Page 146 - Offshore Electrical Engineering Manual

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CHAPTER

Subsea Supplies and 13

Cathodic Protection

INTRODUCTION

To those new to the subject of offshore electrical engineering, using electrical power
on equipment which is totally immersed in highly conducting salt water may seem
somewhat foolhardy. In actual fact, electrical power has been used in submersible
pumps and for underwater arc welding since around the beginning of the last cen-
tury. Providing certain precautions are taken, using electricity under the sea can be
comparatively safe.

SUBSEA POWER SUPPLIES
Permanent arrangements will be required for power to operate valves, switches and
sensors on underwater wellhead manifolds and the new generation of subsea pro-
duction wells which are serviced periodically by special vessels. Where power and
control is required at the manifold from a platform some distance away, an unearthed
supply will be required at a suitable voltage to compensate for the impedance of the
subsea cable. It is usual, however, for power for operation of valves etc. to be trans-
mitted hydraulically with electrical actuation from a secure (UPS) ac supply on the
adjacent platform.
It is recommended that all ac supplies used are three phase and isolated via
the delta connected, unearthed secondary windings of transformers. This provides
the best scope for incorporating protection devices and single earth faults can be
arranged to operate alarms whilst the circuit remains in operation.

DIVER’S LIFE SUPPORT EQUIPMENT
The main requirement for the North Sea diver is heat, both for body and respiratory
gas heaters.
The design of respiratory gas heating, if fitted to the diver’s suit must be such
that there is no possibility of electrocution through the mouth. If the diver’s suit is
heated electrically then the elements of heated undergarments should be screened
such that there is no possibility of the heating elements touching the diver’s skin, or
the elements shorting internally and causing localised heating. Both respiratory gas
heaters and electric body heaters should use a low voltage dc supply no greater than

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