Complex sentence: As the "Deepwater Horizon" sank, the riser pipe that connected the well-head to the rig was severed and as a result oil began to spill into the Gulf of Mexico.
Split into simpler sentences: As the "Deepwater Horizon" sank, the riser pipe that connected the well-head to the rig was severed. As a result, oil began to spill into the Gulf of Mexico.