“While a reported third test of North Korea’s submarine-launched ballistic missile (SLBM) system in December 2015 now appears to have failed, an examination of the video of the initial stages of this launch, along with commercial satellite imagery of the submarine and support vessels in port two days later, suggests that this test was probably conducted from a submerged barge rather than an actual submarine. The failed launch combined with testing from a barge shows that North Korea still has a long way to go to develop this system. Contrary to some speculation in the media, an initial operational capability of a North Korean ballistic-missile submarine is not expected before 2020.”

Source: North Korea Tests a Submerged-Launch Ballistic Missile, Take 3 | The Diplomat