FPSO Design and Conversion: A Designer's Approach
Authors: Terpstra T. , d&apos;Hautefeuille B.B. , MacMillan A.A. 
Published: October 2008

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.

Introduction

FPSO design has shown a fast evolution in recent years. The concept is more and more frequently used for deepwater solutions and in addition new design concepts are being considered. This is especially the case for the development of oilfields offshore Brazil and West Africa. The field developments of benign environments at West Africa have introduced a new generation of FPSO designs. These are designed as barge-like hulls. The introduction of these new designs in combination with topsides weight in excess of 25,000 tonnes leaves the designer with new challenges.

Parallel to these new developments and due to industry need, FPSO design procedures are being reconsidered by all parties involved, e.g. operators, designers and classification societies. The background for these developments lies in the need for establishing more consistent design guidance. On previous FPSO design projects, the way offshore and shipbuilding principles were combined depended mainly on parties involved and could differ between projects.

FPSO concepts have frequently been based on converted tankers. Shipbuilding standards could be directly applied and the hull design could be covered with existing ship Rules. However, specific operational tanker design criteria are no longer applicable to FPSOs as the hull now becomes a floater with storage capacity rather than a sailing tanker. Therefore it is justified to raise the question whether a new build FPSO is to be considered as an offshore structure or that traditional maritime principles are still applicable.