Why is the inside of a semi-submersible ship empty?

The main purpose of designing the interior of semi-submersibles as hollow is to provide greater buoyancy by increasing the volume of water discharge, so as to meet the unique requirements of sinking and floating operations.
The design principle and practical function of the hollow structure inside the semi-submersible can be analyzed from the following perspectives:

Buoyancy Adjustment and Sinking and Floating Function
Semi-submersibles can dive or float on deck by adjusting the amount of water in the ballast tanks. The hollow structure can hold a large amount of ballast water, when it is necessary to dive into the water injected to increase the weight; when drained, the air compression of the hollow chamber is used to restore buoyancy. This design allows the hull to be controlled to sink and float like a submarine, but only partially submerged. 
Carrying Oversized Cargo
The hollow structure expands the overall displacement volume of the hull, and according to Archimedes' Principle (F_{Float =} ρgV _row), the larger displacement volume provides stronger buoyancy to support heavy cargoes such as offshore drilling platforms and warships. For example, the hollow design of the Chinese semi-submersible “Tai'ankou” allows it to carry up to 18,000 tons.
Stability and Safety

The decentralized arrangement of hollow compartments lowers the center of gravity and reduces the effects of wave turbulence;
Separate ballast water tanks allow for precise balancing to avoid tilting when loading cargo.

History and engineering needs
After the 1970s, ocean cargoes became oversized and traditional towing methods were risky. Semi-submersibles of hollow core design, such as the first semi-submersible “Super Hire 1”, solved the overall transportation problem by using the “sea flatbed truck” model.

Conclusion: The hollow structure is the basis for semi-submersibles to realize their core functions, which not only conforms to the physical principle of floating and sinking, but also adapts to the needs of safety and efficiency of modern maritime transportation.