The scopc of work which this paper is based on is the conversion of the semisubmersible drilling rig PETROBRAS-XXIII (Ex-VINNI) from the original mode of operation to a dynamic positioning unit, capable to operate on offshore fields of Brazilian coast from 100 to 1500 m water depths.

Originally such unit was designed by GVA, named GVA 4000, to operate up to 460 m water depth, positioned by means of a 8-point mooring system.

The conversion is based on the brazilian waters environmental conditions and the design criteria is according to DNV rules and the principle of NMD, as stated for class 2.

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