As designer of FPSO vessels ample experience has been gained with both the design of new build FPSOs and the conversion of existing trading tankers into an FPSO. This experience covers FPSO hull design, topsides and topsides support design and the combined application of ship and offshore rules, regulations and standards during the design process.Items discussed in the paper will address the use of direct calculations versus traditional ship rules, and more generally the differences between naval architectural and offshoreorientated approach. A comparison will be made between new build and conversion projects. Integration of topsides and hull design is required, while maintaining sufficient flexibility to accommodate design changes. Moreover the FPSO hull influence on topsides design will be further examined. Attention should be paid to adherence to common shipyard practice.Two examples of FPSO hull design will be presented. One is a new build vessel, and the other a conversion. Both projects are comparable with regard to geographical location and mooring system. For both vessels, aspects of hull design and relation between hull and topsides design will be addressed. Typical aspects of hull design to be discussed are cargo and ballast arrangement, extreme bending moments and fatigue design. The impact of the module support arrangement on the hull design and piping design is evaluated.The general information presented in the paper, and supported by the cited examples, will provide guidance to the industry on how to merge shipbuilding practice and offshore standards. OTC 13210 FPSO DESIGN AND CONVERSION: A DESIGNER'S APPROACH 3