Within the renewable energy industry, power transmission is an important aspect. A great variety of cables, both static and dynamic umbilicals, is being used for that purpose. With the rise of wave energy conversion systems and offshore floating wind, there has been renewed interest in dynamic umbilicals / power export cables.But how do such dynamic umbilicals work, and how do you design the subsea power export cable that fits your project perfectly? At DeRegt, we’ve been making dynamic subsea cables for various industries for over 50 years and created multiple wave energy cables. This experience is also applicable to new industries and initiatives. To understand how dynamic umbilicals / power export cables and their design process work, we explain the various components in this article.
Dynamic Umbilical Construction: the main components
A couple of components are of utmost importance for a dynamic umbilical to function properly. The components in the centre are crucial for power and data transmission, but the outer components are vital as well. In the chapters below, we dive deeper into the inner workings of a dynamic umbilical / power export cable.
- Power Transmission
The “Power” component is an important one for most renewable energy projects. The main material used for power delivery in dynamic umbilicals is copper. The amount of copper is determined by the power and voltage of the cable.
For a voltage between conductors of 5kV or lower, single-layer power cores are used. When the voltage between conductors exceeds the 5kV threshold, we move to triple-layer extruded power cores. These power cores are more complex - with individual earth shields for each power core - and can handle up to 33kV.
Overview of a dynamic power cable
- Data Transmission and Fiber Optics
Besides power like electricity, renewable energy projects like wave energy conversion systems also need to generate and transmit data. Data collection and transmission are done via the same subsea cable as the one generating power. The kind of data that needs to be processed is an essential factor in the design process.
For small amounts of data, small copper wires can be sufficient for the transmission. However, for more significant amounts of data, fiber optics are the preferred method of transmission.
These fiber optics can also be used as sensing cables, which is a relatively new technology. This technique will become more common in monitoring cable health in years to come.
- Sheaths and Tapes
Data and power transmission are done within the core of the cable. Everything around the core is there for strength, safety and support. As the cable needs to be waterproof, the entire core is protected with a layer of watertight polyethylene. Additional water protection can be added with water-blocking tape, if necessary.
In order to get a field distribution as radial as possible, and to limit any enhancement of the electrical stress within the insulation, an inner semiconductive layer (above the conductor) and outer semiconductive layer (above the insulation) are used. The metallic tape, or other earth screen constructions, are mostly used to provide a ground reference: a return path for capacitive charging current and short-circuit current carrying conductor in case of fault.
Since there are many different applications in which subsea power export cables can be used, the degree of protection required is a custom factor in the design process. There are various solutions for the sheaths and tapes needed for each application. If you want more information about your specific application, do not hesitate to contact our cable experts.
The same is true for the armoring of the cable. This component gives the cable its strength, stiffness and most of its protection.
For dynamic umbilicals, it’s vital to have two layers of armor that are torque balanced. This allows the subsea cable to meet equal resistance moving in all directions. Another benefit of double armoring is that sharks can’t bite through them (which sounds uncommon, but it really happens).
Additionally, the armoring is protected from seawater and abrasion by an outer sheath that is extruded around the entire cable.
Dynamic cable with shark bites
- Additional components (besides the cable)
The components described above are crucial for any dynamic umbilical / power export cable. However, the subsea cable forms a connection between other equipment within your renewable energy project, like in a wave energy conversion system. This means other components besides the cable are required for the whole structure to function correctly.
Overview of a wave energy conversion system with its cables and other critical components
a. Landing points and load transfer pieces
The movement of a wave energy conversion system builds tension in the cable. The cable needs a place to unload this tension on both sides. That’s why the cable should be mounted with a load transfer piece to a reinforced part of the hull.
The cable’s attachment point on the seabed should have a strong anchor point to prevent the landing point from moving. There should be sufficient space between the anchor point and junction box/plug to prevent tension or movement on the (grid connection) cable to shore.
b. Terminals and plugs
Connecting the data and electrical wiring can be done in multiple ways. An oil-filled junction box can be made within the device after sealing the cable from the ocean. The connection on the power export side mainly depends on what the device is plugged into. For test areas, these are usually dry-mate or wet-mate connectors.
c. Various configurations to ease tensions
The point where a subsea power export cable transitions from static to dynamic is known as its weakest point. Stress is usually diverted over a larger area to relieve some of the tension on this point. This is done with bend stiffeners. To allow the cable to have enough slack for the various types of movements at sea, floated shapes like Lazy-S or J-lay configurations are used.
Unique opportunities and challenges
Knowing the options and challenges in an early stage of the development process can aid in successful prototyping and deployment. That’s why knowledge about the various components and their functions is crucial for your renewable energy project. Creating a custom cable in this market takes approximately 6 months.
If you have any questions about the options for your offshore energy generator or wave energy conversion system, please contact our experts. You can also schedule a Feasibility Check with one of them to discuss what is necessary to make your project a success:
Every renewable energy system and related challenges are different, so we’d love to hear about your unique opportunities and challenges!