Non-destructive examination (NDE) methods for dynamic subsea cables for offshore renewable energy

Abstract

Offshore renewable energy installations are moving into more challenging environments where fixed foundations are not economically viable, forcing the development of floating platforms. Subsea cables are critical for transfer of the power generated back to shore. The electrical capabilities of subsea cables are well understood; however, the structural capabilities are not, subsea power cable failures accounting for a significant proportion of insurance claims. Cables are challenging to repair, with specific vessels and good weather windows required, therefore making operations very costly. A good understanding of the internal structure of a subsea cable, and interaction between the layers, is integral to the development of robust and reliable, high voltage, dynamic, subsea cables. A requirement therefore exists for non-destructive examination (NDE) of live subsea cables to determine locations, and identify the causes, of faults and classify their type. An NDE framework such as this would assist in planning operations and reduce the risk and cost inherent to delivering offshore power. Improved understanding of subsea cable failure modes and mechanisms could also be achieved through us of NDE during onshore, dry, experimental testing. Three currently available NDE methods are considered, developed for use in other disciplines, for the purpose of structural monitoring of subsea power cables during onshore evaluation testing. The NDE methods were: (a) thermography, (b) eddy current testing (ECT), (c) spread spectrum time domain reflectometry (SSTDR). The methods are assessed with regards to the information that could be obtained from both a static and oscillating cable in pilot physical tests. The results of the testing were promising, with cable motions and interlayer movements being detected by all techniques to various degrees.