Low cycle fatigue of FPSO ship structure

Abstract

The phenomenon of low cycle fatigue (LCF) is characterised by high stress range, close to or above yield, and relatively low number of cycles to failure, typically below 104. In the case of tankers and Floating, Production, Storage and Offloading units (FPSO), nominal stress amplitudes lower than the yield stress may result in plastic strains due to the high stress concentrations that are typical in many of the hulls’ structural details. FPSOs are more susceptible to damage due to LCF compared to tankers, cargo and other ocean going ships. The main reasons are; the unique structure of FPSO in terms of the presence of internal turret and topsides load which affects the structural response of FPSO to dynamic and quasi-static loads, the frequent loading and unloading patterns of FPSO (i.e. unlike oil tankers which are either in full load or ballast condition) which causes the FPSO to experience the maximum hogging and sagging still water bending moment every single cycle and the condition of the sea at which the FPSO is operating (site specific environment) where even benign condition may subject the FPSO to extremely diverse wave induced loads. An increasing number of FPSOs are being used in the oil and gas industry due to the practical advantages they offer as compared to fixed installations, however, many FPSO show signs of cracks at critical locations in the first five years of service. It is believed that this is primarily due to LCF. It is therefore imperative to address LCF at the design stage. Finite Element Analysis (FEA) has been used to demonstrate that extremely high stress levels, exceeding three times the yield stress of the material, may occur at some critical locations during FPSO operations. Due to this ‘new’ form of damage in ship structures classification societies, shipyards and other organizations are addressing the issue of LCF by issuing various guidance notes and recommended practices in order to assess the damage due to LCF. This research contains a very extensive and useful literature review of the state-of-theart in LCF assessment methods available in literature and various class societies. Representative operational loading conditions (most onerous) have been presented for LCF Assessment of FPSO. LCF tests of typical longitudinal attachment were performed. This important structural element is seldom tested compared to the transverse attachment or cruciform. Experimental and numerical results compare well. A novel method of predicting LCF life has been proposed and a new S-N curve is proposed to be used for LCF assessment.