Thesis

Low cycle fatigue in floating production storage and offloading units

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Awarding institution
  • University of Strathclyde
Date of award
  • 2010
Thesis identifier
  • T12611
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Abstract
  • The purpose of this study was to investigate low cycle fatigue damage, from loading and unloading of cargo and ballast, of structural details on a Floating Production Storage Offloading (FPSO) unit. There has been an increased interest in low cycle fatigue in the offshore industry as fatigue cracks have been observed within five years of service delivery. Also FPSOs operate at constantly changing drafts. The draft variation between fully loaded and ballast drafts for some new FPSO designs may be as large as greater than 10 metres. This large draft variation result in additional low cycle fatigue which needs to be taken into account. A detailed finite element analysis was performed to check the low cycle fatigue strength of highly stressed locations and to provide an insight into the mechanism of loading and offloading. The hotspot stress approach and Dirliks rainflow counting method is employed for calculating fatigue damage. Several classification society rules are compared. It was discovered that loading and unloading generate extremely high stresses. A stress value of 1400 MPa was obtained for a critical location in the bottom stiffener detail which is about four times the yield stress of the material. This clearly shows that some critical details are governed by low cycle fatigue and not high cycle fatigue. The analysis has highlighted the fact that it is necessary to take low cycle fatigue into consideration during design.
Resource Type
DOI
Date Created
  • 2010
Former identifier
  • 814892

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