FPSO Integrity: Comparative Study of Fatigue Analysis Methods
Authors: Francois M. , Mo O. , Fricke W. , Mitchell K. , Healy B. 
Published: October 2008

Within the framework of the FPSO Integrity JIP, a comparative study of fatigue analysis methods used by the participant Classification Societies (ABS, Bureau Veritas, DNV, Germanischer Lloyd, and Lloyd's Register) is being performed. The objectives of this study are to compare the results produced by the application of various existing fatigue analysis procedures, both with each other and with field data from the full-scale monitoring of the Glas Dowr FPSO.

In the completed first phase of work, a base case for comparison was defined and each participating Classification Society made predictions of the fatigue loading and damage using its own procedures and software. In this paper, the analytical procedures applied in the first phase are described and their results compared in terms of both the predicted fatigue damage and the intermediate fatigue loading steps. This comparative process is valuable in that it provides insight into the source of any observed differences.

In the ongoing second phase of work, the results of fatigue analysis procedures are being compared to site measurements from the Glas Dowr. This paper presents results from the initial study of the second phase, in which one participating Society (GL) compared analytical results against site measurements for one typical storm.

Given the broad spectrum of methods that were investigated in the base case comparison, reasonably good convergence of the results for certain response quantities is observed. Other response quantities, such as the fatigue damage calculated at individual locations, can vary substantially.

Introduction

The JIP FPSO Integrity, led by MARIN, was initiated in 1996. Its primary focus is a full-scale monitoring and measurement campaign of the FPSO Glas Dowr, operated by Bluewater for Amerada Hess on the Dauntless and Durward field in the North Sea. Further descriptions of the JIP activities and monitoring system may be found in companion papers [1,2].

Within the framework of this JIP, a comparative study of fatigue analysis methods used by the five participating Classification Societies (ABS, Bureau Veritas, DNV, Germanischer Lloyd, and Lloyd's Register) is being performed. The objectives of the comparative study are toCompare the results produced by the various analytical procedures and software proposed by the participants.Identify the differences and gain insight into the source of these differences.Compare existing analytical procedures for FPSOs with field data from the full-scale monitoring of the FPSO Glas Dowr.

To accomplish the above objectives, the comparative study has concentrated on the fatigue loading experienced by the FPSO Glas Dowr hull. In the completed first phase of work, a base case for comparison was defined and each participating Classification Society made predictions of the fatigue loading and damage using its own procedures and software. The task leader (Bureau Veritas) compiled and evaluated the results in order to identify and interpret any observed convergence or divergence between the methods. In this paper, the analytical procedures applied in the first phase are described and their results compared in terms of both the predicted fatigue damage and the intermediate fatigue loading steps.