A Structural Strength and Fatigue Assessment Methodology Applicable to Conversion-Type FPSO Designs
Authors: Caruso Nick , Johnson Michael , Grove Todd 
Published: October 2008

The increase in deepwater development activity has generated interest in floating production and storage (FPSO) facilities. This, coupled with the need of tanker operators to extend the useful life of existing tanker vessels scheduled for phase out by OPA '90, has promoted the need for a reliable methodology for evaluating the existing structure of tanker vessels slated for conversion to FPSO service. Tankers which have been well maintained and which have served relatively benign ocean environments during their trading history can be utilized as offshore storage facilities provided the existing structure has sufficient strength to support the extreme loads and fatigue damage expected during the target service life at the intended site. This paper will describe a methodology for evaluating the structure of an existing vessel taking into consideration the following: fatigue damage accumulated during trading history, route and/or site-specific wave environment, and strength and fatigue demand at the intended site. The procedure for assessing the strength of structural members takes into account new methods of determining required scantlings and considers the corrosion on a member which has already been in service. The estimation of accumulated fatigue damage is obtained through a process of "environmental assessment" in which the wave environment along the historical route and/or site service is applied in a full spectral fatigue analysis. The methodology requires access to a reliable global wave database and the wave database which is used is described and compared to other wave databases used within the industry. These methodologies will be utilized in a case study comparing what would be required for a FPSO conversion in the North Sea, from a fatigue aspect, to that of what would be required for a FPSO conversion in the Gulf of Mexico..

Introduction

During the past number of years there has been an ever increasing trend to convert existing oil tankers into Floating Production Storage Offloading Systems (FPSO). Converting existing tankers is, in most cases, more economically feasible and faster than building new production and storage facilities for the same purpose in somewhat marginal fields. Conversion also allows for the extended use of these single skin tankers which are gradually being phased out for regular oil trading due to increased environmental regulations. In many cases, these FPSOs are designed to be left on site for upwards of twenty years. Complete planning of the conversion is very important in achieving this desired service life without necessitating to come off station for repairs. It has been shown in Ref. 1 that performing repairs while the FPSO is on site can be time consuming, costly, and reduces the overall productivity of the unit. Removing the FPSO entirely off site will also be very costly in terms of the shipyard cost and the time that the unit is not on site performing its intended job. Therefore, by careful planning, structural modifications and renewals can take place at time of conversion eliminating the need to perform these repair items in the future. In most FPSO conversions, the basic structural arrangement is kept intact in way of the cargo block region, whereas in the turret region there are major changes and additions to account for the mooring loads, risers, bearings, etc.