Patent ID: 8752496

Claim:
An autonomous, self-propelled semi-submersible vessel comprising: at least one lower hull section which defines a first water displacing volume, the at least one lower hull section comprising a propulsion system and a dynamic positioning system using propulsion and/or thrusters to control horizontal positioning of the autonomous, self-propelled semi-submersible vessel; a support structure connected to the at least one lower hull section and extending upward from the at least one lower hull section wherein the support structure has a waterplane area which is smaller than the waterplane area of the at least one lower hull section; a deck structure connected to the support structure, wherein the deck structure is provided at a distance above the at least one lower hull section; and a ballast system for controllably varying the draft of the vessel; wherein the vessel is designed and constructed to be operated at a number of different predescribed drafts which are specified for that vessel, the predescribed drafts comprising: an operational draft for carrying out an operation, a transit draft for travelling large distances, and a survival draft for surviving a storm; the semi-submersible vessel having structural reinforcements configured for operation of the semi-submersible vessel at the survival draft, in which survival draft the at least one lower hull section of the semi-submersible vessel is partly above a waterline.