The super-sized 30,000 ton displacement ZAPATA UGLAND has been drilling in the North Sea since. August 1974. This paper summarizes the experienced weather, the associated rig motions and the drilling performance in its operations in the North Sea. This paper highlights the design, equipment and operational aspects of the ZAPATA UGLAND that have been key factors in its splendid performance in the North Sea. A discussion is presented of model test data and the experienced motions and performance. This repot concludes that (1) super-sized semi submersibles, such as the ZAPATA UGLAND, are the optimum type of unit to use in year around operations in deeper waters of the North Sea and (2) self propulsion for a semi submersible is operationally effective and reliable. INTRODUCTION As the offshore industry extended into deeper waters of the North Sea and other severe environmental offshore regions, a new class of super-sized semi submersibles were designed, developed, constructed and put into operations. The criteria for classing a semi submersible as "super-sized" is based upon its operating displacement exceeding the working displacement of the pioneering Mohole platform the prototype and forerunner design for all twin-hull semi submersibles. In 1969, Zapata initiated design studies to determine the optimum characteristics and carrying capacities for a super class design of semi-submersible drilling units. After initial studies of existing and proposed designs were compared with the trend of industry requirements, Zapata developed design parameters for a new unit. These included the following key features:Ability to continue operations in seas of up to 40 to 50 feetAbility to remain moored in seas of up to 100 feetVariable deck load capacity of 3,000 tonsLoad capacity enabling all movements to be made with anchors and anchor chains stowed aboard the vesselSelf-propelled, with a target speed of 9 knots in calm waterMooring system for 1000 to 1500 ft. maximum range of water depths. A detailed study of designs tested by Bethlehem Steel at the Netherlands Ship Model Basin indicated one of its designs was highly suitable for further development according to Zapata's target objectives. In 1971, Zapata initiated a joint program with Bethlehem to modify the design for the parameters Zapata had selected. This program included designing the structure, selection of the lower hull form, model tank tests of the new design and strength analyses concurrently with selection of the optimum drilling and auxiliary equipment. DEVELOPMENT PROGRAM The resulting design consisted of catamaran lower hulls with six stability columns supporting a water-tight upper hull. The lower hulls were given a sea-barge configuration with faired bows to provide optimum heave dampening action and propulsion characteristics. The trim upper hull was designed with clear lines, minimizing the amount of machinery and storage located on the upper deck exposed to the elements.