Are two hulls better than one?

Double hulls are now a mandatory requirement for tankers and there can be no going back on this decision but I do not believe it is the universal answer to tanker safety that some would claim.

It seems strange to me that if the cause of so many accidents and structural failures on these vessels was corrosion due to the lack of inspection and maintenance of the steelwork then effectively doubling the surface area of steel to be maintained will eradicate the problem! Not only is there a great increase in the surface area of steel to be maintained, access to the space between the inner and outer hulls is limited.

But that is not all. One of the main arguments put forward for the adoption of the double hull is its ability to withstand a grounding without resulting in cargo spillage. This may indeed be true in the case of a low velocity impact but is this true for higher impacts, and what is the effect of the energy to be dissipated in such an incident? The terrorist attack on the double-hulled 298,997 tonne deadweight tanker Limburg in 2002 resulted in an explosion that is said to have sent shock waves through the two hulls with disastrous results to engine seats etc. Something similar could well happen with a high energy grounding of a double hull tanker.

There are other hazards that should also be considered. The insulation of the oil cargo from contact with steel cooled by seawater could cause increased corrosion as the cargo remains warm, speeding up the corrosion process. Equally, as many experienced operators of double hull tankers already know to their cost, it is impossible to prevent seepage of gas into the space between the hulls. This results in the risk of an explosion in the void space and the need for this to be inerted. Then when steelwork inspections are required the spaces will have to be gas freed and checked for any residual gases.

Taken together these negative aspects put the adoption of double hulls for tankers into a different light. How much of the decision making was influenced by pollution incidents and what notice was taken of the operational experience of owners of existing double hull vessels? All that can be hoped is that a major disaster does not happen to a double hulled tanker; otherwise a lot of questions will need to be answered.

And now there is a move to adopt double hulls for bulk carriers where a different situation exists. Such vessels are already substantially double hulled and the filling in of the area between the saddle tanks and bottom tanks could prove advantageous in preventing damage to frames from grabs etc. Nevertheless there is a growing dispute between the authorities and some owners, particularly the Greeks, about the imposition of this requirement for vessels over 150m in length.

Whatever the outcome, it is to be hoped that a sensible decision is reached that does not prejudice overall safety for a small improvement in one area as has been the case with tankers. n