1.1 – Fatigue measurement in subsea power cables
Static sub-sea power cables which undergo repeated strains during use are prone to failure through fatigue. Repeated cyclic loading of elements within the structure of the cable encourages crack nucleation and growth to a point where failure can occur. The main weak point is the lead sheath where cracks allow water ingress leading to tree formation and ultimately electrical failure. In both dynamic cables and static cables, the conductor materials and armour wires can also be damaged by the same fatigue mechanisms
The optical fibre which is embedded in the cable structure is also strained during cyclic loading of the cable, and DAS can be used to measure and track this strain. If the relationship between the strain of the cable elements and the fibre can be determined, fatigue damage can be tracked. With a short measurement (1 week or so) the remaining lifetime of the cable can be estimated, but with a permanent DAS installation the remaining lifetime of the cable can be directly measured.
We recently collected data from a sub-sea export cable which shows two sections of cable at the CPS and J-tube which are subject to cyclic loading above the fatigue limit (the only parts of the cable to be so).
We have partnered with the Astute Centre of Excellence at Swansea University to use finite element analysis of cable structures to determine the remaining lifetime of the cables at these locations so that advice on the necessity of intervention can be given. Thanks to Fawzi Belblidia and the team.
Key takeaways: fatigue and remaining lifetime can be directly measured. This is going to be critical for floating wind but on static cables that are exposed to vibration it is essential knowledge.
1.2 – Designing for Fatigue Resistance
Floating wind is struggling with a lack of solid reference points and standards for design for fatigue.
As the need grows for utilisation of offshore wind, the turbine locations are becoming increasingly diverse and challenging. Floating offshore wind in deeper waters, for example, where the turbines are mounted on floating structures with inter-array cables that may need to suspended above the seabed, leads to increased uncertainty in cable reliability. Indeximate recently attended the Renewable UK Floating Offshore Wind conference in Aberdeen where there were several presentations and discussions on modelling the consequences of suspending cables on their reliability and susceptibility to fatigue, etc. But how do we validate these models and assess cable performance in real life environments? This is an open question but Indeximate has the tools to extract this key info.
An interesting case study for fatigue resistance comes from our experiences with tidal stream where exposed cables are subject to daily heightened vibration. We have been working with the SAE Renewables MeyGen tidal array off the North coast of Scotland, the largest planned tidal stream project in the world.
MeyGen were aware of the danger that the tidal stream would present to their cables so insisted on quad-armoured cables which are extremely stiff and well-protected from abrasion and have the added advantage of extra weight, which helps to restrict movement and minimise fatigue. Using an ASN DAS system, MeyGen have instrumented the whole of the tidal array cable network and Indeximate have been processing their data to assess the longer-term effects of this harsh environment.
Looking at the vibration of one of the cables in the array over a three-week period in May 2023 (1), we can see very clearly that some sections subject to high levels of vibration, arising from tidal flow – four times a day every day.
But how much strain is the cable subject to and how do we assess the likely impact of this? We’ve shown previously how we use DAS to measure cable fatigue. In MeyGen’s case the stiffness in the quad-armoured cable restricts the strain on the cable and the measured strain is below the fatigue limit, so we don’t see any indication that the movement is enough to cause significant fatigue. However, if we were to reduce the stiffness by a factor of 4 as would be the case for a single armoured cable, so more strain is transferred to the cable, we can repeat the calculation and see that some of the strain cycle amplitudes could be over the minimum level for fatigue to occur and so fatigue would be observed (2).
Further work is required to refine the strain response and strain-fatigue relationship for different cable types and Indeximate is working with the ASTUTE Centre of Excellence at Swansea University to assess the relationship between the level of strain seen on the fibre and that of the surrounding cable. In addition, practical measurements to assess the response of embedded fibres to cable tension is planned at the OREC Dynamic Cable Test rig.
