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.

The tidal and sea current turbines are one of potential utilization of renewable energies as well as offshore wind ones. However they possibly produce efficiently electricity because of high density of water to air and more steadiness of flow to winds, hydro turbine generation system is less employed than wind one. We are studying an application of Straight Wing Vertical Axis Hydro Turbine Generation Systems in the tidal current. This paper presents the results of hydraulic model test using 2-dimensional channel and field test.

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For the design of offshore wind turbines exposed to wind and wave loads, the method of combining the wind load and the wave load is significantly important to properly calculate the maximum stresses and deflections of the towers and the foundations1). Similarly, for the analysis of the fatigue damage critical to the structural life, the influences of combined wind and wave loads have not been clearly verified. In this paper fatigue damage at the time of typhoon passing is analyzed using actually recorded data, though intrinsically long-term data more than 10years should be used to properly evaluate the fatigue damage. This paper concludes that the fatigue damage of the tower caused by the wave load is not substantial and, thus, the fatigue damage by the combined wind and wave load is only 2–3% larger than the simple addition of the independent fatigue damages by the wind and the wave loads. The fatigue damage of the tower top, which is required to reduce the diameter in order to minimize the aerodynamic confliction with blades, is larger than that of the tower bottom. The fatigue damage at the foundation by the combined wind and wave load is 25% larger than the simple addition of the wind and wave damages, as the foundation is directly exposed to the wave load. For the foundation, the proper structural section can be designed in order to improve the structural performance against fatigue.

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