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.

With more and more offshore pipelines being laid in deeper water, the need for larger laying equipment has increased larger barges, stronger tensioners and longer stingers. The primary reason is that, in deeper water, the pipeline is suspended at a steeper liftoff angle, and consequently a longer over bend portion is required between the liftoff point and the welding stations In order to help relieve stress in the pipeline. If the over bend portion can be curved until the plastic region of the pipe is reached, it will be possible to use shorter stingers and lay pipe in deeper water without any special equipment investment. We took note of these possibilities and conducted integrated experiments consisting of material tests, pipe bending tests and pipe well denting tests. We also developed a continuous beam elastoplastic analysis program, a pipe section analysis program and a cylindrical shell analysis program. Using both test data and programs, we established a precise elastoplastic design method for laying offshore pipelines. Measured results show good agreement with calculated results. This elastoplastic design method accurately predicts the capacity of pipe to bear environmental loads, thereby making it possible to set optimum limitations for laying operations. However, it should be noted that, just above the yield point of the pipe, slight variation environmental loads cause significant changes in the strain and deflection. It would therefore be desirable in actual laying operations to use a stinger that permits control and restraint of pipe curvature and deflection. INTRODUCTION The elastoplastic design method for laying offshore pipelines makes it possible to use shorter stingers, which results in reduced liquid forces acting on the stinger pipeline system. This in turn results in improved operating rates and increased water depths in which pipe can be laid, as well as reductions in cost. Moreover, optimum limits set on laying operations by this design method ensure operational safety. Three problems arise when large strains are induced in the pipe during laying. It is essential for the elastoplastic design method to solve these problems. The three problems are:NonlinearityThe bending stiffness of pipe bent beyond Its proportional limit becomes nonlinear due to the nonlinearity of the steel's stress-strain characteristics and the ovaling of the pipe section. Therefore, a nonlinear analysis must be made of the portion of the pipeline between the lay barge and the ocean floor.Local bucklingIn our experiments, all pipe specimens with a D/t ratio of 56 or over failed due to local buckling. In the case of specimens with a D/t of 56, local buckling occurred after the yield point was reached, whereas specimens with D/t ratios of 78 and 97 buckled locally before their yield points were reached. Thus, the elastoplastic design must give full consideration to local buckling.

A commercial or industrial large city often develops at the hinterland of a deep bay owing to its high defensibility against stormy sea. However, the exchange rate of sea-water between the inside and the outside of the bay is very low, so that the water is much polluted by industrial and domestic waste water in many cases. The Dokai Bay has also the same social problem. In order to solve the problem we propose an idea of improvement of water quality in this paper, which is that clean sea water of Hibiki open sea is introduced into Dokai Bay. Various conditions for water purifications have been attempted by using the diffusion simulations of COD. It has been clear that introduction of clean sea water (3m3/sec.) into the innerpart of Dokai Bay is very effective for purification.