Behavior of Water in Vertical Well with Bottom Opening of Ship, and its Effects on Ship-Motion

Abstract

Recently some ships have been constructed with a vertical well which have the bottom opening for the purpose of raising and lowering the equipment. Some examples are vessel-type drilling rigs, vessels carrying small submersibles and so on.
This paper shows the behavior of the water in the well, and its effects on the motion of the ship, when she is running.
Experimental studies were performed under two different sets of conditions.
(1) The motion of the free water surface in the well was studied, when the ship-model was fixed stationary in the stream.
(2) The effect of the motion of the water in the well on the motion of the ship-model was studied, when the model was towed.
The following conclusions were obtained.
1) When the ship-model with a well is fixed stationary in the stream, it can be observed that the water surface moves regularly in it. This phenomenon is similar in many respects to the motion of a cylinder which is elastically supported by springs in the stream.
2) The type of water motion, whether up and down, right and left or forwards and backwards, depends on the velocity of the stream and the shape of the well.
3) The water motion can be seen at the dimensionless stream velocity of 0.2<2U/lω₀<1.6, where U is the stream velocity, lis the length of the bottom opening and ω₀ is the natural circular frequency of the water moving in the well.
4) The experimental values agree well with the results of the theoretical calculation on the heaving amplitude of the free water surface in the well, and on the increased drag of the ship-model by the motion of water in it.
5) In order to reduce the motion of water in the well, it is very effective to fix short flanges on its vertical walls a little below the water surface.