Coating armor wires of subsea power cables, umbilicals, and power umbilicals in bitumen is a common industry practice due to bitumen's excellent and field-proven anti-corrosion properties. The temperature dependent, viscoelastic behavior of bitumen influences the cables' and umbilicals' mechanical properties. This paper presents the derivation of an analytical model of the capacity of bitumen-coated armor wires. Capacity refers to the allowed combinations of axial cable tension and cable bending curvature for which the armor wires remain within their capacity criterion, which is a certain percentage of their yield limit. Examples demonstrate that the armor wires' capacity is sensitive to bitumen's temperature and to some extent to the frequency of the cable oscillations. The derived model is suitable for implementation in spreadsheets and in scripts.
Subsea power cables are used to connect power grids overseas, while umbilicals are used to connect offshore oil and gas production units.
Power umbilicals include power phases in addition to traditional umbilical elements such as tubes, electric signal cables, and fiber optic signal cables. The power phases are typically used for energizing subsea units, such as compressors.
Subsea power cables, umbilicals, and power umbilicals may include steel armor wires for carrying axial load. Steel armor wires may also be used to tune the cable's submerged-weight-to-diameter ratio. The armor wires are usually arranged in two or four armor layers located outside the other cable elements.
The armor wire steel is commonly galvanized. However, this may be insufficient corrosion protection as the armor wires are submerged in seawater for decades. Therefore a common industry practice is to coat the armor wires in bitumen. Bitumen is a bottom fragment from crude oil distillation and is field-proven to be highly effective as corrosion protection for steel armor wires in subsea cables and umbilicals.
Bitumen is a viscoelastic material, i.e. its mechanical properties are partly elastic, partly viscous. The viscoelastic properties of bitumen are highly temperature-dependent. At higher temperatures bitumen behaves as a lubricant, possibly reducing shear forces between armor wires compared to dry friction. As the temperature decreases bitumen becomes more sticky and sets up larger shear forces.
