Cyclic Loading of Full-Size, Tubular K Joints
Authors: Bouwkamp J.G. 
Published: December 2007

This paper describes a study of 12 full-size, tubular K-joints with and without full weld penetration subjected to alternating loads. Results indicate that under load chord-wall stresses basically were unaffected by weld defects while branch-wall stresses increased. The fatigue life of the joints was reduced 50 percent.

Introduction

Environmental conditions have been studied extensively to improve the structural reliability of offshore drilling and production towers. Analytical procedures have been developed to describe the dynamic interaction among wave, structure, and foundation. Earthquake ground motions can be considered accurately and the potential linear and nonlinear responses of the structures can be determined. Sophisticated analytical procedures are available to assess tubular joint design. Material requirements and construction procedures can be specified to assure structural quality.

Despite significant progress in analytical capabilities and quality control, the ultimate service life of offshore structures affected by fatigue resistance of the tubular connections remains an important consideration. Major emphasis has been placed on evaluating the behavior of various joint configurations under both static and cyclic-alternating loads through experimental research. Although the studies clarified several aspects of joint behavior and resulted in improved tubular joint design, they almost exclusively considered the influence of the structural configuration affected by the relative flexibility of the tubular joint members and such joint elements as gusset plates, stiffening rings, and wing plates. Despite these studies, little or no information exists regarding the influence of weld defects on joint life. Thus, a number of full-scale, tubular K-joints under alternating loads, with and without complete weld penetration, were investigated. Joints with repaired defects and those that were part of an actual offshore tower also were studied.

Because of the basic configuration of a typical jacket structure and the local stiffening effect of the column wall by the branch members, the studied K-joints included not only the horizontal and diagonal brace members of the loaded truss, but also short sections of the brace members of the truss in the adjacent normal plane. To simplify the load arrangement without reducing the validity of the results, only loads in one plane were introduced.

Four types of weld condition were tested to evaluate the influence of weld penetration on cyclic load life of the joints. These conditions were introduced in four properly constructed joints (Specimens 1, 2, 3, and 4), four defective joints (Specimens 7, 8, XX-7, and XX-8), two repaired joints (Specimens 5 and 6), and two joints from a working structure (Specimens A-1 and E-2). Weld details are shown in Fig. 1.

Test Specimens

The joint specimens were constructed from tubular sections with the same geometry as the joints removed from an actual structure. Each specimen was composed of a 41.5-in.-OD column section with a 1.25-in.-thick wail and a 36-in.-OD pile with an 0.75-in.-thick wall. The annulus between the two tubes was grouted with a cement grouting material to enhance the necessary integrity of the column section.

JPT

P. 649