Fatigue assessment of trailing suction hopper dredgers

Abstract

Since the 19th century hopper dredgers have been used for civil operations such as land reclamation and water way maintenance. In the past 25 years a trend of scale enlargement of hopper dredgers driven by economic and productivity reasons is seen. Scaling of the designs has effect on strength and fatigue assessment as well on settling of particles which affect the rate op production. This thesis is focussed on the fatigue assessment of the hopper dredger. Cracked damages in coaming and bottom door areas have been found in relatively large hopper dredgers that are classed by Bureau Veritas. According to Bureau Veritas rules for steel ships fatigue assessment is required if the ship has length greater than 170 m but damages have also been observed in hopper dredgers with a smaller length. This indicates that the hopper dredger design demands for a specific fatigue assessment procedure. The hopper dredger has a unique characteristic load profile consisting of loading and unloading cargo several times a day in combination with small wave heights due to reduced freeboard regulations. Special interest is given on the effect of the loading and unloading cycle in the corners of the bottom discharge openings. It is suspected that the effect of this characteristic cycle may be underestimated and requires an alternative assessment procedure. The main question is what the contribution of this cycle is and how it should be assessed. The relatively low frequency of the cycle raises the suspicion of low-cycle fatigue and this phenomenon is therefore investigated. It is concluded that the loading and unloading cycle has a major contribution to the fatigue damage in the bottom opening corners. In fact 97% of the fatigue damage is induced by the dredging cycle and therefore this cycle could be seen as the sole cause for fatigue cracks in the bottom opening corner structural details. The high stress ranges confirm the suspicion of a low-cycle fatigue phenomenon and therefore further research is done on how to approach this type of fatigue. A method is proposed to take into account bi-axial cyclic plasticity and material hardening based on the use of a maximum principal hot spot stress range. Linear-elastic stresses are corrected to pseudo elastic stresses and effects of SN-curve selection, material selection, bi-axiallity and residual stress are analysed. Based on the found pseudo hot spot stress ranges it is concluded that the linear-elastic stresses in the analysed hopper are not high enough to demand for such a low-cycle fatigue assessment approach. The bi-axial ratio between the two multi-axial plane stress components has no effect on this conclusion.