To respond to the needs for operational supplies for the ultra-deep water mooring of semi-submersibles, the conversion of an ageing single hull tanker is proposed in which the transverse bulkheads in the center tank area are eliminated and the corresponding deck portion is lowered do provide a single loading space served by a hoisting crane located at approximately mid length. The structural capacity of the proposed conversion is tested against rule defined vertical bending and combined horizontal bending and torsion by means of a coarse mesh three dimensional finite element model. Whereas torsional strength is assured, longitudinal strength is rendered insufficient due to the significant reduction of the midship section modulus. It is however shown that with a reduction to 60% of the rule still water bending moment longitudinal strength may also be assured. Since the rule still water bending moment is provided as a guidance only, such reduction may be acceptable considering on one hand that the loading and unloading operations may be performed with great precision with the available hoisting crane serving the single loading space and on the other that due to the additional ballasting flexibility provided by the preservation of the original pump room a preventive reduction of the still water bending moment may be induced by appropriate ballast control.