Page 307 - Offshore Electrical Engineering Manual

294 CHAPTER 2 Electrical System Earthing

LIGHTNING PROTECTION

Offshore structures and ships are not immune to lightning strikes just because they
are located in the sea. Lightning tends to strike at the highest point, so any metal
structures such as drilling derricks, telecommunications masts, flare stacks or crane
booms may be vulnerable. Although lightning is less likely to strike over open water,
there is still a risk to ships, rigs and platforms. In fact, changes in global weather pat-
terns due to global warming may produce lightning strikes in areas where lightning
has been historically rare. As offshore wind farms consist of a group of high towers,
it is certainly necessary to provide lightning conductors and surge arrestors to protect
equipment in the nacelles.
The main threats from lightning on both fixed and mobile (floating) oil installa-
tions are

1. direct ignition of explosive gases in hazardous zones,
2. current surges and voltage spikes causing damage to critical production and
safety electrical safety equipment (telecommunications, process control and
monitoring systems, fire and gas systems, etc.)

The main threat on wind turbine nacelles is damage to the control system due to
current surges and voltage spikes.
Note: Danish and German offshore wind farm statistics indicate that the damages
to control systems in the newer and larger wind turbines occur less frequently, prob-
ably as a consequence of improved lightning protection (see Reference 12).