From the link: "A semi-submersible obtains its buoyancy from ballasted, watertight, pontoons located below the ocean surface and wave action. The operating deck can be located high above the sea level due to the good stability of the concept, and therefore the operating deck is kept well away from the waves. Structural columns connect the pontoons and operating deck.[1]"

So, gnu, are you saying that the pontoons melted/ruptured/broke off in the explosion and that's why it sank?