No abstract

There are situations where a marine structure is subjected to stress cycles of such large magnitude that small, but significant, parts of the structural component in question experiences cyclic plasticity. Welded joints are particularly vulnerable because of high local stress concentrations. Fatigue caused by oscillating strain in the plastic range is called “low cycle fatigue”. Cycles to failure are typically below 104. Traditional welded joint S-N curves do not describe the fatigue strength in the low cycle region (< 104 number of cycles). Typical Class Society Rules do not directly address the low cycle fatigue problem. It is therefore the objective of this paper to present a credible fatigue damage prediction method of welded joints in the low cycle fatigue regime.

The effect of weld toe grinding on fatigue life of welded joint has been studied by a number of investigators who have established that improvement factor on fatigue life is greater for higher tensile steels as compared with mild steels [1]. To establish the effect of weld toe grinding on the fatigue strength of joints made from DH36 steel, transverse welded specimens were tested under constant amplitude loading. This work was carried out by DAEWOO Shipbuilding and Marine Engineering Co. within Lloyds Register/DSME Joint Project. It was shown that the improvement factor on fatigue strength due to weld toe burr grinding for DH36 steel is above 1.5 for high cycle fatigue range.

Agbami FPSO is a floating facility for production, storage and offloading of oil at OPL 216/217 offshore Nigeria of the central Niger delta. The FPSO is positioned with spread moorings in water depth of about 1500 m for the service life of 20 years. As per the design requirement, the hull of FPSO shall be designed to meet ABS’s SFA (Spectral Fatigue Analysis) notation and the seagoing condition. Seagoing is a mandatory condition additionally required by Client, assuming a navigating vessel of North Atlantic. But, to conservatively assess Agbami for 60 years at its Onsite except specific details was taken into account, which is related to the uncertainties in S-N data & Palmgren-Miner’s damage rule and etc. Normally the off-western Nigerian sea has swell-governed environment and the multi-peaked spectral characteristics with wave and swells. For the spectral fatigue analysis, Jonswap spectrum for wind wave & Gaussian spectra for swells were taken into consideration as a representative in case of Onsite. Thus, in order to achieve a single damage, the combined spectral method for Onsite was introduced on the basis of EMDC’s FPSO FMS (Fatigue Methodology Specification). On the contrary, Walden’s wave scatter diagram as recommended by ABS for Seagoing was applied. The forward speed of FPSO in Seagoing case was also taken into account by WASIM, a hydrodynamic tool of DNV and compared to the results of PRECAL, ABS’s hydrodynamic code to ensure the validity of the analysis. The post-process to calculate fatigue damage was carried out using the in-house program to analyze fatigue of FPSO. The results from the analyses were found that the Seagoing led consequentially critical fatigue damage for most of hull structures more than Onsite that has moderated sea state.

The structural stress (SS) method developed by BATTELLE has been studied based on small or mid size scale specimens. In order to apply new method on actual project, the new method such as SS should have application results on actual project. However, SS method didn’t have a lot of application data compared to class procedure using hot spot stress (HSS). In order to find that the SS method for the evaluation of fatigue life can give reasonable results when it is applied under the same loading suggested by classification societies, fatigue lives derived by class and SS method were compared. ABS & DNV’s simplified fatigue analysis method were adopted to check the validity of SS method. Before applying complicated loading of class, static loading case was applied since the class method has their own correlation factor for wave loading. And then simplified fatigue analysis was performed with more complicated loading cases. From the results of fatigue life calculation, it can be said that SS shows reasonable fatigue lives with respect to HSS or notch stress based fatigue lives.