Stresses at Intersection of Tubes: Cross- and T- Joints
Authors: Toprac A.A. 
Published: December 2007

The recurring problem of joint failures in offshore structures, coupled with the catastrophic collapse of many of these structures during Hurricane Hilda (1964), has forced reconsideration of techniques presently used to design structural joints. This paper presents results of studies conducted for this reconsideration. Only right-angled T-joints and cross-joints where a brace was welded directly to the undisturbed surface of another tube (the chord) were considered. Tests were performed on seven T-joint and 12 cross-joints specimens of two types which were designed to give a broad but practical range to the geometrical parameters believed to be of significance to the problem. These parameters were expressed in dimensionless form. Each specimen of the T-joints was instrumented with both electrical resistance strain gauges and deflection dials. After the elastic tests were concluded, each specimen was loaded to failure by applying an axial tensile load to the brace. The cross-joints were tested to failure without measuring elastic strains. Theoretical solutions for the stresses in a cylindrical shell due to a distributed radial load over portions of its surface were numerically evaluated. The experimental results are compared with calculated values obtained using Bijlaard's and Dundrova's theoretical analyses.

Introduction

During the last 15 years a large number of structures were built for the offshore exploration of oil both in the Gulf of Mexico and along the Pacific Coast. In all these offshore platforms, the tubular shape was extensively used since this shape provides outstanding strength in proportion to its weight, compared to other shapes, when all directions (x, y, z) are considered. Past use of tubes was hampered because of connection details. But with the advent of welding, it became no more difficult to connect tubular shapes than to connect rolled shapes. However, some problems have been encountered in actual construction in making welded joints that are adequate and yet economical. In trying to connect tubular members, designers have used a varied number of methods; consequently, some of the joints seem to be very expensive in detail while others appear weak and underdesigned. An inspection of offshore platforms owned by different companies indicates the use of a large number of joint arrangements and configurations. Such a variety of joint types in current service indicates the wide divergence of opinion that exists on how such joints should be designed, stiffened and strengthened. Fig. 1 shows the various types of connections that have been used in practice. This paper reviews the present state of art on tubular joints of the T and cross type, and presents results of experimental and analytical work being conducted at The U. of Texas and elsewhere. It is believed that such information will be of great help to designers for the formulation of proper design procedures for joints.

CONNECTION PROBLEM

Recently, cracks have been observed in several welded connections in offshore structures. Fortunately, until about 1/2 years ago, these failures have been localized and have not resulted in the collapse of structures or loss of life. For example, one platform installed in Louisiana during 1955 in about 60 ft of water had been subjected to four tropical hurricanes. This platform successfully withstood these forces and was later removed because operations at this location had been completed. Many joint failures were observed during the removal.

JPT

P. 695ˆ