A deep sea ROV is desired to be light from the viewpoints of reducing a tension on an underwater cable and possessing the adequate movability by thrusters in water. On the other hand, when moving by a crawler system on the seabed, such a lightweight will influence on its movability characteristics. As an initial investigation experiments using an actual ROV were conducted in a water tank. And it was observed that the ROV ran in wheelie in some cases and almost fell down in extreme case in spite of the fact that it could run stably on land. In order to clarify the results fundamental theory of stability of the ROV in steady running is presented. The theory gives the discriminant chart of stable running for the combination of the center of gravity and the center of buoyancy. Experiments using a model were conducted to verify the theory. In order to increase the stable area and also to change meaningly the center of gravity, this paper proposes a method to virtually increase the weight and to change the center of gravity by using thrusters. And preliminary experiments were conducted to confirm its advantageous effect.
In order to reduce tension on a cable and process movability, a deep sea remotely operated vehicle (ROV) is designed to reduce its weight. However, a lightweight ROV is apt to wheelie when running by means of a crawler system. To run stably in counterpoise, the combination of the center of gravity and the center of buoyancy should be in an adequate area called the “stable area,” which by theory can be obtained as corresponding to the weight and the buoyancy. The stable area becomes small as the weight is light. The combination of the center of gravity and the center of buoyancy is designed to be in the stable area. However, it is important for the ROV to run forward and backward, which results in changing the discrimination of the stable area. This sometimes causes the center of gravity and the center of buoyancy to be outside the stable area. Thus, it is advantageous to increase the weight only when running by crawler system and to change the center of gravity meaningfully. Furthermore, the flipper-type crawler system is advantageous when running on the sea floor with inclination or undulation. This paper proposes a method to virtually increase the weight and to change the center of gravity by using thrusters. This paper also describes the flipper-type crawler system that improves movability when running on the sea floor with inclination or undulation. Furthermore, we conducted preliminary experiments in a water tank using a small-size ROV having four thrusters and a crawler system, a normal-type crawler, and a flipper-type crawler system, to confirm the advantages.
The crawler driven ROV is considered as one of the probable systems for seafloor exploration or seabed resources development [1][2][3][4]. However the movability of crawler driven ROV on the sea bottom is not explained clearly compared with the ones on the land [5][6][7][8][9]. The experimental investigation on the crawler based ROV’s movability suggests that light weight ROV are easy to run in bow up condition and sometimes are possible to turn over. Therefore it is important to make it clear what kind of condition is to be satisfied for the normal run when the ROV moves on the sea bottom with crawlers. In this study, a simple dynamic model for the ROV which runs steadily on the inclined smooth sea bottom has been developed and the condition which should be satisfied for normal running has been derived. We consider that the forces acting on ROV are gravity, buoyancy, reaction from sea bottom, thrust and hydrodynamic resistance and treat them as concentrated loads. From the balance condition of forces for longitudinal and vertical direction and pitching moment, three kinds of relation are obtained. We consider that force reaction point from sea bottom should be inside between the fore and rear wheels for the normal operation. With this condition, the relation to be satisfied between the location of gravity center and center of buoyancy is obtained. To validate this conditional equation, the model experiments are carried out. The ROV model contains several weights and floating materials so that the longitudinal center of gravity and buoyancy can be changed by moving the weights and floats. Changing the longitudinal location of buoyancy center of the model from backward to forward, the limiting normal running condition for each longitudinal location of gravity center are measured. The obtained experimental results agree well with the theoretical ones. The presented conditional equation to be satisfied for normal running is considered to support the basic planning of a crawler driven ROV.
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