Effective FPSO/FSO Hull Structural Design
Authors: MacMillan Adrian 
Published: October 2008

There has been a struggle with the hull structural design for Floating, Production, Storage and Offloading (FPSO) vessels due to conflicting approaches between the maritime and offshore industries. Typically the maritime industry follows the empirically based Rule approach whereas the offshore industry tends to use first-principle methods. FPSO structures are an integration between the two industries where each party needs to be aware of the basis and limitations of their respective approaches. By providing designers and operators with some background and applicability of tanker design requirements and DNV's recently introduced Offshore Standards for ship-shaped units it will allow the two industries to merge and achieve the efficient design and analysis of FPSO hull structures.

Using existing traditional tanker design as a basis the design process is defined by firstly considering the environment in which the vessel shall operate. DNV considers two primary environmental categories: harsh and benign. The designer is then provided with guidance for selecting the appropriate environmental category, based on a combination of environmental criteria and vessel particulars. This will assist designers and operators with establishing the appropriate design requirements.

Designers are provided with guidance for using direct calculations to supplement the traditional tanker design requirements. Loading issues such as: greenwater, slamming, bow impact, sloshing, motions and non-collinear environments are evaluated, comparing the use of empirical formulas versus direct analysis methods.

Although the guidance is aimed at FPSOs the procedures are equally applicable to all ship-shaped offshore units, including Floating, Storage Offloading (FSO) vessels.

Introduction

Currently, there is a wealth of accumulated tanker design, construction and operational experience in comparison to that available for ship-shaped floating, production, storage and offloading (FPSO) vessels. Therefore, it is relevant to consider this experience when planning the design for an FPSO. Simultaneously there are also some important differences between an FPSO and a tanker that need to be considered. In order to develop a rational and economic design the available tanker knowledge and experience should be considered with the corresponding structural requirements: Class Rules. For those requirements that are empirically based it is important to be aware of their basis and possible associated limitations.

Most FPSO projects shall be built in traditional shipyards that rely on very efficient fabrication processes. These yards may have up to 50 vessels being constructed each year and any interruptions in the fabrication schedule can impact onseveral projects with severe consequences. It is then critical to integrate, as far as possible, with the current yard procedures.

To begin this process it is important to highlight the similarities and differences between an FPSO and a tanker to enable all designers and operators to plan the design process to ensure minimal interruptions while still maintaining a tight building schedule. This will allow for the appropriate selection of analytical tools so as to reduce the uncertainty in the design and construction process.