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ABSTRACT The 32000 t semi submersible RS 35 designed by ERNO Raumfahrttechnik and partners is a new concept for rough weather operation. It is characterized by a deep submerged dodecagonal displacement structure with four slightly slanted columns supporting a 55.5 × 55.5 m deck. The paper deals with sea keeping and mooring tests on one hand and towing tests for two headings and three draughts (transit, survival and operation) on the other hand. The experiments were performed with two models at a scale of 1:50 and 1: 100. The results of the sea keeping tests, i.e. the transfer functions of heave, surge and pitch prove the excellent response characteristics of this design. Especially in the period range of 5-12 sec the resulting platform motions are extremely small since the forces acting on the submerged torus are nearly cancelled by the forces on the columns. A keypoint of the paper is the comparison of test results of the two models including a discussion on the non-linear damping characteristics for several resonance conditions (heave, surge and pitch of unrestrained and moored semi submersible). Further, an extensive analysis of drift forces has been carried out. Finally, in towing tests at several draughts the drag characteristics of both models have been evaluated. Due to the unusual shape of the structure only a 5-knots transit speed can be achieved with the 24 000 hp propulsion system. The resistance of the ring structure with diameter Dt itself roughly equals the resistance of a single cylinder having the same length as the torus diameter and towed transversely. Thus the resistance of the structure - independent of the towing direction - is about half of the transverse resistance of a comparable twin hull semi submersible. This feature, in conjunction with the excellent sea keeping characteristics, qualifies the RS 35 semi submersible for highly sophisticated deep water operations. INTRODUCTION Germany with its specialized industries has adopted a role as one of the North Sea neighboring countries developing and constructing new systems for offshore oil and gas exploration and exploitation. This report deals with one of these new concepts, the semi submersible RS 35 which was jointly proposed and developed by ERNO Raumfahrttechnik, Bremen, Ingenieurgemeinschaft Meerestechnik + Seebau GmbH, Hamburg, and Schiffswerft und Maschinenfabrik Max Sieghold, Bremerhaven . The project was partly supported by the Federal Ministry of Research and Technology, Bonn. The main objective of this paper is to highlight some hydromechanical qualities of this design, i.e. its seaway behaviour at operation and survival draughts, and its towing resistance during transit. Design procedurewere assisted by preliminary model tests at a scale of 1:100. Finally, with a 1:50 scale model an extensive experimental program has been accomplished, thus enabling comparative scaling considerations.