Manufacturing a subsea cable
: Energy supply is a crucial pillar of the developed world. The infrastructure required to extract, capture, transmit and distribute energy is enormous.
To demonstrate just a small fraction of this global operation, the North Sea is currently home to more than 180 offshore oil platforms, and with the UK using over 19 boe (barrel of oil equivalent) per head in 2015, a sophisticated system of power supply and subsea control has to be continually developed and secured.
Cables and umbilicals are a vital element in this process, both in the transmission of large amounts of power under the ocean, as well as in the supply of hydraulic, electrical and optical control to offshore installations. At JDR, we specialise in the design, manufacture and installation support of offshore cables to deliver power and control in subsea environments.
How are subsea power cables and umbilicals made?
Most subsea power cables are comprised of a power conductor, made out of copper or aluminium, which transmits power at specific voltages and currents dependent on requirements. Outside of this layer there are additional insulation, protective and outer layers.
Following extensive design and customisation according to clients’ requirements, the manufacturing process begins with the construction of the conductor. Typically small wires of metal are tightly wound into a larger conductor core, which forms the central section of the cable. An insulation layer is then applied over the conductor and is made of either EPR (ethylene propylene rubber) or XLPE (cross-linked polyethylene). Both materials are well-suited to high-voltage power cable applications and have been commonly used for many years in subsea applications. They are lightweight, which makes them excellently suited to longer delivery lengths and both of these insulation materials have outstanding electrical and mechanical properties.
A three-phase subsea power cable typically incorporates three insulated conductor cores, as well as other components such as fibre optic cables.
Subsea production umbilicals on the other hand, almost always include many different components to suit a greater mix of power and control functions needed offshore and subsea. For example, thermoplastic hoses provide hydraulic valve control functions and low voltage cables are used for communications, both of which JDR manufactures in-house. Alternatively, hydraulic functions can be provided by steel tubes, when the water depth or distance to shore of a development requires a different approach. In short, every umbilical is different and is designed to meet specific customer requirements.
The final stage of the insulated internal layer involves a horizontal and vertical lay-up machines. At JDR, our machines are used to twist these internal components into a helix to form the cable or umbilical bundle. The process of twisting enables the composite bundles to be bent much tighter than if the function were straight, enabling the power cable or umbilical to be shipped and installed with greater ease. Watch this video to find out more about our horizontal machine.
Given the harshness of the environment in which a subsea cable must survive, they have to be developed with strength in mind. This is where the armouring layer comes in. Frequently made from coiled galvanised steel, this layer provides mechanical protection tensile strength and impact protection.
The use of steel in the armouring layer also gives the whole cable considerable weight for stability on the seabed. This is an important consideration in the design of all subsea cables, most notably because they must withstand strong and frequently unpredictable seabed currents, for example in stormy North Sea conditions.
Finally an outer sheath or roving encases all the other layers. A sheath is frequently made from a continuously extruded thermoplastic material such as polyethylene, which has the physical strength and moisture resistance capabilities ideal for subsea cables. Alternative, polypropylene rovings may be used depending on the application.
Historically medium voltage power cables have been produced up to voltages of 33 kV (36 kV Max) in a ‘wet-design’ configuration, that is without a metallic radial water barrier. Designs at higher voltage have typically required a metallic radial barrier such as an extruded lead sheath at voltages of 66kV and above. JDR’s new ‘wet-design’ high voltage 66 kV subsea cable is made without the need for a lead sheath while still ensuring long-term operations. This means the cable is lighter, thus reducing the associated capital costs for installation.
Whether in a ‘wet-design’ or sheathed, the final stage of the subsea cable manufacturing proves is to coil the finished product onto the cable laying vessel in preparation for deployment.
This blog provides a brief overview of the complexity of cable manufacture, to find out how JDR can help you please get in touch.