Table of Contents
Introduction
Submarine Cable Construction
Conductors
Submarine cable conductors are normally stranded copper as it has a better conductance and corrosion resistance than aluminium. Note however that some submarine cable projects consist of both copper and some aluminium conductor sections and multiple cable rating calculations may be required.
Conductor areas may be non-standard sizes. Submarine cables are not normally bought off the shelf and so conductor areas, rather than being built to standard values (i.e. those in Standard IEC 60228), are made to meet the needs of a project. For example, non-standard conductor sizes of 790 and 1410 mm2 have been used and up to 3500 mm2 are manufactured.
The conductor material selection (copper or aluminium) is based on the following considerations:
Electrical conductivity – A higher conductivity leads to a smaller conductor size for the same current-carrying capacity. Smaller conductors leads to less material being used for the other cable layers such as the insulation, screen, sheath or armour wires. A smaller cable size also leads to an increase in cable length which can be supported onto the installation vessel.
Note that a special solution which reduces AC losses and capacitive charging currents for longer transmission is to operate the submarine cables at lower than the power system frequency (i.e. 16.7 Hz) however to date there have been no recorded installations using this methodology.
Mechanical strength and flexibility – Submarine cables must be capable of physically withstanding mechanical forces during both installation and operation. Copper conductors have higher tensional strength compared with aluminium conductors.
Corrosion resistance – Corrosion is a significant problem for power cable installations in subsea environments. Copper has better corrosion resistance compared with aluminium.
Swelling agents are used in between the conductor layers to provide water tightness and avoid water ingress during a fault.
Weight considerations – The weight of the cables themselves impacts the tensional force exerted on the cables at the vessel drum reel which may be excessive.
The weight of pure aluminium is about 1/3rd of that of pure copper. Therefore for the same conductor area the weight of aluminium is much less than copper. However, the current rating of aluminium conductors is less than that of copper.
In general, for high voltage cables the weight difference between a cable with aluminium or a cable with copper conductors is minimal for the same current-carrying capability.
Conductor Screen
Insulation
Metallic (Water Blocking) Sheath
Armour
Outer serving
Installation Conditions
Depth Of Burial
Thermal Resistivity Of Subsea Soils
Subsea soils usually have a relatively low thermal resistivity of 1 C.w/W or lower.
The thermal resistivity of soils depends on the soil base material, the dry density, distribution of grain size, the compaction, the moisture content and the content of organic materials.
The thermal resistivity of subsea soils does not vary much and due to being water saturated the thermal resistivity is quite low.
The Table below provides assumed thermal resistivity for common subsea soil types.
| Soil Type | Thermal Resistivity (C.m/W) | |
|---|---|---|
| Reference | [1] | [2] |
| Gravel | 0.55 | 0.33 - 0.5 |
| Sand | 0.2 - 0.59 | 0.4 - 0.67 |
| Clay/Slit | 1 - 2.5 | 0.56 - 1.11 |
IEC 60287-3-1 provides the following guidance based on standard practice by country outlined by the following table.
| Country | Description | Thermal Resistivity (C.m/W) |
|---|---|---|
| Finland | For submarine cables where the soil is completely saturated with water | 0.4 |
| Netherlands | Sub-soil water level near to cables | 0.5 |
| Sweden | For submarine cables where bottom is covered with sand Where nothing is known about the seafloor |
0.4 0.6 |
Note it is important to have actual and sufficient samples of subsea soils for a project along the route of installation. This is because there may be sections of soil containing high organic content and these have much higher thermal resistivity.
Submarine soils with exceptionally high thermal resistivities of up to 1.4 K.m/W (generally caused by the presence of high organic content) have been reported.
Thermal Resistivity Measurements
Ambient Temperature
Current Rating Calculations
Conclusion
References:
[1] CIGRE TB 610, “Offshore Generation Cable Connections”, 2015.
[2] CIGRE TB 640, “A guide for rating calculations of insulated cables”, 2015.
[3] Worzyk, T., “Submarine Power Cables – Design, Installation, Repair, Environmental Aspects.” Springer 2009.
[4] IEC 60287-3-1:2017 Electric cables – Calculation of the current rating – Part 3-1: Operating conditions – Site reference conditions.
[5] ASTM D5334-14 Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock by Thermal Needle Probe Procedure.
[6] IEEE 442-2017 – IEEE Guide for Thermal Resistivity Measurements of Soils and Backfill Materials.
