ABSTRACT: The integral-equation method is applied to calculate the effects of tank-wall reflections upon the hydrodynamic forces acting on a model of offshore structure. The Green function satisfying the tank-wall boundary condition is provided by first considering an infinite number of mirror images and then seeking a closed-form analytical expression for the resultant infinite series. By the analysis of energy and momentum conservation, the formulae are derived, giving damping coefficient, wave-exciting force, and drift force in terms only with the Kochin function. Numerical computations are performed for a structure, composed of four vertical circular cylinders with horizontal base, both in the open sea and in a towing tank. It is shown that the tank-wall effects on the second-order drift force is greater than those on the linear forces and resultant motions. INTRODUCTION Measurements of the hydrodynamic forces on models of offshore structures such as semi-submersibles and tension-leg platforms are usually carried out in a towing tank with parallel side walls. If the tank width is not large enough, we must expect some degree of tank-wall effects to be included in the results of experiments. In order to clarify the degree and nature of the effects of tank-wall interference, a number of theoretical studies have been made so far. Ohkusu (1975) considered first-order wave forces and second-order drift force on vertical circular cylinders which are arranged in multiple rows and an infinite number of piles. Since Ohkusu" s theory was confined only to the case where each cylinder extends to the sea bottom, there exist no evanescent-wave components. These two works were not done for the problem of tank-wall effects, but mathematical formulation is equivalent to that of tandem cylinders placed on the centerline of the wave tank.

ABSTRACT: The principle of the Pendulor is based on a resonant phenomena of a pendulum with ocean waves. The swing moting of the pendulum is used for running an electric generator. A 5kW/8kW(output/input) coastal test device was installed at Muroran harbour, based on the results derived by the studies of an early stage. After eighteen months operation, the device collapsed because of being attacked by "- storm having ten times as much power of the design rate. So, a new Pendulor was redesigned to prevent incoming over power into it, applying a wave damping method. The new device has been running for four years even after encountering subsequent storms, and also proving a 40% overall efficiency. INTRODUCTION This Pendulor device was lnvented In 1979 by the study group of Muroran Institute of technology. (Hideo Kondo et al, 1980) (Tomiji Watabe et al,1983) Its principle is similar to the sensitive work of an antenna. As travelling sea waves are inducted into a water chamber of a caisson, the waves are altered standing waves At the node of the waves, all energy is concentrated into horizontal reciprocating water flow. By the flow, a pendulum which has a pressure plate and hangs there, is driven at resonant condition, and then, the motion of it is converted to a generator through a hydrostatic transmission. We named the pendulum, Pendulor. A coastal test Pendulor system(5KW output/ 8KW lnput) was designed and installed at Muroran harbour In 1983 after a series of fundamental studies having been done. During the test, the Pendulor once collapsed because of being attacked by an unpredictable storm having ten times as much power of the design rate. So, a new Pendulor was redesigned to prevent incoming over-power into the system, applying a wave damping method.

ABSTRACT: This paper is concerned with the optimum design of stringer-stiffened cylinder found in tension leg platforms based on the component reliability. In the context of structural design, a major goal is to achieve a balanced design between safety and cost, that is, the design should probably based on the structural reliability. The total cost is minimized, which is composed of the structural cost and failure cost, with constraint on the allowable reliability index and some design requirements drawn from various design codes. The numerical results with varying the number of design variables considered in the optimisation procedure are derived and optimum designs are compared with the minimum weight designs from the view point of safety. The present results provide a more efficient design than the rule-based design in use at present and it is drawn that the minimum weight design may lie on the unsafe side. The paper ends with a discussion of the present results and an extension of the concept toward a more economically efficient design of such kind of structures. INTRODUCTION In view of the inherent random nature of the loading and the strength as well as the imperfect structural analysis and load estimation, the structural optimization needs to be based on the structural reliability. the last two decades we have seen much work on the reliability- based optimum design linked with the classical deterministic optimisation formulations. It has been proposed for some times that a more rational criterion for structural design is that safety be represented by the reliability or alternatively at the failure probability of a structural, and hence the reliability constraint should likely be included in the re-design and the optimisation process to get a more efficient design (Kishi et al,1985, Frangopol,1985, Corotis and Soltani,1985).

ABSTRACT: It was found by Chaplin and by the the wave forces acting on cylinders submerged in regular and irregular waves decrease with increasing Kc number in number region (1<Kc<3). In the present paper, the effect of the steady motion of a circular cylinder on the reduction of the wave forces is experimentally and theoretically investigated in regular waves. The experimental results show that the inertia forces acting on the cylinder are significantly affected by the constant speed, and the tendency of the inertia coefficient depends on the direction of motions. The cause of this result is investigated by simulations of the vortex shedding patterns. INTRODUCTION It was first found by Chaplin (1984a, 1984b) that the wave forces acting on a circular cylinder horizontally submerged in regular waves dramatically decrease with increasing Kc number due to VISCOUS effect in very low Kc number region (1<Kc<3). In such low Kc number region it had been believed that there is no VISCOUS effect on the inertia force in plane oscillatory flow as shown by Bearman et al (1985) and Sarpkaya (1986). The authors (1988a, 1990) investigated the phenomenon experimentally and theoretically, and pointed out that the reduction of the wave forces is caused by the circulating flow created by separated vortices from the cylinder which is in much greater scale than those in sinusoidally oscillating flow at the same Kc number. They also found that the same phenomenon appears for a square cylinder, a flat plate and a three-dimensional semisubmersible (Ikeda et al, 1988a, 1988b). In irregular waves the same reduction of the wave forces occurs (Otsuka et a1, 1989, 1990). In the design of an offshore structure the current and the drifting motion should be taken into account an estimating the hydrodynamic wave forces acting on it.