ABSTRACT

Using scale models one-sixtieth of the size of a deep-sea platform built off Aga, in the Sea of Japan, we conducted a series of laboratory tests on wave and current forces acting on a fixed platform. In conducting the experiments, we used a water-wave tank in which the models were subjected to wave periods varying from 0.9 seconds to 5.0 seconds and wave heights from 5 cm to 42 cm. We measured both vertical and horizontal components of wave forces, taking the effect of the buoyancy variations on the vertical components into consideration. As for current forces, we measured their horizontal components only. Three jacket models and one boat landing model were used in toe experiments. Results were evaluated in terms of wave forces coefficients by using the Stokes fifth-order approximation wave theory. Current forces were evaluated in terms of drag coefficients. We also analyzed in terms of relations between Reynolds number (103-3×104), wave steepness (0.003-0.1), and a ratio of the model?s width to the wave length (0.01–0.6). It was concluded that the drag and inertia coefficients of the fixed platform can be estimated by the Stokes fifth-order wave theory to be approximately 0.6 and 1.6, respectively.

INTRODUCTION

Many field and laboratory experiments have been conducted to find wave forces on piles, 1–3 and small- and finite-amplitude wave forces have been analyzed by using the Morison formula. We too conducted wave and current force experiments on relatively simple structures -e.g., horizontal pipes, vertical piles, stingers, dolphins and sea berths -- and obtained wave force coefficients by using the small-amplitude wave, solitary wave and Stokes third-order wave theories. More recently, we conducted experiments to obtain current and wave force coefficients for jacket structures as framed structures. For wave force coefficients, we used the Stokes fifth-order approximation wave theory.4 This report describes these experiments and test results.

Model-Testing Equipment

These experiments were conducted using a water-wave tank. Since it was impossible to generate currents in the tank, each jacket model was moved in the water to test its resistance to currents by using a flat car, which traveling on tracks installed above the tank, hoisted and towed the model at a uniform velocity or a uniform acceleration.

Models

In consideration of the capacity of the water-wave tank and the various conditions at the test site, Froude models to a scale of 1 to 60 were employed. These consisted of one jacket without conductor pipes, one with 15 conductor pipes, and one both with 15 conductor pipes and a boat landing (Fig. 1). A 1/20-scale boat landing model was also used. All models were made of plastics.

Wave Force Dynamometer

Since the center of external loads applied to the jacket changes constantly as in the case of wave forces, the force is given as the difference between bending moments at two points (mathematical equation)(available in full paper) Both the-horizontal and vertical forces can be determined at the same time by using two bridge circuits each consisting of four strain gauges.

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