Abstract: A building structure having a roof opening which is covered by a retractable closure. The retractable closure includes a plurality of panels which are pivotally mounted about the margin of the roof opening. The panels are sized and shaped so that they fit together and cover the opening when in their closed positions, and when the panels are in their open positions the roof opening is completely unobstructed. Tracks on the roof support the panels and resist vertical load forces. Support towers and guy lines are provided to bear a part of the load forces as the panels are closed. Drive means is provided for opening and closing the panels conjointly or independently to achieve a range of roof opening capability.

Abstract: A drilling unit is disclosed which incorporates a submersible streamlined hull supporting a streamlined upper hull above the surface of the water by support columns and trusses. The unit is directionally controlled by transverse thrusters and in-line propulsion screws in the submersible hull which are operated by a control system to maintain station keeping over the wellhead at an optimum heading for minimizing the effects of environmental forces such as wind, current and wave action. The control system senses the vessel's heading and surge and sway motions with respect to the wellhead reference and converts this information into error signals. The bow and stern thrusters and in-line propulsion screws are then operated to produce thrust forces which reduce the respective errors. In response to the generation of a stern thrust the control system iteratively calculates a change in the bow target point position on the target circle which will make the stern thrust trend toward zero.