This article outlines major parts of cable construction and supports for selection of electrical cable in Oil & Gas Onshore Offshore Plant.
The rated voltage designation of the cable according to IEC is U0/U kV or U0/U (Um) kV.
- Uo: rated power frequency voltage between conductor and earth or metallic screen.
- U: rated power frequency voltage between conductors.
- Um: maximum value of the “highest system voltage”.
Conductor is critical part of cable construction since conductor properties will effect Plant economic operation. Power cable uses copper or aluminum material in their highly refined form as conductor. However, Copper is preferable in oil industry because of its softer, lower loss and lower voltage drop. Normally, conductors shall be stranded wires in accordance with Class 2 of IEC 60228. Where more flexible cable bending is required, flexible conductors (Class 5 or 6) shall be used.
See also: Electrical Standards • Electrical Cable Standards
Both thermoplastic and cross-linked insulation material is allowable. Below are list of materials for Low Voltage (Um = 1,2kV) and Medium Voltage (Um = 3.6kV and above) cable insulation:
- Polyvinyl chloride (PVC)
- Cross-linked polyethylene (XLPE)
- Ethylene propylene rubber (EPR)
- High modulus or hard grade ethylene propylene rubber (HEPR)
XLPE or EPR is normal choice unless specified by Client ITB. PVC is not recommendable when comparing with XLPE because of below reasons:
- Allowable max. temperature in normal and fault operation is lower than that of XLPE (normal operation: 70°C against 90°C, short circuit: 160°C against 250°C). Due to that fact, PVC cable size become larger.
- Dielectric constant is higher than XLPE and hence the capacitance is larger. Max. allowable length is shorter than XLPE insulation cable when the cable is used for control circuit working on AC voltage.
- PVC is environmentally less friendly than XLPE.
For offshore installation, PVC material is not preferred by standards (i.e. IEC 60092-353 and 354).
0,6/1 (1,2) kV cables shall have metallic screens surrounding the cores collectively.
1.8/3 (3,6) kV and above cables shall have metallic screens surrounding the cores either individually or collectively. In order to size metallic screen properly, engineer will inform earth fault current and fault clearing time to Vendor. Vendor then proposes type and size of metallic screen.
Metallic screen will consist of one or more tapes, or a braid, or a concentric layer of wires, or a combination of wires and tapes.
Where solvents might present in the cable surround soil, engineer will perform oversheath selection with due care. In that case, lead sheath is normal choice. However, lead is one of the hostility materials for soil.
Metallic amour provide protection for mechanical damage. Types of metallic armor are as follows:
- Round Wire Armour
- Flat Wire Armour
- Double Tape Armour
- Metal Braid Amour (offshore only)
Round or flat wire shall be of galvanized steel, copper or tinned copper, aluminum or aluminum alloy. At the same time, Tapes shall be of steel, galvanized steel, aluminum or aluminum alloy.