Page 184 - Practical Design Ships and Floating Structures

Practical Design of Ships and Other Floating Structures 159
You-Sheng Wu, Wei-Cheng Cui and Guo-Jun Zhou (Eds)
0 200 1 Elsevier Science Ltd. All rights reserved

DESIGN RECOMMENDATIONS FROM THE FPSO - FATIGUE
CAPACITY JIP

I. Lotsberg
Det Norske Veritas, Veritasveien 1,1322 Hhk, Norway

ABSTRACT

A joint industry project on the Fatigue Capacity of FPSOs has been carried out with 19 participants
from oil companies, designers, shipyards and classification companies in the period 1998 -2000. The
main objective of the project was to reduce the risk for fatigue cracks and fracture in FPSOs. The
project provides small scale and full scale S-N data on typical fatigue sensitive details in FPSOs and
tankers. It also provides improved information on the important link between S-N data and finite
element analyses for fatigue life assessment. Finally it provides data to assess more reliable the risk for
unstable fixture of FPSOs when fatigue cracks are present in the structure. This paper provides links
to other sources where a more detailed description of some of the main achievements have been
presented.

KEYWORDS
Fatigue, FPSO, Fatigue testing, S-N data, Hot spot stress, Finite element analysis, Fracture.

1 INTRODUCTION
A joint industry project on the Fatigue Capacity of FPSOs has been carried out with 19 participants
from oil companies, designers, shipyards and classification companies in the period 1998 - 2000. For
background and a more detailed description of the scope of work for this project see Lotsberg (2000).
For an overview of work performed see also btsberg (2001). (FPSO = Floating Production Storage
and Offloading unit).
The main objective of this project was to reduce the risk for fatigue and fracture in FPSOs.
Finite element analysis (FEA) is becoming a design tool also for fatigue assessment of FPSOs. In order
to have a reliable analysis procedure it is important that there is a proper link between the calculation
of hot spot stress and the hot spot stress implicit the S-N curve that is used for assessment of fatigue
damage. Therefore hot spot stress evaluation from FEA and corresponding S-N curves were important
issues in this project. Also FEA of a number of FPSO details were performed to derive stress
concentration factors that can be used for efficient fatigue design of FPSO structures. These items are
described more in detail in this paper. Unstable fracture may initiate from fatigue cracks at welded
connections with materials showing a low fracture toughness. This issue was also investigated in this
project. Reference is made to Pisarski (2001) for further details.