The American ESBs were successful and three are in service with two more under construction and three more planned (depending on the navy budget). The American ESBs are built to commercial (not military) ship standards and use mixed military and civilian crews. They can be equipped with defensive (anti-missile) systems and cost about $500 million each.
In late 2016 the U.S. Navy received its first ESB (Expeditionary Mobile Base), the USNS Lewis B. Puller, and conducted sea trials successfully before the ship entered service in 2017. The ESB type ship was previously known as MLP (Mobile Landing Platform) or AFSB (Afloat Forward Staging Base). The first American ESB was a 78,000-ton ship that is basically a modified oil tanker. The flight deck can handle the heavy (33 ton) CH-53 transport helicopters as well as MV-22 tilt rotor aircraft. A structure on the hanger deck contains mine-clearing gear used by the helicopters (like four mine-clearing sleds towed by the helicopters). The hangar deck carries boats and unmanned surface and underwater vehicles weighing up to 11 tons (the max that the cranes can handle.) The ESB crew consists of 101 navy personnel and 44 civilians. There are accommodations for an additional 298 personnel (commandos, marines, specialists of any sort). There are plans to add a gym and other facilities for troops. The flight deck can also operate many types of UAVs. There is a lot of cargo space for supplies and fuel. The ship is highly automated and before conversion only needed a crew of 34 civilians.
The first two ESBs were called ESDs and were half the size of the other five ESBs in service or under construction. The first of these smaller ESD type ESBs was completed in 2013 as ESD-1 Montford Point. ESD stood for Expeditionary Transfer Dock (formerly T-MLP) ships. Montford Point successfully completed its sea trials and entered service in early 2014. The second ESD (ESD-2 John Glenn) followed. Instead of building more of this design the navy switched to a different (but larger and similar) design; ESB. Originally the navy sought to use the ESD design as floating bases to support commando type operations ashore. The ESDs are 34,500 ton vessels that, in effect, serve as seagoing piers for situations where there is no friendly port handy. Each is 239 meters (785 feet) long and has up to 2,322 square meters (25,000 square feet) of deck space for storage of vehicles and aircraft.
The ESD looks like a container ship with the main deck lowered to approximately the height of a dock. These transports are called semi-submersible because the vessel being carried can be "floated" aboard the transport, which is designed to submerge their transport deck, which is closer to the water surface than the rest of the ship, to load and unload the smaller ship carried.
On the side of the ESD are mooring fenders (so cargo ships can, literally, tie up like at a dock). The T-ESD also has ramps for getting cargo from ships or a dock. Cargo would be transferred to landing craft or LCAC (air-cushion high speed landing craft which can carry 60 tons of cargo). The ESD can also partially submerge itself so that its deck is underwater. Landing craft can then move over the deck and the ESD can bring its deck back out of the water so the landing craft can be loaded. T-ESDs are to each carry three LCACs. You could not base helicopters on the ESD and that was a major reason for switching to the larger ESB variants. These can carry helicopters and MV-22 tilt-rotor aircraft. The navy considered increasing the fire resistance of part of the deck so that the ESB could also handle vertical takeoff F-35B. This was not done because of the expense and the fact that existing (LPD/LHD) amphibious ships could handle the F-35B. The ESB supplemented the LPD/LPH type ships as well as providing seagoing bases for special operations forces or marines doing similar work. In effect it expanded the navy amphibious ship fleet with cheaper, but larger, ESB ships that handled longer-term logistical support of troops ashore.
Work on the ESD/ESB concepts began earlier. In 2012 an American LPD (amphibious ship with a dock in the rear for small boats), a 17,000-ton Austin class amphibious ship (USS Ponce), was rapidly (five months) modified to be a support ship (for mine-clearing mainly, but also disaster relief or supporting commando operations). Normally these LPDs carry a reinforced battalion of marines, plus six CH-46 helicopters and 39 landing craft (24 of them AAV infantry fighting vehicles). The Ponce normally has a crew of 420, plus space for 900 marines and vehicles or cargo. The shipyard conversion saw a lot of berthing spaces for the marines converted to work areas (for headquarters or training). Since the converted ship got by with half as many crew, the crew quarters were remodeled to make them roomier and more comfortable. The modified Ponce was equipped to support smaller mine-clearing ships and helicopters as well as serving as a floating base for MH-53 mine-clearing helicopters. There is also more communications gear and special equipment, like UAVs and UUVs (unmanned submarines the size of torpedoes used for finding mines). Its designation was changed from LPD-15 to AFSB-15.
This was not a new concept as in 2006 an Austin class amphibious ship was sent to the Indian Ocean without the normal complement of marines. That ship was instead used as a floating base for UAVs and SOF (special operations forces). A similar task was assigned to a navy carrier in 2002, to support SOF operations in Afghanistan. In 2006, it was believed that an amphibious ship was also supporting SOF operations ashore in Somalia or Iran. The Austins entered from 1969 to 1971 and the Ponce was the last to do so. Austins are normally armed with two 25mm autocannon, two 20mm Phalanx for anti-missile work, and eight 12.7mm machine-guns. The Ponce retired in 2017 but by then the ESDs were in service and the first ESB went to work. In 2018 mine clearing personnel began training on ESBs so mine-clearing could become a standard capability of these ships.
China already has some of the larger (50,000-ton) semi-submersible heavy transports for carrying smaller ships or boats long distances. This is safer and cheaper than sending these smaller vessels on long trips many of them are not designed for.