Modern oil tankers are pretty sturdy

Dennis Blair and Kenneth Lieberthal argue that it is harder to close the world's shipping lanes than many people think: Smooth Sailing. The World's Shipping Lanes are Safe in the May/June Foreign Affairs (subscription required to read more than 500 words).

Part of the reason is that there are a lot of oil tankers, and that they're pretty sturdy.  In a section of the paper titled, "Floating Fortresses," Blair and Lieberthal write:

Recent trends concerning oil tankers make it even more difficult to disrupt maritime oil shipments. The size and the strength of the global tanker fleet have increased markedly over the last two decades. From 1980 to 2006, the number of tankers grew from 2,516 to more than 10,400, and the average capacity of each tanker increased by 400 percent, with a disproportionate amount of the new tonnage having been added in recent years. Single-hulled tankers are being phased out in favor of more resilient double-hulled ones.

The greater number of tankers traveling at higher speeds and in more congested shipping lanes makes it increasingly difficult to identify and intercept them. This is especially true for submarines, which have a limited ability to identify surface ships and have only a small onboard supply of torpedoes and antiship missiles. If a submarine attacks a ship using just two torpedoes, it will have exhausted more than a tenth of its standard arsenal. Meanwhile, other potential targets nearby will disperse, forcing the submarine to relocate. A single conventional-power submarine (Iran has only three) facing no opposition could realistically expect to damage about half a dozen oil tankers in a busy sea-lane several hundred miles wide over the course of a month, disrupting at most a tiny fraction of the oil deliveries made during that period. Conventional-power submarines, moreover, are relatively slow and cannot catch modern tankers, which travel at 15-20 knots.

Mines and conventional-warhead missiles are even less effective now against large modern tankers than they were in the past. During the Iran-Iraq War, several oil tankers ran over mines in the Strait of Hormuz, but they sustained little damage due to their size and the protective effect of the liquid petroleum they carried (petroleum is not explosive in the airless tanks, and its weight holds the hulls in place). Even the most modern antiship missiles have relatively small warheads that are designed to damage the sensors and weapons systems of surface warships but are not capable of sinking or disabling a large tanker. Most missiles shot at a tanker would explode on its large deck, causing minimal damage. Even if they penetrated the deck, they would explode inside tanks where the liquid oil or the water in ballast would absorb the blast without igniting. In order to disable a modern-day tanker, an attack would have to include a salvo of eight to ten missiles with conventional warheads; a sustained campaign would quickly exhaust the missile stockpile of a medium-sized military power.

It's not clear from this if torpedoes are more or less of a threat to an individual tanker than mines or missiles, but the authors clearly think that there are important limits on the ability of most nations to target significant numbers of tankers with torpedoes.

Blair is the former Commander in Chief of the U.S. Pacific Command, so he's in a position to know what he's talking about.

Comments

[Robert Petersen]

I am not a naval expert but I think I would like to point at a few things which I consider to be obvious even to non-military or non-naval experts. Especially regarding the situation in the Persian Gulf, where a war could erupt pretty soon thanks to the Iranian nuclear program: While oil tankers today might be more difficult to sink they still constitute extremely large, slow-moving targets.They are not protected the same way like aircraft carriers and there are missiles specifically designed to "kill" a carrier. So oil tankers are vulnerable. Especially in a "lake" like the Persian Gulf. Given the fact that the oil market today is very stressed even the destruction of one or two oil tankers would send a powerful message to the oil market and I can't see how it is possible to protect all of them. Unless you use the convoy system like during the two world wars. The argument that US marines than secure the area around Hormuz to prevent land attacks against ships (which was mentioned in the article in Foreign Affairs) is not convincing. After all: How much success did American ground forces experiencing in Iraq since 2003? Nope, while oil tankers might be better today I wouldn't take the chance and ignore their obvious vulnerabilities.