When working with cables in shallow water, relatively less of the cable will be submerged while the rest of it is wound on a drum on the vessel. In deep water applications, where relatively more of it is deployed, water provides a cooling effect for the cable when it is subjected to electrical currents.
In contrast, in the shallow water applications, layers of cable are spooled on a drum on the vessel and these layers are unable to benefit from the cooling effect. Eventually, the heat caused by the electric current flowing through the cable builds up.
Moreover, the increased amount of cable layers further amplifies the maximum reached temperature within the cable. This phenomenon is explained by ‘Joule heating’:
where P is the electric power in watt (W), I is the current in amps (A) and R is the cable resistance in ohms (Ω).
It is vital to verify that the temperature of the cable will remain lower than a set maximum temperature while it is subjected to electrical power, as the cable components have temperature dependent properties.
DeRegt has therefore developed a bespoke thermal analysis tool for cables that are subjected to electrical power and are spooled on a drum. This tool allows DeRegt to simulate the temperature and the heat transfer within and between the spooled cables. Estimating the maximum temperature that the cable will reach adds significant value to the entire cable design process. This type of analysis is crucial in certain cable design and application conditions. DeRegt has developed this analysis tool in response to the need in the market.
The developed thermal analysis tool consists of modelling and simulation processes. The following steps are performed during thermal analyzing:
- Modelling Process
- Creating a cable model.
This includes simplification of the cable design and calculation of the related properties (diameter, weight, density, conductivity, specific heat, conductor resistance etc.). This model is drawn in 3D as a slice of layered cables (Figure 1). The previously defined properties are assigned into this 3D model.
Figure 1. Model- a slice of the layered cables on the drum (a)
- The heat power is calculated by using the current and duty information combined with the previously calculated resistance.
- Creating a cable model.
- Simulation Process
- Defining the boundary conditions.
- Running the simulation (Figure 1).
- Evaluating the results (Figure 2).
|Figure 2. Result of the thermal analysis|
Thermal structural analysis is performed by applying the finite element method to calculate the temperature distribution within a solid structure, which is due to the thermal inputs (heat loads), outputs (heat loss), and thermal barriers (thermal contact resistance) in the model. Thermal structural analysis solves the conjugate heat transfer problem with the simulation calculation of thermal conduction, convection, and radiation.
This thermal analysis tool is then validated against real test values.