Author: Rosie O’Connor, Data Scientist
Historically, the fatigue experienced by a cable has been a value only calculated or modelled after a catastrophic failure has occurred. This reactive approach leaves operators in a state of perpetual uncertainty, unable to quantify the hidden accumulation of damage until the cable’s structural integrity has already been compromised.
To bridge the gap between reactive analysis and proactive management, Indeximate utilises Distributed Acoustic Sensing (DAS), turning the asset’s existing fibre optic components into continuous strings of virtual sensors. By monitoring DAS data over weeks, months and years, Indeximate can produce a robust profile of cable health through various metrics. This article focuses on two of the primary mechanical metrics within our Scattersphere® platform: Summed Strain and Fatigue.
The Summed Strain metric acts as a baseline for the cable’s health. It measures the total strain experienced by each section of the cable per day and returns a daily average figure. By standardising this measurement into strain rather than raw optical data, we allow operators to easily compare cable health accurately across different sites, installation types and time periods.
Figure 1: An example strain metric plot, showing the average daily strain along a cable.
Summed Strain provides immediate visibility into the “hotspots” along a route, identifying which specific areas are experiencing the highest levels of mechanical stress. However, the true value of Summed Strain is revealed through long-term monitoring. By observing these patterns over months and years, we can identify seasonal cycles, where strain might fluctuate with predictable weather or tidal patterns, as well as gradual changes in the cables strain profile.
Figure 2: An example strain time series, showing the behaviour of the Max strain point in Fig 1, over a year of monitoring
This analysis gives operators the information needed to move beyond guesswork, allowing for precise andproactive interventions at the exact locations where the cable is most at risk, ensuring maintenance is targeted and effective.
Simple assumptions can be made to give an indication of how close these dynamic strain levels are to becoming the limiting factor for the service life of the cable.
Fatigue
Where Summed Strain monitors daily mechanical strain, the Fatigue algorithm monitors the material integrity of the asset. It specifically targets the accumulation of damage in the cable’s metal components, such as the steel armour layers, which are susceptible to cracking under repeated mechanical stress. Unlike electrical faults, which are often binary (the cable works or it doesn’t), fatigue is a cumulative ‘silent’ process. Accumulated fatigue in lead sheath, caused by poor installation design, quality control or material specification, is often an important risk element in static cables. This degradation cannot be detected visually, and when failure comes it is sudden and terminal. Our algorithm provides the only way to see this damage as it accumulates on the seabed, rather than once a failed cable has been retrieved.
To calculate the fatigue experienced by the cable, we monitor the dynamic strain on the cable and, using anestimate of the cable component’s fatigue limit, identify and count damage inducing levels of cyclic strainamplitude. Fatigue is tracked spatially; by monitoring each section of the cable separately, we provide a high-resolution map of the fraction of ‘fatigue life used’ along the entire length of the asset. Assuming parallel conditions, we can predict remaining life from the fatigue life used. Continuous monitoring improves the confidence is such measurements.
Deeper analysis beyond measuring levels can probe the relationship between strain and fatigue and experienced conditions by correlating temporal peaks in strain with specific METOC conditions. This allows asset owners to investigate what specific aspects of installation design and cable conditions are pushing cables to fatigue.
From Reaction to Prediction
By combining Summed Strain and Fatigue, we can create a comprehensive risk profile: while Summed Strain is essential for detecting gradual exposure or seasonal patterns in the cable’s strain response, the Fatigue algorithm identifies the rate of material degradation. Combining these metrics produces a chronological digital history of the cable. Long term monitoring replaces traditional post-failure examinations with a live analytical framework, turning raw strain data into actionable intelligence and a proactive tool for real-time failure prediction and prevention.
This shift is a financial imperative. When a cable fails, the direct costs of emergency vessel mobilisation and specialist repair teams are only the tip of the iceberg. The most significant financial blow often comes from the lost energy production that occurs while an asset is offline. In the offshore sector, where lead times for repair vessels can be months and weather windows are narrow, a single failure can result in millions of pounds in lost generation. By moving to planned, data-driven maintenance, operators can schedule interventions during periods of low wind or planned outages, significantly reducing the Levelised Cost of Energy (LCOE) and protecting the asset’s overall yield.
For cable operators, the true value lies in long-term monitoring. By observing these intertwined metrics over months and years, operators can distinguish between low level, predictable fluctuations and concerning, permanent shifts in structural integrity. This insight allows for precise, proactive interventions at the exact locations where the cable is most at risk, ensuring maintenance is targeted effectively and securing the long-term value of these critical infrastructure investments.
As offshore wind moves toward more complex environments (including floating offshore wind, where cables are subject to even higher dynamic loads), having a baseline of “normal” behaviour becomes vital. By establishing this baseline today through long-term monitoring, operators are not just preventing the next failure; they are building the foundational knowledge required to design the more resilient subsea power grids of tomorrow.
If you want to learn more about our Summed strain and Fatigue metrics, and how they can help your cables,contact us at: https://indeximate.com/contact-us
