Multiaxial Fatigue due to Wave Induced Accelerations in a Flex-lay Tower Structure

Marine and offshore structures are continuously exposed to different loading types. Cyclic loads results in cyclic stresses in structural components, which may result in degradation of the structural integrity. This process is called fatigue. Fatigue damage of structural components usually occurs without any —global— visible sign. Ultimately, damage in critical components of a structure may lead to catastrophic fracture, and must therefore be prevented. Currently, for uniaxial fatigue of welded joints several design techniques are available which are approved by classification societies. Research indicates that uniaxial fatigue life prediction methods results in a lifetime overestimation when they are applied on structural details exposed to multiaxial stress states. Experiments shows differences in calculated lifetime and real lifetime up to a factor 14. A new method based on the Verity structural stress and a path dependent maximum range (PDMR) cycle counting algorithm proposed by researchers of Battelle was implemented in a software by the author. As study object a welded detail in a flex-lay tower structure installed on a vessel is used. This new method, based on the Verity structural stress and PDMR cycle counting directly takes into account the effect of the multiaxiality. Comparison of the results of the new proposed method is made to the methods presently prescribed by Eurocode 3 and IIW classification codes. Present methods instruct algebraic summation of the damage due to normal and shear stress in case of a multiaxial loaded welds. For the method by the codes the hotspot stress in combination with rainflow cycle counting are applied. For a proportional loaded weld in the flex-lay tower, the new proposed method shows a factor 3 higher damage than the presently methods described. A non-proportional stress state suitable for analysis is not found in the flex-lay tower; therefore a realistic one was obtained by modification of proportional data. Comparison of the proportional and non-proportional stress histogram shows that on average the non-proportional stress state has more cycles in the region of the mode and higher ranges at the cost of the cycles at the ranges positioned left of the mode. For the non-proportional loaded weld, the new proposed method shows a factor 4.2 higher damage than calculated using the current class codes. The research shows using an advanced method, significant higher damage levels are calculated, which may be validated in future experiments. The focus of this research was on the loading side i.e. use of correct stress components, cycle counting method and implementation thereof in software. The author recommends to research the resistance side as well, for example types of S-N curve to be used for proportional and non-proportional loaded welds.

Subject

Multiaxial fatigue
PDMR
Verity Structural Stress
Fatigue

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Embargo date

2040-03-31

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Student theses

Document type

master thesis

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file embargo until 2040-03-31