes in the internal pressure and set up radial pressure gradients. These pressure variations are superimposed on the external hydrostatic pressure. Consequently, a high external pressure presumably suppresses cavity formation. However, if the ambient tempera­ture is low—which is typically the situation for a cable directly exposed to sea water at large depths—a load turn-off still causes the internal pressure to abruptly fall to low levels. When imitating load turn-offs in a cable at 90 m depth and with a 10 °C ambient temperature, the pressure in the innermost part of the insulation dropped to levels where voids may form and remained so for several days. A cable buried in the seabed experiences a warmer and more stable ambient than an uncovered cable, making it less susceptible to cavity formation.
KEYWORDS
Cable insulation – Mechanical properties – Oil filled cables – Power cables – Underwater cables