Design and Conversion of FPSO Mystras
Authors: Terpstra T. , Schouten G. , Ursini L. 
Published: October 2008

A recently completed FPSO conversion project shows some novel features in FPSO design. An existing trading tanker was selected for conversion into a 48,000-bopd FPSO unit, for a site offshore Nigeria in a water depth of 70 meters. While engineering of the FPSO was already underway it was discovered that the field production characteristics were underestimated.A crude oil throughput of 80,000 bopd was required, which corresponds to a medium size process installation. This happened to be in line with the contractor's "generic FPSO" design and left the designers with the challenge of fitting this medium size process installation on a relatively small sized tanker hull.

To accommodate the increased size of the production facilities, sponson extensions were fitted over a considerable length of the vessel's side. By the use of sponsons the vessel's main deck area, ballast tank volume and buoyancy were increased considerably. However installation requirements for the spread mooring system, large central pipe rack, separate cable tray supports and marine deck piping still resulted in a congested deck space, which demanded a thoroughly planned lay-out of the main deck arrangement, pushing it beyond existing limits, resulting in an innovative design.

This paper addresses and details the design decisions and their backgrounds, also reviewing alternatives considered. Special attention is given to the life extension calculations of the hull structure and the effect on these of adding sponsons and the cantilever supports. And finally, practical experience during vessel conversion and construction from the conversion yard in Dubai is discussed.

Introduction

Early December 2000 first preparations were made for the preengineering for a small sized FPSO with a production capacity of 48,000 bopd and a storage capacity of 1,000,000 bbls for Agip Energy & Natural Resources (Nigeria) Limited (AENR). In March 2001 a letter of intent was issued by AENR to FPSO Mystras Producao de Petroleo Limitada (FMPP) - a Joint Venture (JV) of Saipem and SBM - with a tight contractual first oil date of December of that same year. An existing Suezmax tanker "Mystras II" (140,000 dwt) was purchased and laid up waiting for the conversion to commence. Due to the early first oil date required, a separate fast track solution was selected. The existing FPSO Jamestown, also owned by FMPP, was converted and fully refurbished between September and November 2001 in Portugal with minimum modifications to the existing process facilities. Hereby the contractual date of end December 2001 could be achieved and the FPSO Jamestown would remain on site until the FPSO Mystras- with full crude oil production capacity, would become available.

Early 2002 first field production data received from Jamestown proved better than initially expected and justified exploring the possibilities of considering a larger unit for the field. It was decided to retain the Mystras hull, but to fit larger topsides on it, which left the vessel designers with a number of challenges and implementation of some novel design features.