Conversions vs. Newbuildings: General Arrangement and Strength Issues in FPSO Design

The conversion of ageing tankers is a tempting alternative to newbuildings when FPSO units are planned. The low cost of an already classified hull and an existing pumping facility compensate for the repair and updating costs a conversion may demand. Newbuildings on the other hand allow for improved production plant and storage tank lay-out. The trade offs between conversions and newbuildings are analyzed from the point of view of strength and general arrangement. Converted tankers carry a serious strength drawback since the emptied engine space enhances the high buoyancy region at the stern which significantly increases bending moment amidships. In general one or more cargo tanks must be kept at partial loading to compensate for the increased bending moment. A new building lay-out is analyzed in which the tanker engine space is completely eliminated. Utility systems are resized and placed on the main deck aft of the production plant. Accommodation spaces are also resized and rearranged in order to fit into the reduced deck space available. The gains in strength are quantified and the lay-out implications are discussed in detail. A comparative study is presented between a conversion of a hypothetical tanker and an equivalent new building.