Experience Based Data For FPSO's Structural Design
Authors: Wang Ge , Spong Robert 
Published: October 2008

There is a lack of systematic data suitable for conceptual and preliminary design of ship-shaped FPSOs. At the same time, there is an increasing interest in their applications, and the number of FPSOs is growing. This combination makes the development of relevant design data both necessary and very useful. The ever growing interest in applying risk and reliability approaches in designing and inspecting FPSOs, also calls for many data inputs, ranging from corrosion rates to proven designs. Based on this, design approaches can be refined and inspection plans can be rationalized or optimized.

This paper presents some structural strength related data from trading tankers that may be used in the structural designs or selection of designs for FPSOs. These include main dimensions of hull, design loads for different installation sites, scantlings of primary structural members, corrosion wastage and the hull girder properties of corroded hulls.

Corrosion wastage data for FPSOs is either scarce or in a format that is not very usable. The experience gained from trading tankers can be used to establish design standards and planning inspection and maintenance before sufficient corrosion data becomes available. A corrosion measurement database of 140 trading tankers is introduced. Trends are analyzed as well as discussions related to the reduction in hull girder strength over time. Some of the potential limitations of the data presented are due to the differences between tankers and FPSOs. The influences of FPSOs' aging on the local and global strength are demonstrated, taking into account buckling and ultimate strength. Implications of corrosion on FPSO's safety and inspection planning are presented based on time variant reliability approaches.

The collected experience based data will facilitate the development of rationalized approaches to the decision-making, FPSO selection, and inspection.

Introduction

There are approximately 85 FPSOs currently operating around the world [7, 8]. There are between 8 and 10 FPSO units coming on-line per year worldwide and 80 or more FPSOs planned or under study for future field developments [2]. Of this fleet of FPSOs, approximately 65% are converted from trading tankers, primarily older vintage single hull tankers.

Compared to the trading tankers, FPSOs have very limited experience, with an estimated total cumulative operating experience in the range of only 500 - 600 years. This is compared to other offshore installations such as fixed platforms, which have well over 100,000 combined operating years of experience with many platforms still in operation 10-20 years after their 20-year life. In comparison, FPSO installations tend to be in their infancy, with only 15% of the fleet having been in operation for more than 10 years. Because of this, there is a lack of systematic experienced based data for conceptual and preliminary design, as well as assisting in the evaluation of the long-term performance specific to shipshaped FPSOs. At the same time, there is an increasing interest in their application, and the number of FPSOs is growing. This combination makes the development of relevant design data both necessary and very useful.