ロッドの強度からみた海洋掘さくの安全作業限界

抄録

In order to find the limitations of safety operations in off-shore borings, the authors' previous theory on the lateral vibration of the boring rod in the sea has been checked by the model experiments and improved by the new method for evaluating the factor of damping due to the water surrounding the rod as well as by the application of the virtual mass of the water. Furthermore, the maximuiri bending_stresses of off-shore boring rods have been calculated by the above-improved theory, and the limitations of safety operations in off-shore borings have been presented by comparing those bending stresses with the fatigue strength of the rod-material.The main results obtained are as follows:
1) The previous theory provides the natural frequencies of the rods which are higher by about 16% than the actual frequencies. However, these discrepancies could be almost reduced by applying the two-dimensional virtual mass of the surrounding water to the theory.
2) Owing to improper evaluation of the damping factor of the surrounding water, the previous theory gives the bending stress almost twice as large as that of the practical rod at the resonance of its lateral vibration in case of the deep sea. However, the theory combined with the new method for obtaining the equivalent viscous damping factor from the drag coefficient, which is already known, is very useful for estimating the bending stress of an off-shore boring rod.
3) Within the limits where the buckling of the rod does not occur, the range of safety operation of an off-shore boring is wider in case of deeper depth of the sea and narrower in case of greater tensile load axially applied at the top of the rod. Besides, it can be said that the current rule of thumb is very insufficient for telling the limitation of safety operation in an off-shore boring.