Subsea power cables have to endure harsh conditions for decades at a time without failure or damage. A key factor in designing a cable is to understand the thermal losses to the surrounding seabed to make sure its performance is up to standard.
Thermal Analysis of Offshore Cables
Offshore cables can typically be found either snaking along the seabed to transfer electrical power between countries, or more typically linking an offshore wind power site to the mainland. They remain in place for years and during that time have to withstand a pretty harsh environment.
At the bottom of the sea, they have to endure high pressure, a corrosive environment and are buffeted by the residual effect of surface waves. On top of normal wave action, storms can also generate huge forces on the cable system as the cable is picked up by the currents and vibrates as fluid vortices shed off it (The offshore dynamics of cables is something we also work with, but that isnt the focus of this article).
Typically in order to protect the cable it is covered with a flexible cable protection system (CPS – sometimes also known as a Cable Armour System, or CAS) that completely encloses the cable. Normally made of some kind of polymer, while great at preventing damage to the cable, it has an outstanding thermally insulating effect.
And as we all know, resistivity increases with temperature, so the thermal management of the cable and its interaction with the sea and sea bed surrounding it is important to prevent excess power losses from the wind farm to the shore.
We use SimFlow with OpenFOAM to solve these problems, running CFD simulations with conjugate heat transfer between the solid media of the different materials inside the cable and the surrounding ocean and seabed, combined with some customised scripting tools for verification checks.
Using this method, cDynamics can accurately evaluate the thermal interaction between the cable, CPS and surrounding environment and ensure our clients’ cables will perform as needed during their many years of service.
For this example, I created a mockup of a cable and cable protection system arrangement with regions of different material properties that are very typical of most commercial systems.
I simulated this arrangement in two conditions, one buried rather shallowly below a gently undulating seabed and the other in a free fluid volume.
The difference here is obvious between the conduction of heat from the cable to the seabed (but still showing the major heat producing regions from the power transmission lines within the cable) and the induction of convective currents in the free fluid volume as a result of heat transfer from the cable to the surrounding water.
The fluid velocity is not all that much, but there is at least some small gradient produced as a result of the heat transferred.