17.2.2 Risk assessment
Tables 17.1 and 17.2 show that lightning strikes are associated with 25–45 per cent of
damage in tank accidents, and that tanks are more prone to lightning-related damage
than any other piece of equipment in the oil industry. In general, the risk to life and
property is so high when tanks and vessels containing flammable materials are
concerned, that the provision of every means possible for protection from the consequences
of a lightning discharge is essential, unless the tank or vessel has been
specially designed and is situated in a place chosen specifically to limit the effects
of a catastrophe. If an external lightning protection system incorporating air terminations
is to be installed for tanks and vessels in the open field, in which the lightning
current flow can cause a serious problem to the skin of the tank, the system should be
isolated (because the thermal and explosive effects at the point of strike may cause
damage to the tank, its content and the surroundings, as mentioned in Chapter 8);
otherwise, a non-isolated system can be applied in which several air terminals are
installed onto the structure, and bonded to the body. If an external lightning protection
system with an air termination system is to be installed for installations containing
tanks and vessels, the system might or might not have to be isolated according to the
specific requirements.