FPSO Integrity: Comparative Study of Fatigue Analysis Methods

Abstract

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.

Sensing and Understanding Fatigue Lifetime of New and Converted FPSOs

“…The uncertainties of hydro-structural analyses will be here illustrated by a benchmark which was carried out within the FPSO Integrity JIP by five major CSs (François, 2000). All CSs (ABS, BV, DNV, GL and LR) received the same set of data consisting of: It has to be pointed out here that the benchmark is not very rigorous and that the obtained stresses would form an input to the fatigue analysis.…”

Design Criteria and Inspection Strategy for FPSO's

“…The methodology and associated software HYDROSTAR and NSO have been extensively verified, and results compared with full-scale measurements onboard the FPSO Glas Dowr [7]. This is however a subject of continuing R&D activities.…”

Review of Risk Assessment for FPSO Structural Design

“…Despite the fact that many of the steps taken and techniques adopted are widely accepted in the engineering community, it is clear that they are on-going subjects for extended research projects. Francois et al, [39] provided an insight into some of the difficulties inherent in the application of the 'standard method' across the certifying authorities. This last paper also begins the process of comparing a range of fatigue load calculation findings and measurements for mid-ship deck strains.…”

“…Given the general nature of these results, it would be wise to acknowledge the likelihood of substantial discrepancies between any fatigue calculation model and reality. [39] also draws attention 'the magnitude and complexity of this type of analysis. Advanced hydrodynamic and structural analysis software must be coupled with powerful computing resources so that large models with many load cases may be processed.'…”