Fatigue Analysis of FPSO Topside Module Joints

Customer Challenge

During the independent verification process, finite element analysis of topside module roll brace connections identified higher stress concentration factors (SCF) than had been defined in clients original fatigue basis of design for roll brace/ beam interface.
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Operator client wished to better understand the estimated fatigue life of joints considering the higher SCF. FPSO was to passively weather-vane and as a result the proportion of the on-station life that the vessel would experience head seas, beam seas and quartering seas was unknown. 


 

Key Skills Exhibited

Experience in FPSO topside structural behaviour and quasi static structural analysis was used to build upon an existing analysis model file that would output stress ranges for roll brace connection.

First principles numerical manipulation of force range outputs at joints using spreadsheets was used to sum cumulative fatigue damage and calculate predicted fatigue life for different proportions of relative headings for sea states in accordance with DNV-RP-C203.
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Experience working with IVB/ Class society was used to open-up discussions to agree fatigue life calculation methodology with IVB before committing significant manhours to the job. This managed the risk to the client’s budget.

Customer Benefit

• Project was turned around quickly which was key to minimising delays to fast track project.
• Engineers worked closely with IVB throughout to agree acceptable methodologies and assumptions to guarantee swift close out IVB process for topside modules. This was key as modules were installed on vessel and a protracted IVB process had the potential to delay sail away date and summer window to hook up FPSO.