Recent Trends in Research on Tubular Connections

Abstract

Abstract This paper presents three general approaches towardsthe solution of the stress distribution and the behaviorof tubular connections as used in offshore well drillingstructures. First, the possibilities of using plastic models andphotoelasticity techniques in evaluating the stressdistribution in gusset plate connections are analyzed. Theresults of photoelastic studies on the stress distribution inthe in-plane gusset plate of two-dimensional joints areprevented. The influence of the configuration of thegusset plate (with and without cut-outs) is discussed. Second, the paper deals with recent developments andapplications of computer programs to analyze connectionswith directly inter-welded tubes and with gusset plates. The possibilities and limitations of these programs arediscussed. Stress patterns analyzed with these programsare presented for different joint configurations. Finally, the basic test procedures and results of a teston a tubular joint under static and alternating loads arediscussed. Introduction The effective design of tubular connections asencountered in offshore well drilling towers or floatingplatform has been complicated basically by the radialflexibility of the tube walls. This flexibility is a source ofsevere stress concentrations which can initiate an earlyfailure of these joints. In an attempt to reduce the influence of the wallflexibility of the column tube, certain joints are presentlydesigned by inter-welding the incoming branch members. In those designs, the force acting normal to the chordtube can be reduced considerably. A second group ofjoints incorporate gusset plates or stiffening rings tostiffen the column wall and to distribute the incoming branchmember forces over a larger part of this wall. A thirdapproach to improve the stress distribution in a tubularjoint is to increase wall thickness of the columnmember. This can be achieved by simply applying a thickerwall section in the vicinity of the joint. A fourthpossibility to restrain the radial flexibility of the tube wallis to fill the column member with concrete. Also, a single, cast steel seat welded to the column tube can be usedto improve the stress distribution in this wall. Althoughall these designs improve, in general, the state of stressin the column wall, the altered stiffness often causes thedevelopment of critically stressed areas in the webmembers. At the same time the actual design in mostinstances is decisively influenced by the site which governsthe depth and controls the forces produced by waves, earthquakes and ice flow. Also, the towing, erection andfoundation requirements of offshore structures can affectthe actual design selection. Because of the complexity of the structuralconfiguration of tubular joints, the stress analysis of theseconnections has necessarily been based on simplified andoften crude assumptions. For the earlier and smaller typeof connections with about 4-ft diameter columnsections, the primary problem was to evaluate the relativestiffness of the column wall section and the load transferbetween joint members. Due to the recent developmentsof offshore drilling structures with increasingly largerconnections (e.g., column diameter, 32 ft; webmembers, 8.5 ft) this problem has become even more critical. One design philosophy for such large joints followsa member-to-member connection with radially heavyreinforced column sections. This radial stiffness can beattained by closely spaced and intensively stiffenedhorizontal diagrams together with vertical stiffeners. Asecond solution incorporates a concrete-filled sectionbetween the outer and inner walls of the column tube. Another philosophy considers large gusset-plated joints. The problem in these joints is to develop an effectiveload transfer between the branch tubes and the gussetplate and to minimize the stress concentrations in themember walls as effectively as possible. Several conceptsare followed to achieve this gradual transfer betweenweb-member walls and gusset plates. Because the number of joints in these huge platformsis limited compared to the over-all size, a proper designof these joints is even more important than was the casefor the many joints in the smaller, but multi-leggedtowers. A failure of one of those ultra-large joints could wellcause the complete collapse of such a structure. Underthese circumstances an accurate analysis of the largejoints is of the greatest importance, together withinformation regarding the expected behavior of such jointsunder critical alternating load conditions. Recent applications of photoelastic model techniqueshave proved to be quite effective in evaluating the elasticbehavior of such joints. Although a complete study ofsuch connections is quite well possible with present-dayphotoelastic techniques, it might often be feasible andnecessary to limit the objectives and to restrict theseinvestigations to the study of a specific aspect of the joint. Another very promising avenue of approach to solvethe complex stress pattern in these connections seems tobe the recent development of digital computer programs. JPT P. 1491ˆ