FPSOs: Design Considerations for the Structural Interface Hull and Topsides
Authors: Krekel M.H. , Kaminski M.L. 
Published: October 2008

This paper discusses the structural interface on FPSOs between the hull structure and the topsides modules. It identifies the most common topsides foundation concepts applied on FPSOs, and discusses the consequences of each configuration for the layout of the unit, the design of the hull structure and the topsides. The information needed by the hull designer and the topside designer is identified. Moreover, the differences between shipbuilding and offshore construction design practices are discussed, and it is identified where and how these fall short for FPSO purposes. Topics that are addressed are overall safety, operational aspects, such as tank ntry and mechanical handling, and the design specifications or the hull and the topsides modules. In order to control the chedule and costs of FPSO projects, fabrication of the hull nd topsides should be allowed without impractical or unduly trict specifications imposed on the shipyard or the topsides abricator. At the same time the traditional design pecifications for hull and topsides design may fall short to over the functional needs for FPSO service.

Introduction

To date, FPSOs have been in operation for several decades. Initially this development concept was selected for marginal fields in remote and environmentally benign locations. With he further advancement of sub-sea completions, flexible risers and turret mooring systems, the FPSO made its way in the nineties to harsher environments and larger development chemes that were previously uneconomical. Comparatively little investment had to be made in production-and export acilities, whereas the investment still had a residual value after depletion of the field as it could be re-deployed lsewhere. Furthermore, contractors emerged that were offering lease schemes, lowering the up front investments even further. With the shift of new discoveries towards ever deeper water, the FPSO is becoming more and more the efault development platform for deep water for the years to come. Traditionally, an FPSO consists of a converted tanker with the production facilities, or topsides, mounted on deck. After only a limited conversion the oil tanker will fulfil all unctional demands for storage and offloading of the produced oil. The most common project strategy is to contract large locks of work, such as hull conversion, topsides and mooring system, to independent specialized contractors parallel in time. This is possible since the concept of an FPSO is robust: space n a tanker deck is ample, and mono-hulls are relatively eight insensitive. The downside of this approach is that (contractual) interfaces are created that need to be carefully anaged in order not to jeopardise the successful completion of the project. Over the years, two types of FPSO projects ave evolved.

Conversions the ‘classic’ approach is to convert a ‘vintage’ tanker. This comprises an extensive Repair and Life Extension (R&LE) program, after which a conversion will take place to accommodate the mooring system, production acilities, utility systems and offloading system.