Dynamic investigation of semi-rigid tubular T-joints

Offshore structures are fabricated from tubes or circular hollow sections. Tubular joints which are created at the intersection of the circular members experience complicated structural problems such as stress concentration, ultimate strength, joint stiffness and fatigue life. Some aspects of tubular T-joints regarding joint stiffness and stress concentration are investigated in this thesis. The methods employed in these investigations are based on dynamic theoretical and experimental modelling techniques. Semi-rigidity of tubular joints leads to different results when compared with the common assumption of rigid joint analysis. However, due to the lack of an efficient modelling method, tubular joints are still assumed to be rigid in most analyses. A method based on the model of a beam with end springs is used in this study to derive the bending and axial stiffness of joints. This method can be distinguished from other techniques, as it employs the natural frequencies rather than static measurements. A n extensive Finite Element analysis is carried out to establish a series of stiffness equations for inplane bending, out of plane bending and axial deformation of brace in tubular T-joints. The proposed parametric equations include diameter to thickness ratio which is not presently considered in any other formulae. The analysis results show that including diameter to thickness ratio makes a significant difference in the stiffness of tubular T-joints. The validity of the Finite Element analyses is investigated by experimentally testing eleven steel T-joints. The effect of dynamic loading on strain or stress concentration factors in tubular joints are also investigated in this thesis. Stress concentration factors are used to calculate the maximum stress values at the tubular joints. The results show that strain or stress concentration factor is frequency dependent, when strains at tubular joints are subjected to dynamic forces. This parameter has not been considered in the other studies of stress concentration of tubular joints. Another aspect of tubular joints investigated herein is the effect of joint stiffness consideration on the fatigue life estimation of offshore towers. Results of the analyses of a 100m tower in this study show an average difference of 30% in fatigue life due to the flexibility consideration of tubular joints. Further detailed investigation though is required into the joint stiffness effect on fatigue life estimate of tubular structures