Strength of Contact Part of Rack Tooth for Leg Fixing Device of Juckup Rigs

Abstract

The rack teeth of a leg fixing device, which is indispensable for a jackup rig to operate in harsh environment, such as the North Sea, take severe load and the Hertz contact pressure of the contact part exceeds the yield point of the material. In this case, plastic deformation of the contact part gives favorable effect on the distribution of load among mutually contacting several pairs of rack teeth. Accordingly, it is very important to study on the strength of contact part of rack tooth for static and cyclic loading.In this paper, static tests and cyclic contact tests on scale models of rack tooth contact part are performed. Then three kinds of analyses (3-dimensional, plane stress and plane strain analysis) are performed on the static contact model by using the finite element method for elasto-plastic contact problem. Following conclusions are obtained from a comparison between the results obtained from the calculation and the static test as well as from the results of the cyclic contact test.(1) Comparing the calculated results of three kinds of analyses by the finite element method, it is made clear that the effect of the stress in the direction of plate thickness on the stress and strain distributions on the contact area is predominant.(2) From the comparison between calculations and experiment concerning strain, maxi-mum displacement, residual displacement and contact width, it is shown that the results of the 3-dimensional analysis correlates quite well with the experiment ones.(3) From the results of the 3-dimensional calculations on the contact model with various thickness of the plate, it is found that the feature of the transition from plane stress state to plane strain state is made clear.(4) Cyclic contact test up to 2×104 cycles shows the characteristic of the behavior of displacement and strain of the models. It is clarified that there will be no problem as for the strength of the contact part even under such high contact pressure σH equal to Brinell Hardness HB as σH/HB=1.0.