The first submersible ballistic missile test stand barge was set up in 2014 and is based at the Sinpo South Shipyard.
This barge supports the development of the Pukguksong-1 SLBM and the SINPO-Class experimental ballistic missile submarine.
The fitting-out dock is for when vessels need systems such as pumps or electrics to be installed and minor work to be completed before being placed into service.
Since the last set of images were taken in December, the barge now has a small floating crane moored alongside it.
The purpose of the work is unclear, but it is probably related to final stages of repair before declaring the barge operational.
It comes as a nuclear scientist who has frequently visited North Korea warned that the nation could have an atomic bomb capable of fitting onto a missile.
Director of Los Alamos National Laboratory Sig Hecker said they could also have between 30 and 60 small mobile nuclear warheads.
Speaking to CBS News’ 60 Minutes, he said: “What’s even more important than the 30 or 60, is how small can they make them.”
Photos from 2016 show the rogue regime’s leader posing with what he claimed was a nuclear warhead small enough to be mounted on a missile.
Asked what it looked like, Hecker said it resembled a fission bomb.
He said: “This to me is one that I would call a spherical fission bomb, in other words, the atomic bomb.
“It looks like a simple bomb. However what I found most important about this is the size of it, it looks to be about 60 centimetres.
Mr Hecker was shown another photo of Kim’s weapons from September when North Korea unveiled a sleeker warhead design.
The regime said it was a much more powerful thermonuclear weapon.
Mr Hecker added: “That shape is consistent with what we would call the two-stage thermonuclear weapon. What that essentially means is sort of a modern hydrogen bomb.”
Asked whether it could be mounted to a missile, Hecker said: “They definitely want us to think so.
“In the background, they actually show the warhead positioned in the nose cone of the missile which we interpreted to be an ICBM.”
Kim Jong-un’s repeated tests of his atomic arsenal have caused tensions to heat up in Asia, bringing condemnation from numerous powers and fanned World War 3 fears.