Cyclic Loading of Full-Size Tubular Joints

ABSTRACT A test program on twelve full scale tubular joints is described. The effect of-weld defects was studied under both static and cyclic fatigue loads. The joints consisted of 41.5 in × 1.25 in. column sections with a grouted pile section of 36 in. × 0,75 in. Branch members were 18 in × 0.50 in. for the horizontal and 18 in × 0,625 in. for the diagonal sections. Weld defects in the welds between branch and column walls resulted in an increase of the branch member stresses as compared to the stress condition in properly fabricated joints. As a result of the particular defects the fatigue life of the joints was reduce by almost 50%. Strain deterioration and crack progression were observed and correlated. Linear elastic fracture mechanics fatigue failure predictions were compared with observed fatigue test data. INTRODUCTION In order to improve the structural reliability of offshore drilling and production towers environmental conditions have been studied extensively. Analytical procedures have been developed describing the dynamic interaction between wave, structure and foundation. Earthquake ground motions can be considered accurately and the potential linear as well as non-linear response of the structures can be determined. Sophisticated analytical procedures are available to assess the tubular joint design. Material requirements and fabrication procedures can be specified to assure structural quality. Despite the significant progress in analytical capabilities and quality control, the ultimate service life of offshore structures as affected by the fatigue resistance of the tubular connections still remains a factor of considerable concern. Major emphasis has been placed on evaluating through experimental research the behavior of various joint configurations under cyclic alternating loads. Although these studies clarified several aspects of joint behavior and resulted in improved tubular joint design, they almost exclusively considered the influence of the structural configuration as affected by the relative flexibility of both the tubular joint members and other .joint elements (e.g., gusset plates, stiffening rings, wing plates, etc.). Despite these studies little or no information seems to exist regarding the influence of weld defects on the joint life. Hence, it was with this specific objective in mind that a number of full scale, common, jacket type tubular joints with and without weld defects, have been investigated under cyclic load conditions. The studies also included joints with repaired defects and joints having been part of an offshore tower. Because of the space frame configuration and the associated relative stiffness of both the column and brace members, the joints were made up of typical horizontal and diagonal brace members of the loaded truss, as well as the brace members of the truss in the adjacent "normal" plane. The latter members were limited in length. To simplify the load arrangement without reducing the validity of the results, only loads in one plane were introduced.