When people talk about offshore wind power, they often focus on massive turbines, foundations, and installation vessels. Yet hidden beneath the sea lies one of the most critical—and least visible—components of the entire system: the optical-electrical composite submarine cable.
Optical-Electrical Composite Submarine Cables
Today, most offshore wind farms no longer rely on simple power cables. Instead, they use optical-electrical composite sea cables, which combine high-voltage power transmission and fiber-optic communication into a single integrated system. In many ways, these cables act as both the “blood vessels” and “nervous system” of offshore wind farms—delivering energy while simultaneously transmitting data and monitoring system health.
What Is an Optical-Electrical Composite Submarine Cable?
An optical-electrical composite submarine cable is a specialized marine cable designed to:
Transmit large amounts of electrical power from offshore wind turbines to offshore substations or onshore grids
Carry optical fibers for communication, control, and real-time condition monitoring
By integrating power conductors and optical fibers into one cable, offshore wind projects achieve:
Higher reliability
Reduced installation complexity
Lower total system cost
The Layered Structure of an Optical-Electrical Composite Submarine Cable
Layered Structure of an Optical-Electrical Composite Submarine Cable
Unlike standard land cables, offshore composite sea cables are engineered with 12–14 carefully designed layers, each serving a specific function to withstand extreme marine environments.
Below is a simplified breakdown of the cable structure—from the core outward.
1. Power Conductor: The “Electric Highway”
At the center of the cable lies the copper conductor, responsible for carrying high-voltage electricity.
Key characteristics:
Made from electrolytic copper with purity ≥ 99.95%
Annealed, oxygen-free copper strands for flexibility and conductivity
Smooth, clean surface with no burrs or defects
To prevent longitudinal water penetration, the conductor is designed as a water-blocked conductor, typically incorporating water-blocking materials between strands. This ensures that even if the outer layers are damaged, seawater cannot travel along the conductor and cause widespread failure.
2. Insulation and Electrical Shielding: The “Safety Barrier”
Surrounding the conductor is the insulation system, which determines the cable’s voltage rating and electrical performance.
Conductor Screen
Semi-conductive cross-linked layer
Ensures uniform electric field distribution
Prevents partial discharge
XLPE Insulation Layer
Made from cross-linked polyethylene (XLPE)
Provides primary electrical insulation
Thickness increases with voltage level (e.g., 220 kV → 500 kV)
Insulation Screen
Semi-conductive layer outside the insulation
Further smooths the electric field
Typically includes longitudinal phase identification tapes
Together, these layers form the electrical insulation system, essential for safe high-voltage transmission under harsh offshore conditions.
3. Water Blocking & Metal Sheath: The “Deep-Sea Pressure Chamber”
To survive long-term immersion in seawater, additional protective layers are required.
Radial and Longitudinal Water-Blocking Layers
Semi-conductive water-swelling tapes
Prevent moisture ingress toward the insulation
Maintain electrical integrity even after minor damage
Metal Sheath (Typically Lead Alloy)
Seamless lead alloy sheath with ≥ 99.6% purity
Acts as the ultimate moisture barrier
Resists corrosion, radiation, and external pressure
Prevents relative movement between the cable core and sheath during vertical installation
The metal sheath also contributes to short-circuit current carrying capacity, working together with the armor layer.
4. Armoring System: The “Mechanical Armor”
The seabed is a hostile environment—strong currents, anchors, fishing gear, and rocky terrain all pose risks.
Steel Wire Armoring
High-strength galvanized steel wires (typically ~5 mm diameter)
Provides tensile strength during laying and operation
Protects against external mechanical damage
For floating offshore wind farms, where cables experience constant movement, double-layer armoring may be used to handle complex bending and fatigue stresses.
Bitumen coatings are often applied to improve corrosion resistance and bond the layers together.
5. Optical Fiber Units: The “Intelligent Nervous System”
This is what makes the cable optical-electrical.
Within the cable’s internal gaps are fiber-optic units, typically containing:
12, 16, 24, or more single-mode fibers
Functions of the optical fibers:
Turbine control and communication
Data transmission across the wind farm
Real-time condition monitoring
Using technologies such as BOTDR and BOTDA, these fibers enable distributed temperature and strain sensing, allowing operators to:
Detect overheating
Identify mechanical stress
Locate faults precisely and early
This capability is essential for offshore wind farms located far from shore, where maintenance access is limited and expensive.
6. Outer Sheath: The Final Protective Layer
The outermost layer typically consists of:
Bitumen
Polypropylene (PP) yarn or serving
This layer provides:
Additional mechanical protection
Abrasion resistance
Corrosion resistance
Reduced marine organism attachment
It is the cable’s last line of defense against the marine environment.
Why Is the Structure So Complex?
The complexity of optical-electrical composite submarine cables is not accidental—it is driven entirely by extreme operating conditions:
High voltage: Massive power transmission requires flawless insulation
Deep-sea pressure: Metal sheaths and armoring resist water pressure and installation tension
Corrosive environment: Saltwater and microorganisms demand highly resistant materials
Water ingress risk: Even minimal moisture can destroy insulation
Smart operation needs: Integrated optical fibers enable real-time monitoring
Thanks to this multilayer “layer-cake” design, modern submarine cables can operate reliably for over 25 years on the seabed.
The Future of Offshore Wind Submarine Cables
As offshore wind projects move into deeper waters and farther offshore, cable technology continues to evolve:
Higher voltage levels (up to 500 kV)
Flexible HVDC systems
Stronger mechanical designs
More advanced sensing and monitoring capabilities
The optical-electrical composite submarine cable is no longer just a power carrier—it is a high-tech energy and data infrastructure backbone.
Conclusion
Optical-electrical composite submarine cables are the unseen heroes of offshore wind power. By combining power transmission and intelligent monitoring in a single, robust system, they ensure that offshore wind farms operate safely, efficiently, and reliably—year after year beneath the sea.
As offshore wind continues its expansion into deeper and more challenging environments, these “nerves and blood vessels” of the system will only become more advanced and more critical.
