Ultra deep water scenarios are still showing up offshore Brazil. New recent oil discoveries in the Tupi and Jupiter oil fields are over more than 3000m water depth. These new scenarios require new technology for their development. A useful methodology is though the use experimental test in ocean basins. However, the depth of all world ocean basins is not enough to accommodate recommended scales for floating bodies and moorings and risers for such water depth. Hence, line truncation is necessary. The present paper proposes a new idea to surpass this limitation. The work suggests the use of the so called magnetic bases to avoid some problems that occur with a blunt truncation of lines. These magnetic base devices are projected to obtain with the truncated lines, the same full scale properties such as restoring forces and also the same viscous dissipation. This paper describes the procedure followed to study a magnetic base application. Due to availability of a ITTC semisubmersible, the first step was to make this ITTC model to work as TLP. The next task was to design the truncation of its lines using the concept of magnetic base. Subsequently, the TLP project with full and truncated lines mooring system in what concern theirs natural frequency of response, the decay behaviour and the response in irregular waves were addressed numerically using a non-linear time domain program.

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