Abstract: A method of severing a tubular structure can include producing a stream of plasma between a plasma arc cutter and the tubular structure, and displacing the plasma arc cutter relative to the tubular structure, thereby cutting through the tubular structure. A plasma arc cutting apparatus can include a transport vehicle, a boom extending from the transport vehicle, at least one plasma arc cutter, and at least one actuator that displaces the plasma arc cutter relative to the boom. Another method of severing a tubular structure can include conveying a plasma arc cutter to the tubular structure with a transport vehicle, and displacing the plasma arc cutter relative to the transport vehicle.

Abstract: In a subsea drilling operation, a riser isolation tool may be installed inside a marine riser between the subsea wellhead and the rig floor to provide a conduit having a higher pressure rating than the original riser itself. In some embodiments, the riser isolation tool includes a tubular body and, extending therefrom, a seal stinger sized to be slidably received in a receptacle seated in the wellhead. Additional apparatus, systems, and methods are disclosed.