Static And Fatigue Tests On T-Joints Stiffened By An Internal Ring
Authors: Sawada Y. , Idogaki S. , Sekita K. 
Published: October 2008

With offshore structures constructed of steel tubular members, the greatest stresses occur in joints, making them the weakest parts of the entire structure. This paper presents the results of static and fatigue tests conducted on tubular T-joints. Test specimens included stiffened and unstiffened joints. The former were complex joints stiffened by means of an internal ring in the chord. The load was applied to the branch in the axial direction. Test results showed that a stiffening ring used in the chord significantly increases both the static and fatigue strength of the T-joint. These tests were also useful in establishing the static and fatigue design method for T-joints stiffened by an internal ring.

INTRODUCTION

Steel pipe is widely used for the structural members of offshore structures because it exhibits excellent resistance to fluid forces caused by currents and waves. Other advantages of steel pipe include buoyancy and the non-directionality of section properties. But steel pipe is susceptible to local loads applied vertically to its surface, a drawback that makes tubular joints the weakest points of an offshore structure; Moreover, since offshore structures are constantly subjected to fluctuating loads due to wave forces, joints in which significant stresses occur are prone to fatigue failure.

Because tubular joints are of intricate geometrical configuration, it is difficult to determine their stress distributions and ultimate static strength by means of analytical methods. Therefore, laboratory tests have been used as the primary means of obtaining these data. API 1 and AWS 2 design criteria for tubular joints, which are now widely used in the design of offshore structures, were established through laboratory tests, as reported by Marshall.

We conducted static and fatigue tests to evaluate stiffening methods for tubular joints. In these tests, we used unstiffened T-joints as well as complex joints stiffened by a ring placed inside the chord. This paper reports the test results for the static and fatigue strength of T-joints under axial loads on the branch.

In designing offshore structures, ultimate static strength and local strains must be estimated, and the appropriate S-N curve selected. Accordingly, we made these studies a part of our tests.

The method of stiffening with an internal ring can be applied to any steel tubular joint having a diameter large enough to permit a stiffening ring to be welded on the inside.

STATIC TEST

Testing Method and Test Results

We used two unstiffened and seven stiffened joint specimens. In each of the seven stiffened joints, a ring of a different width wall thickness and material was placed inside the chord. The specimen configuration is shown in Fig. 1. Fig. 2 shows the test setup. The specifications of the specimens are shown in Table 1, together with tensile test results for the steels used.

Internal rings for stiffened joints, with the exception of specimen R-1-3, consisted of four pieces built up by butt welding, as shown in Fig. 1. For comparison, a single-piece ring was used for specimen R-1-3.