Chinese efforts to claim all of the South China Sea and then keep all foreign naval forces out have usually been explained in economic terms. There are a lot of valuable fishing grounds in that area, not to mention oil and gas deposits under the sea bed. But another benefit is to provide secure patrol areas for its noisy (and thus easy for nearby anti-submarine forces to detect) SSBNs (nuclear powered ballistic missile carrying subs). While it is 13,000 kilometers from the South China Sea to Washington, DC and the longest range of SLBMs (submarine launched ballistic missiles) is 11,000 kilometers (for the American Trident missile), a longer range SLBM could be built. That would be easier than mastering the much more complex technology of making SSBNs quiet enough to avoid detection by American subs and aircraft. While this sounds like a desperate solution, China does not have too many other good options.
In the last year several Chinese JL (Julang) 2 SLBM (Sea Launched Ballistic Missile) were test fired. The results were not encouraging. The 42 ton JL-2 has a range of 8,000 kilometers and would enable China to aim missiles at any target in the United States from a 94 class SSBN cruising off Hawaii or Alaska. Each 94 boat can carry twelve of these missiles, which are naval versions of the existing land based 42 ton DF-31 ICBM. China has had a lot of problems with the JL-2, which was supposed to have entered service four years ago but kept failing test launches.
In part because of the lack of a reliable SLBM, no Chinese SSBN has ever gone on a combat cruise. But the subs have been very unreliable as well. China has so far produced two generations of SSBNs. In the early 1980s, the Type 92 was launched but had a lot of problems and never made a patrol. It only went out for training in Chinese coastal waters. Only one was built. In the last decade the Type 94 showed up. This was believed, in the West, to be the Chinese SSBN that would go on patrol but that never happened. Turns out that the Type 94 also had lots of technical problems.
This sad saga began with the Type 93 class SSN (nuclear powered attack sub), which looks a lot like the three decade old Russian Victor III class SSN design. The first Type 93 entered service in 2006. The Type 93 was the basis for the Type 94 SSBN, which looks like a Victor III with a missile compartment added. Taking a SSN design and adding extra compartments to hold the ballistic missiles is an old trick, pioneered by the United States in the 1950s to produce the first ever SSBNs. The Chinese appear to have done the same thing with their new Type 93 SSN, creating a larger Type 94 SSBN boat of 9,000 tons displacement. Priority was apparently given to construction of the Type 94, as having nuclear missiles able to reach the United States gives China more diplomatic clout than some new SSNs. The first 94 entered service three years ago. But it has still not gone to sea equipped with nuclear missiles.
Having already sent the first two new, 7,000 ton 93 class SSNs to sea, China was apparently underwhelmed by their performance. Not much more is expected from the 94s. The 93s were too noisy and had a long list of more minor defects as well. It's unclear how many 93s will be built, probably no more than 3-6. More resources are apparently being diverted to the next SSN class - the 95, and the next SSBN, the Type 96.
The Type 93 and Type 94 were both over a decade in development and construction. Work began on the 94 class in the 1990s. For years all that was known was that the Chinese were having technical problems with the new design. The 94 is a modern SSBN, using technology bought from Russia, plus what was developed by the Chinese in their earlier nuclear submarine building efforts. While the Chinese have had a hard time building reliable and quiet nuclear subs, they are determined to acquire the needed skills. You do that by doing it and eating your mistakes. U.S. intelligence experts believe that China is now concentrating on the design of the new Type 96s. China has made progress in developing more reliable land-based ICBMs which means they have the technology to build similar SLBMs.