WASHINGTON — With President Trump and Secretary of State Mike Pompeo asserting that Iran was behind the explosions on two tankers in the Gulf of Oman on Thursday and threatening military retaliation, the United States Central Command released a surveillance video showing what it said were Iranian sailors removing a weapon called a limpet mine from one of the ships.
Although the operator of one of the tankers said on Friday that the vessel had been struck by “a flying object,” expressing doubt that a mine had been attached to its hull, the United States stuck to its explanation.
Here’s what we know about limpet mines.
What are these things?
A limpet mine is a type of naval mine that is usually manually placed by a swimmer, diver or frogman on the underside of a ship’s hull.
Named for a type of mollusk, or shellfish, that sticks to rocks and is hard to remove, limpet mines are similarly stuck fast to their targets, usually with magnets.
Unlike larger mines that lie on the sea floor, or buoyant mines that are anchored to the bottom by a chain or cable, limpets are usually not intended to sink a ship. Rather, they are designed to cause a “mission kill,” immobilizing the targeted ship and taking it out of action.
Are these weapons new?
No. Modern limpet mines have been around at least since World War II, and most navies around the world likely have them in their arsenal. A United States Navy limpet attack on an enemy ship would be carried out underwater by a SEAL platoon. The limpets they use were designed in the early 1970s and have been updated only modestly in the decades since.
How do they work?
The mines usually contain small explosive charges and explode through a device called a fuze set by the person who attaches it to the target. Typically these fuzes function via time delay, either through chemical processes or mechanical means.
Some models incorporate a secondary anti-removal fuze that will cause the mine to explode if someone tries to take it off the hull of the vessel.
How do you defend against them?
United States Navy ships have protocols for what they call “anti-swimmer” watches. They post lookouts who walk on the weather decks and keep an eye on the water below.
They look for bubbles released by a diver, or anything that does not look like fish or other marine life. If they see something suspicious, they could be authorized to drop a concussion hand grenade, which produces a large blast wave but little fragmentation.
Since sound travels about four times faster in water than in air, the effects from such a weapon could travel quite far. The underwater blast would hammer at someone’s sinuses and lungs and would wound or kill a diver.
Then the ship would call in an explosive ordnance disposal, or E.O.D., team to respond. The Navy has a permanently deployed E.O.D. element in Bahrain, and the nearby aircraft carrier the U.S.S. Abraham Lincoln would have a small team aboard as well. Technicians from either Bahrain or the carrier could respond to the tankers fairly quickly by helicopter.
What would the E.O.D. team do?
It would start by deploying a pair of swimmers to quickly search the ship’s hull at the water line.
If they do not find anything, they would leave the water and be replaced by two E.O.D. technicians kitted out with scuba equipment.
Then those two divers would search the parts of the ship most likely to be targeted by limpets.
What parts of the ship are those?
The parts that would likely be attacked are the steering gear (the rudders and rudder posts) and the propulsion elements (shafts and propellers).
How do you get rid of them?
This is not something that civilian mariners can do, since the mines might explode when someone approaches and tries to remove them. But it is a core mission of Navy E.O.D. technicians and allied “clearance divers.”
Generally, you move as quickly as you can, and you work as remotely as possible, but the situation still calls for accepting and managing extreme risk. The situation is urgent, so the technicians try to be as safe as they can while being mindful that the limpets can explode and kill them at any time.
What can be seen in that video?
It’s not really very clear.
A competent military force would not generally send a large team to approach a live limpet. It was standing room only in the smaller boat seen in the video.
For safety’s sake, one would want to approach any unexploded munition with the absolute minimum number of people required to accomplish the mission. In addition, the supposed limpet mine was above the waterline.
Why is that important?
Because, by design, limpets take advantage of being surrounded by water to accomplish their task.
When placed against a ship’s hull underwater, the limpet has dense water behind it, and most likely has an air void on the other side of the hull plating. The ocean tamps and helps reflect the explosive blast, which like all types of energy seeks the path of least resistance. Thus, more of the explosive force is directed into the ship itself.
Even a small limpet can open a surprisingly large hole in a ship. But if a limpet detonates above the water line, it will most likely punch only a small opening in the hull, which would do little to affect the ship or its ability to operate.
How could a diver get close enough to place a limpet on a tanker?
Civilian tankers have a high tolerance for taking other vessels within a few hundred yards or so on the open ocean. In the Persian Gulf, tankers may allow small boats called dhows to get even closer at slow speeds.
So it is possible that a small boat could have moved right up to these tankers and that someone on the boat could have slapped a limpet on the tanker without drawing too much attention, especially if it happened at night.
Warships, however, are much more strict about not letting other ships or boats get close to them at all, and often have to maneuver to avoid them.
The most likely place to be attacked by a limpet is while a ship is moored, or when it is sitting at anchor. The ship is basically a sitting target then — as were four tankers that were attacked off Fujairah in May.
It is much harder to place a limpet onto a ship that is moving through the water, so if a boat was used to approach a moving tanker, that might explain why a limpet was found above the waterline.