Armouring in Offshore HVDC & HVAC Power Transmission Cables for Wind and Solar Farms

The integration of renewable energy sources, such as wind and solar farms, into the transmission network is a critical step in the global transition towards sustainable energy.

Offshore wind farms, in particular, require robust and reliable power transmission systems to deliver electricity to the grid. These systems typically utilize High Voltage Direct Current (HVDC) or High Voltage Alternating Current (HVAC) cables, which must be armored to ensure mechanical protection and stability in the harsh offshore environment. Armoring is crucial for maintaining the integrity and performance of these cables, which are exposed to various mechanical stresses and external threats.        

I have summarized in brief types of armoring used for power transmission cable, importance of customized armoring design and its impact on power quality and grid stability in this post.

A. Types of Armoring in Offshore Power Transmission Cables

A1. Double Layer of Armoring Wires (DWA):

The Double Layer of Armoring Wires (DWA) is a common method of providing mechanical protection and tension stability to offshore power transmission cables.

This technique involves wrapping the cable with two layers of armor wires, typically made from galvanized steel or other corrosion-resistant materials. The dual layers are wound in opposite directions, which helps to balance torsional forces and enhance the overall tensile strength of the cable.

DWA is preferred for its ability to provide high mechanical protection and tension stability, making it suitable for cables that traverse long distances under the sea.

A2. Rock Armor:

Rock armor involves the placement of large rocks around the cable after it has been laid on the seabed. This method protects the cable from external threats, such as fishing activities and anchors, by physically shielding it.

Rock armor is often used in combination with other armoring methods to enhance protection in areas where external threats are particularly high. However, rock armor does not provide tension stability; it primarily focuses on mechanical protection against external impacts.

A3. Counter-Helical Armored Cables:

Counter-helical armored cables are designed with layers of armor wires wound in opposite helical directions. This design counteracts torsional forces, preventing the cable from twisting under tension.

The counter-helical design also enhances bending stiffness, which is critical during the installation process when the cable is subject to various mechanical stresses.

These cables are particularly effective in maintaining torsional balance and are preferred in installations where high levels of tension and torsion are expected.

A4. Flat Armoring Wires:

Flat armoring wires offer a unique approach to cable armoring by using flat rather than round wires. This design increases the contact surface area between the armor and the cable, providing enhanced tension stability and resistance to crushing forces.

Flat armoring wires are particularly useful in applications where the cable is subject to high compressive forces, such as in deep-sea installations or where the cable is buried under heavy sediment.

B. The Importance of Customized Armoring Design

The armoring of offshore power transmission cables must be meticulously designed with respect to the tension stability, external threat pattern, and protection requirements for each sector of the planned cable route.

Offshore environments vary significantly, with different sections of the route exposed to varying levels of mechanical stress, seabed conditions, and external threats such as fishing nets, anchors, or even subsea earthquakes.

Therefore, a one-size-fits-all approach to armoring is inadequate.

B1. Tension Stability and Mechanical Protection:

The design of the armoring directly influences critical cable properties, such as bending stiffness, tension stability, torsion balance, and the choice of handling and installation methods.         

For example, a cable with inadequate tension stability may experience excessive stretching or elongation, leading to conductor fatigue and eventual failure. On the other hand, a cable with high bending stiffness may be difficult to install, particularly in areas with complex seabed topography.

The torsional balance provided by counter-helical armoring is essential for preventing the cable from twisting during installation, which could compromise its structural integrity.

B2. Optimizing Armoring Lay-Length:

The armoring lay-length, or the pitch of the helical winding of the armor wires, must be optimized with respect to the expected tensional forces, the tension stability of the conductor, and the torsional requirements of the cable and its installation.

A shorter lay-length increases bending stiffness and torsional resistance but may reduce flexibility, making the cable more challenging to handle and install. Conversely, a longer lay-length provides greater flexibility but may compromise torsional stability. Therefore, achieving the right balance is crucial for ensuring the cable’s long-term reliability and performance.

Impact on Power Quality and Grid Stability

The armoring design of offshore power transmission cables can have both positive and negative effects on power quality, grid voltage, frequency, and waveform.

For instance, improper tension stability can lead to cable deformation, which may result in impedance mismatches along the transmission line. These mismatches can cause reflections of the electrical signal, leading to distortions in the waveform, voltage fluctuations, and frequency instability. Furthermore, excessive torsional stress during installation or operation can damage the conductor, leading to partial discharges or even cable failure, which directly impacts power quality and grid reliability.        

To manage grid impedance effectively, it is essential to ensure that the armoring provides consistent mechanical protection without introducing unnecessary inductance or capacitance into the transmission system. This requires precise engineering and material selection during the cable design process.

To ensure the successful integration of renewable energy sources into the grid, effective planning and contract management are essential. Tools like Oracle Primavera P6 can be invaluable for project management, allowing for detailed scheduling, resource allocation, and risk management. Primavera P6 can help project managers coordinate the complex tasks involved in cable installation, from manufacturing to laying and commissioning, ensuring that all aspects of the project are completed on time and within budget.

The use of the NEC4 contract framework can also support effective project execution by providing clear terms and conditions for contract management, dispute resolution, and risk sharing. NEC4 encourages collaboration between the client, contractor, and other stakeholders, which is critical for the successful delivery of complex offshore projects.

Recent innovations in cable armoring include the development of lightweight, high-strength composite materials that offer enhanced mechanical protection without adding excessive weight to the cable. These materials, such as advanced polymers and hybrid metal-composite designs, are being integrated into next-generation HVDC and HVAC cables, improving their durability and performance in challenging offshore environments. Additionally, sensor-embedded armoring is being explored, allowing for real-time monitoring of cable health, tension, and temperature, which can significantly reduce the risk of failure and extend the cable’s operational lifespan.        

Considering the complexities and challenges associated with offshore cable armoring, how do you think the integration of advanced sensor technologies could further enhance the reliability and performance of these critical infrastructure components???

This post reflects my personal knowledge and is for educational purposes only.

#Renewableenergy #PowerCables #OffshoreWindEnergy #HVDCPowerCables #PowerQuality #BESS #GridCodeComplianceStudies

Reference source:

High Voltage AC (HVAC) and High Voltage DC (HVDC) Transmission Topologies of Offshore Wind Power and Reliability Analysis

Biswas et al. - 2021 IEEE Green Technologies Conference (GreenTech) - 2021

Recommendations for the electrical and electronic equipment of mobile and fixed offshore installations, Institution of Electrical Engineers, 2nd. 1992

Electric Cables Handbook Third Edition; G.F. Moore, 1997

Anaya-Lara, O., Campos-Gaona, D., Moreno-Goytia, E. L., & Adam, G.P. (2014). Offshore Wind Energy Generation: Control, Protection, And Integration to Electrical Systems.