Designing fatigue-proof subsea cables

Challenge

Ocean Power Technology (OPT) set out to pioneer grid-scale wave power with “Power Buoys.” The cable had to last 25–30 years offshore. Over 100 million wave cycles, while carrying three-phase power and fiber-optic data and surviving continuous motion at the buoy/termination interface. Conventional designs weren’t built for that lifetime or motion profile. And while the tech needed to be field-proven, the economics for very large farms weren’t there yet. So the system also had to be practical for off-grid deployments where grid connection doesn’t exist.

Solution

DeRegt partnered with OPT from day one so the cable wasn’t an add-on but a core part of the buoy system. Electrically, the umbilical delivered three-phase power at 3.3 kV plus fiber-optic data. Mechanically, fragile fibers were placed near the neutral axis for maximum bend resilience. At the buoy interface, DeRegt engineered a custom bend-stiffener and a smart floating support: buoyancy pressed the stiffener against the structure, removing the need for diver-installed fastenings and reducing local bend strain.

Fatigue was the make-or-break. Instead of waiting decades, DeRegt combined S-N (stress–life) modeling, built from steel-wire fatigue data. With accelerated testing at elevated loads to predict life at operational loads. A dedicated “rotating float fatigue” setup then validated how the cable behaves under continuous wave-induced motion. Together, these methods qualified a design horizon on the order of 100 million cycles.

Deployment needed to be as robust as the design. Cables were pre-terminated, pressure-sealed, and fitted with cam-lock style quick couplings to avoid risky offshore assembly and speed installation. The result was a plug-and-play cable system that blended proven standard components (for reliability and serviceability) with custom protective and dynamic features tuned to the buoy’s motion.

This co-creation approach (electrical + mechanical + termination), validated by analysis and targeted tests, produced a cable system that could genuinely survive the ocean, not just the lab.

Result

The project and the product were a success. OPT’s system was deployed (e.g., Spain and Hawaii), and the custom cables passed extensive lab and at-sea qualification. Demonstrating durability consistent with multi-decade operation. While the economics for very large grid-scale farms weren’t yet viable at the time (hence no mega-arrays), the technology proved itself and has since been used where it makes immediate sense: remote, off-grid locations such as islands or offshore assets that need dependable power without a grid connection.

Importantly, the core idea of large farms has recently regained momentum and is once again considered a credible option for the energy transition. That renewed attention builds on the technical groundwork from projects like this: validated dynamic design, fatigue-proof terminations, and installation practices that turn wave power from concept into deployable infrastructure.

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